GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Acceptable Voltage Drop on Neutral Service Conductor

maine_tyler | Posted in General Questions on

I’ve been dealing with load fluctuations when other loads turn on/off, and not just large motors start ups. I even have an electric kettle (Fellow Stagg) that will cause lights to flicker when it goes into temp control mode, and not just lights on that branch circuit.

I took some measurements and my conclusion is that my service conductor neutral has unreasonably high resistance.

This is a U.S. residential 120/240 spit-phase 200amp service with aerial service drop.

So my questions are:
1) What is an allowable (or acceptable) voltage drop across the neutral service conductor (from the transformer to the service entrance breaker panel). My service drop is indeed very long as I’m in the sticks. At what amperage should the service drop be calculated? It’s a 200amp service.
2) Can you check my calculations to make sure I’m not messing something up. I will post those below as a comment.
3) My plan is to call the utility. Should they be amendable to coming out to look at this? Anything that I should prepare to convince them? I’m trying to avoid dropping a bunch of useless cash on an electrician to come out and tell me I need to call the utility.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. maine_tyler | | #1

    See uploaded image for the edison-3-wire circuit diagram (a poorly drawn one). Note the math is a bit off when you go around the top loop, but I'm using my measurements, and it's quite close.

    With L2-N loads shut down, and with 10.5A of resistive load on L1-N, I get a reading of 113.8V (L1-N) and 127.8V (L2-N). L1-L2 voltage at this time was ~242V. These measurements are all taken at the service entrance panel, so all the branch wiring is being treated as part of the load (and I did confirm further voltage drop along the branch wiring).

    While running a 240V L1-L2 resistive load of 24A, I measured voltage drop of 5V, which should roughly translate to 0.21 ohms of resistance for the loop, so 0.1 ohms for each hot leg.

    This shows (roughly) a drop of 6.15V on the neutral alone. 6.15/120= about 5.1% drop at only 10.5 amps.

    1. maine_tyler | | #20

      Just to make my calculation process a bit clearer for posterity:

      242V from L1-L2 is with essentially no load, which is why the source leg voltages are given as 121V in the above diagram.
      With a 24A load L1-L2, I measured 237V, which is how I determined a volt drop of 5V on the 'hots'.
      I then did the Ohms law calc to come up with the roughly 0.1 ohms of resistance per hot leg (this does assume equal drop on each half of this 240V loop, which is technically unknown). Those resistance values were then used as knowns in the above diagram, along with the known voltage and current readings. The highlighted V-drop on the neutral is then calculated using Kirchhoff's voltage law (sum of voltages in a closed loop = 0). Measurements were taken pretty close together in time and 'no-load' voltage measurements were taken intermittently to confirm a mostly steady 242V no-load state. All loads were resistive.

  2. Expert Member
    BILL WICHERS | | #2

    My guess is you have a loose connection somewhere, which is something that needs to be fixed. Can you run the following measurements for me (you'll need two 1,500w electric heaters or hair dryers for this):

    1- Disconnect ALL LOADS from the system, except for two receptacles on opposite legs that are close(ish) to the panel to make testing easy.
    2- Measure the voltages L1-N, L2-N, and L1-L2 with all loads disconnected.
    3- Put one 1,500w load on L1, measure the L1-N voltage and L2-N voltage. L2-N will look a little higher than what you measured in #2
    4- Move the 1,500w load to L2, measure the L1-N and L2-N voltages again, this time L1-N will look a little higher compared to #2.
    5- Put the second 1,500w load on L1, measure the L1-N, L2-N, and L1-L2 voltages.
    6- Disconnect both 1,500w loads and measure L1-N, L2-N, and L1-L2 again.

    Now put everything back together, and post those measurements. From those measurements, I can give you a good idea of what is likely to be going on. It would be helpful to know the approximate length of your service drop too.

    From what you've posed so far, you either have a long run of undersized cable for your service drop, one or more loose connections, or a combination of those things. The measurements I asked for will help to narrow things down. Note that you want to try to take all those voltage measurements as closely together in time as possible to avoid normal line voltage variations from impacting your measurements too much.

    One other thing you could look for AFTER running those measurements I asked for is to leave the load on either L1 or L2 (but not both together) for a period of 15 minutes or so and see if you see the voltages noticeable change over that period. If you see volt drop gradually increasing with time, that might mean a loose connection that is getting worse as it heats up.

    Once we have things narrowed down a bit, I can give you some ideas of what to say to the utility. If you have done things right, and know how to talk to them (and it's not difficult), they'll send someone out to check and they should fix things if it's a problem with THEIR equipment. If it's a problem with YOUR equipment, you'll need an electrician to do the work. Classic places for loose connections are the lugs in the meter can and the lugs on the main breaker, and between the main breaker and the busbar in the panel. Meter can lugs would be the first place I would check myself. Anything at the pole is the utility's problem to fix, and they're usually pretty good about that. If the service drop is too small for the length, that can be more difficult to get the utility to fix, and it might be your responsibility anyway if it's underground.

    BTW, it's really important that you disconnect all loads except for the two heaters when making these tests. Reactive loads like motors can confuse these kinds of measurements. Electric resistance heaters (which includes hair dryers) are RESISTIVE loads, which are nice and linear loads that will help you to get good measurements.


    1. maine_tyler | | #4

      I will think about doing what you describe, but isn't that essentially what I've already done and shown here?
      I should mention that I measured L1-L2 with no loads and that's where I got the 242V. I then added a sizeable resistive load (electric kettle) to L1-N and took current and voltage readings which are shown in my above diagram. There was negligible current on L2-N, but even so I included it in my math.

      I'm happy to do more testing, but I don't quite understand what more it will reveal vs what I have already done, which if I am not mistaken, has shown the voltage drop across the service conductor neutral for a given resistive load current. If I am mistaken, please point out where or why it is inadequate.

      >"Classic places for loose connections are the lugs in the meter can and the lugs on the main breaker, and between the main breaker and the busbar in the panel. Meter can lugs would be the first place I would check myself. Anything at the pole is the utility's problem to fix."

      I was thinking about the connections between the service entrance and the pole. Isn't the meter owned by the utility? Would that be their responsibility to access? I thought prying into the meter was a big no-no.

      My plan to troubleshoot those connections was going to be to place an extended multimeter lead on the neutral service conductor upstream of the meter (there is easy and safe access at the weatherhead), and another lead on the neutral bus bar in the main panel. Any loose connections between those points would show up as voltage drop when there is current (which I would measure as well). I haven't done this yet. I sort of thought all those connections belonged to the utility, or am I mistaken about that?

      1. Expert Member
        BILL WICHERS | | #5

        The idea with the extra tests is to both make sure it wasn't just random utility variation you were seeing (which it probably wasn't, but you want to be sure), but the big one is you need to put a load on L1, measure voltages, than place an IDENTICAL load on L2 and measure voltages again. You're looking for differences between the two. You also need to make sure there are no other loads on the system at the time that could be messing up your measurements.

        Normally the neutral will show about one half of the total volt drop on a circuit, because the neutral is one half of a loop. If the neutral is at fault, you should see the same problem when putting the load between the neutral and either leg. I would not entirely trust the measurements until having tested both legs like this.

        Note that it used to be common to use a service cable with a smaller neutral compared to the two "hots". This isn't the case anymore, but I don't know how old your cable is. Your 24A load is also showing quite a bit more volt drop than I would expect for that amount of load on a 200A service. Those other measurements will help to determine if there is anything else going on. Some of the volt drop is going to be on the primary and in the transformer too, so it's not entirely volt drop along the service conductors.

        The meter is owned by the utility, but it's usually your responsibility to install the meter base and associated wiring between the meter and the panel, and between the meter and the service drop. Sometimes you have to run cable all the way to the pole too, especially if it's an underground service. If you have volt drop problems, once you've figured out what's going on I'd call the utility. Utilities don't usually charge to come out and check this sort of thing.


        1. maine_tyler | | #7

          I'm happy to test the other 120 volt leg to get a clearer picture, but given the volt drop I measured on the 240V load compared to the 120V, it's clear that the neutral is far worse off than either hot. But like you say, even the 240V load V-drop is a bit high, and you are right that technically I don't know if the two 'hots' are dropping equal voltage.

          I will go measure the service drop distance, but I'd say its approximately 300-400 feet. Aerial drop. Probably kinda old (at least the transformer looks it).

          Thanks for the thoughts. I do think ultimately I will be calling the utility.

          1. Expert Member
            BILL WICHERS | | #9

            300-400 feet is a *long* way. I have about a 300 foot underground run at my house, all 2/0 aluminum (I'd rather it be 4/0, but it is original from way before I was here). See if your transformer has a big number on it -- a single number like "10" or "25" is usually the KVA rating of the transformer. If it's something really small like "5", then the transformer is probably part of the problem as 5KVA is only a bit over 20A at 240V. I rarely see smaller than 10KVA anymore on anything except small stuff like street lights these days, 25KVA and 50KVA are much more common for residential services (my house is fed with a 25KVA transformer, for example). If you have miles and miles of primary (high voltage) wiring between your transformer and the substation, that will contribute some volt drop too, but usually not much.

            My guess is one or more loose neutral connections, and I'd suspect so-called "mechanical" (screw-type lugs) first here. Check for corrosion too, and that the lugs are *tight*. Utilities sometimes use split bolts, but it's much more common for them to use "compression" connectors, which are big crimp terminals that are not prone to problems. Split bolts ARE commonly used between the aerial service drop conductors and the conductors coming down the mast to the meter base though.

            Old wire is usually fine as long as it hasn't been physically damaged. The conductive metal part of wire doesn't really degrade with age (for the most part), only the insulation does. Old wire will still work fine as long as it doesn't have bad connections on the ends, assuming the wire itself is physically in decent shape.

            Call the utility, tell them you have low voltage. You want your "low voltage" to be something below 110 volts, since the acceptable range is typically defined as 110-125v. The way to do that is to turn on everything you can when they are testing, to present maximum load to the system, and to result in the highest volt drop. It's their problem to deliver the correct voltage to your meter base at whatever load your service is supposed to support, so they shouldn't give you a hard time if they see you turn on all your stuff prior to them making measurements.



          2. charlie_sullivan | | #11

            As far as what to turn on when the utility shows, turn on stuff all on one phase, like you've been doing, but a little more so you get the voltage clearly below 110. But also, unplug anything that might be damaged from the other phase because that voltage will go high when you do that.

  3. walta100 | | #3

    When you say “large motor” just how large are we talking about?
    Most service providers do not allow motors over 7 HP on residential services. I this motor happens to be over the limit you may want to choose your words carefully when complaining about your neutral problem.

    Note any motor over 1.5 HP is likely to run better when set up for 240 volt operation.


    1. maine_tyler | | #6

      "not just large motors start ups"

      In other words, I get the fluctuations with all kinds of much smaller loads that I wouldn't expect, unlike large motors which I would expect to see this type of behavior on startup.

  4. charlie_sullivan | | #8

    Your diagnosis from your testing is correct. Bill's list of possible problems is also correct. The additional testing he recommends would narrow things down some, but I don't think you need proof that rigorous to move on to next steps. You mention safe access at the weatherhead to make a long-lead measurement from there, but the hazards there are significant. The neutral should be close to ground, but you've got a bad connection, and if that became an open, you could get at least 120 V on the N, perhaps more in some scenarios. And you are upstream of any protection. So I think the next step is to call the utility, even though you have access there and know how to interpret the results.

    If the utility says all is good on their side, an option is to have them disconnect the service temporarily so you check everything that's on your side without danger, and reconnect later that day. Or you can call an electrician.

    (I'm being vague because what's included in yours vs. theirs can be a little different in different regions.)

    1. Expert Member
      BILL WICHERS | | #10

      > "The neutral should be close to ground, but you've got a bad connection, and if that became an open, you could get at least 120 V on the N, perhaps more in some scenarios."

      The neutral should BE AT GROUND at the ground/neutral bonding (connection) point at the main disconnect, which is often the shared ground/neutral busbar in a panel that also has the main breaker in it. If the neutral goes open, the apparent voltage on the neutral is related to the difference in current drawn from L1 and L2. If both legs have equal loads (assuming unity power factor here), the neutral will still be around zero volts. If one leg has a higher current draw compared to the other, it will "pull" the neutral towards that leg, meaning that the leg with the higher current draw will show a lower voltage to neutral, and the voltage between the leg with the higher current draw will show a higher voltage to neutral. Rergardless, an open neutral is a Bad Thing.

      I did have a site once that had a 277/480V service (a commerical building), and the neutral went open when the transfer switched over to generator during a test run (due to a wiring error on the first test). This resulted in much excitement -- all the big loads (three phase motors, big UPS systems, transformers) were fine, since none of those things care about the neutral. What was very unhappy was the 277v lighting, specifically the ballasts, which explained to us their displeasure in very noisy fashion as soon as the TVSS (surge protectors) on some of the panels started to fail (which took a few tens of seconds). We lost around a third of the ballasts on the lights in the building. I still remember holding the transfer switch's "reset to normal" switch down when the ballasts started to pop... Fun times. Afterwards, I had to have a talk with the electricians (I was the design engineer supervising the test, the electricians had connected the generator's neutral in the wrong place).

      You absolutely want to get any neutral problems fixed ASAP.


  5. maine_tyler | | #12

    I shouldn't have a problem getting voltage below 110 on a leg, but is that all the utility cares about?

    I was under the impression some sort of percentage drop was what mattered. Or what about the fact that when one leg drops really low, even if not quite below 110V, the other leg goes really high (when imbalanced)? Isn't that almost a bigger concern?

    My biggest fear is that it's just the length and size of the line or the transformer size and they're going to refuse to upgrade... we'll see I guess.

    1. Expert Member
      BILL WICHERS | | #13

      The "percentage drop" is a code recommendation (not a requirement). The electric code doesn't really apply to utilities the same way it does to the house though -- the utilities have different requirements from the NESC and some statutes. The utility just cares about the delivery voltage, the voltage they deliver to your meter base, which has to be within the acceptable range.

      Loading one leg heavily and the other not at all is a sort of worst-case scenario for what the apparent neutral "voltage" might be. If you load both legs evenly, the neutral will be approximately centered at about zero. I would heavily load both legs, just monitor things as you do so to avoid problems. Remember that the apparent neutral voltage depends on the loading on BOTH legs, and if the load is equal, the neutral will be zero. The neutral is derived from the center tap on the transformer, so it naturally wants to be in the middle at zero (since it's grounded).

      The utility will upgrade/replace the transformer if required, but the service drop may be your responsibility. If it's an aerial drop, the utility is probably responsible for it. Regardless, it's better to know what and where the problem is than to not know.


  6. Patrick_OSullivan | | #14

    > Or what about the fact that when one leg drops really low, even if not quite below 110V, the other leg goes really high (when imbalanced)? Isn't that almost a bigger concern?

    > My biggest fear is that it's just the length and size of the line or the transformer size and they're going to refuse to upgrade... we'll see I guess.

    Floating neutrals are an incredibly common problem for the utility to diagnose. They happen "all the time". Get them out, let them do their thing. As far as length and size of the drop, not sure what the division of responsibility is with your utility, but in NJ, the utility is responsible for the aerial, so an undersizing would be their problem (but this is really likely a loose connection).

  7. Expert Member

    A lot of good information in this thread! However I would find it hard to believe a loose connection was causing the drop without some sort of other indicator like intermittent connectivity loss, or symptoms when it's hot / cold outside.

    Ideally I wouldn't want my service to dip below 118V under max load, or be over 125 open circuit.

    300-400' is a long way. The most likely cause IMO is an undersized conductor, or highly imbalanced loads. You should be able to rule out imbalanced loads.

    Be sure to report back! I'm more following than providing anything useful.

    1. Expert Member
      BILL WICHERS | | #16

      Loose connections can occur in such a way that they just present higher resistance to the circuit without being intermittent. If a loose connection of this type were inside a commercial panel, it's something we would find with an IR (thermal) camera, since it would also run hot. If that loose connection is outside on a pole, it might not be noticed, even if it's running pretty hot. Thermal changes can sometimes make a difference, but unless it's a lot, no one is likely to notice the link between the weather conditions and the amount of volt drop on the circuit.

      Unbalanced loads shouldn't be the issue here, because of the way the tests were made. The entire purpose of the neutral is to deal with what is known as "phase imbalance", so if the loads were all perfectly balanced, you wouldn't even need a neutral. Obviously that "perfect balance" isn't ever going to happen in practice with a random mix of 120v loads, so you need the neutral to provide a return path for whatever current, resulting from the load imbalance between legs, doesn't cancel out between the loads on the "other" legs.

      Standard is 110-125v. It's rare to see a utility delivering more less than about 120v or so to a meter base though -- utilities usually run a little high on average to provide some extra wiggle room on their distribution circuits. The closer you are to the substation, the higher this "little over" will appear.

      BTW, an undersized conductor would present the same volt drop on L1 or L2 as it would on the neutral, possibly a very small amount more if you have an old cable with a downsized neutral (which used to be allowed as I mentioned earlier).


      1. Expert Member
        KYLE WINSTON BENTLEY | | #17

        That was my thinking Bill, the neutral was a reduced size compared to the two legs.

        1. Expert Member
          BILL WICHERS | | #18

          I'd have to go back and check to be sure, but I don't think even the old "reduced size neutral" cable was allowed to have a neutral reduced by more than one or two wire guage sizes (i.e. a 2/0 neutral with 4/0 line conductors). That wouldn't make enough difference to show the level of volt drop difference the OP is seeing between L1/L2 and N.

          The idea with the old cable was that the neutral is only there to cary the imbalance current (which is true), and there will always be SOME amount of cancellation because you'll always have SOME loads on each leg. Basically, the thinking was, you'll never have stuff so out of balance that the neutral will see ALL of the line current, so the neutral can be a little smaller and still be safe.

          Later on, the code writers found out that "stupid happens", or maybe just things don't awlays go as planned, and decided to require full size neutrals for residential services.

          Note that for commerical three phase services, you can still downsize the neutral IF you can show that it's big enough to handle the maximum possible imbalance current. I've often done this when the only single phase load on the 480v system in a building is the lighting, which can usually be easily calculated as to what the max possible current would be. My own rule is to never size the neutral smaller than what the code requires for grounds though, so that's usually what I end up using for the neutral in these cases -- which is still WAY oversized, but still a lot smaller than the phase conductors.


  8. maine_tyler | | #19

    This voltage drop calculator:

    suggests that even if my neutral service was 1/0 aluminum, I should only see about 1.2% V-drop at 320 feet, 11 amps.

    I'll be calling the utility soo, and I'll update once I figure more out.

    I agree with Charlie in post #11; I will see the most drop when the neutral is carrying the most current, which will be made possible by running loads on only one leg (though I could add some 240V loads to increase the drop on the hot side of that leg without reducing neutral current).

    I set up my Lantern Power monitor so each hub is on its own phase leg (I actually did this in part because I knew I had voltage fluctuations and wanted more accurate measurements). And I've grouped all the loads by L1, L2, and 240, so it's easy to know which leg is being loaded and by what by looking at the app.

  9. maine_tyler | | #21

    Well the utility jumped on it quick.

    They must have been thinking like most here because apparently they checked all (or most of) the connection points first. Apparently it was a head scratcher. Turns out the neutral line itself had some sort of abrasion/wear/reduced cross section or something.

    I wasn't there and didn't talk to them, so the specifics beyond that are rather unknown. I guess they thought a branch wore it down or something (though no branch was touching any longer).

    I haven't actually checked voltages, but visually the issue seems to be better.

    I found this on the ground. I'm going to guess that means I have a 1/0 neutral. (Is it ever any smaller?) I guess I should still expect slightly higher than average V-drop, but at least it won't be as bad.

    I don't suppose if I ever moved my shop here (wood working equipment) they would upgrade the line gauges at my request?

    1. Expert Member
      BILL WICHERS | | #24

      The neutral is probably the same size unless it’s a pretty old cable. That connector you found is a mechanical type splice, somewhat similar to a split bolt except what you found holds the wires side by side instead of one on top of the other. The clamp you found is commonly used to make aerial connections between the service drop and wires from the mast where the transition between cable types near the weather head.

      If you move your shop, they’ll probably just tap you off of the same cable. I doubt they’d upgrade it unless you could demonstrate the existing cable was incapable of supporting the load.


    2. Patrick_OSullivan | | #25

      Did they re-run the drop or just re-splice?

      1. maine_tyler | | #27

        There is an intermediary pole, so they spliced and ran new from that pole to the weatherhead.

    3. charlie_sullivan | | #46

      Best plan for woodworking equipment would be to wire it for 240. You then avoid neutral issues, and the current is lower, so the voltage drop is lower, and even lower still as a fraction of the supply voltage.

      If you had high-current 120 V stuff that was problematic, you could add a neutral-forming autotransformer near the service entrance, but that's probably more expensive than upgrading the conductors.

  10. cs55 | | #22

    1/0 for 200a service would be wild.

    1. Expert Member
      BILL WICHERS | | #23

      Actually, 1/0 aluminum for an aerial service drop for 200A service is pretty typical. 4 gauge would be for a 100 amp service. An ampacity chart for triplex (the usual twisted looking aerial cable you often see) is here:

      The reason for the higher than expected rating is that you are in “free air” with aerial cables, so the wire can cool off better. You also can run up to the full 90*C (or sometimes even higher) temperature rating of the insulation too, instead of being limited to the 60* or 75*C temperature columns the way you usually are for regular building wire. The compression connectors the utilities use are usually rated to either 90*C or 105*C.

      If the wire was separate conductors and not triplexed, you’d even be able to get a little more out of it. I used to use a rough rule of thumb for single conductors in free air of “a hundred amps per aught”, which sounds silly but is actually pretty close. Single 4/0 copper SO cable (flexible power cable) conductors are good for 405 amps, for example.


      1. Patrick_OSullivan | | #26

        Story checks out. I have a "brand new" 200A drop and it is 1/0 AWG triplex.

        Bill knows this, but for the benefit of others, the utilities play by a different set of rules (literally). They are governed under the National Electrical Safety Code, not the National Electric Code, and are not bound by ampacity requirements of the NEC.

  11. maine_tyler | | #28

    Well I'll have to check the actual voltage numbers when I get a chance. I did still notice some lighting sway this morning with that kettle... but better.

    I've known that the utility plays by different rules, but never quite understood why we've set up a system where the electrician needs to run fat 4/0 (Al) for 8 feet of feeder and the utility runs 320 feet of 1/0. I get that in the air there is more cooling, but the wires are still insulated (not the neutral of course). Doesn't make a ton of sense to me. Shouldn't it be about total system V-drop?

    1. charlie_sullivan | | #32

      The hard rules in code are to avoid overheating. Overhead lines have free airflow and it's safer for them to run hot where they aren't touching other materials that might be damaged. There are of course also voltage drop considerations, but those aren't hard rules, but rather guidelines, in electrical code.

      1. maine_tyler | | #37

        Interesting, I did not know that! But I guess it makes sense. Though in this day of sensitive electronics... does it?

  12. gusfhb | | #29

    I might have this completely sideways, but my understanding is the neutral 'is' ground electrically.
    Like bonded in the panel, and the panel is grounded to earth ground via a big spike at the service entrance.
    So, how much current is going to flow through the neutral conductor to the pole?
    Maybe check the state of that main ground?
    Sure it is possible to electrically pull the earth up locally but that I have only heard of in big factories with 100 hp motors poorly wired.

  13. maine_tyler | | #30

    "So, how much current is going to flow through the neutral conductor to the pole?"

    The difference between L1 and L2 current. Neutral carries the imbalance. So in my example, I had around 10 amps on one leg and close to zero on the other, which means all 10 amps flows on the neutral.

    It's the AC equivalent of an Edison 3-wire circuit.

    Are you operating under the assumption charge 'goes to ground'? That's a big misconception. Charge goes to 'source' which in this case is the transformer. We may need to blame elementary school physics teachers (and lightning) for this widespread misconception.

    Earth can (in simple circuit analysis terms) be thought of as a conductor. It does have capacitance, but for simplicity I think viewing it as a conductor in this situation works fine.

    So that makes it a parallel conductor to the neutral conductor, and current will flow through both as parallel paths, in accordance with ohms law (resistance of the paths). When the neutral is high impedance due to some loose connection or something, the voltage potential between the pole transformer ground and the main panel ground is higher, therefore more current will flow through the earth conductor. If the neutral conductor was a perfect conductor, there would be zero voltage potential between these ground points, but in the real world there is always some very small amount.

  14. Expert Member
    BILL WICHERS | | #31

    I'm going to add a bit to what Tyler said in #30, above, which is otherwise pretty complete. There is a concept of "Earth" and "ground", and they are NOT the same thing. It may also help to use the telecom terminology of calling the "ground" the "return", so we have in a phone exchange (for example), two wires, labeled as -48v (or battery) and the other is the "return". That "-48v" is important too, which I'll get to in a moment since sometimes it's easier for people to understand a DC system.

    The neutral carries that "return" current that is going back to the transformer. It is connected to the center tap of the winding on the transformer that feeds your house. That means you have, in effect, two hots, one positive and one negative (not really, it's AC, so it's a phase difference and not polarity, but it's easier to explain with DC, and it works out about the same anyway). The neutral is "zero", since it's halfway between the + and -.

    Let's assume you have a 200v system, with -100v, L1 -- neutral -- +100v, L2. You have 100v between either L and ground, but 200v between the two L's. Now let's assume you have a 1,000w, 200v load, which means it will draw 5 amps at 200v. You also have a 100w, 100v load, which will draw 1 amp at 100v.

    Connect the 1,000w load. It connects to L1 and L2, but not the neutral. You now have 5 amps on both "sides" of the circuit, so both L1 and L2 carry 5 amps. There is no current carried by the neutral. Now add the 100w load onto either L, but we'll use L1 in the example. That 100w load adds 1A to both "sides" of it's circuit, so now L1 carries 6A (5A from the 1kw load, and 1A from the 100w load), but since there is no matching 100w load on L2, there is an "imbalance" of 1A, which is carried by the neutral. If you add another 100w load on L2, you'll see 1A on L2, and the two 100w loads are now effectively in series, so the neutral carries zero amps as the two 100w load currents cancel at the "zero" formed by the neutral. If you move that second 100w load to L1, you now have 7A on L1 (5A from the 1kw load + 1A for 100w #1, + 1A for 100w #2). The neutral now carries the 2A "imbalance" current from the sum of the two 100w loads, and L2 carries only the 5A from the 1kw load.

    Current does not "return to earth", but it can "return to ground", since "ground" in this case is an electrical concept, and it doesn't mean the "earth ground". The connection to Earth ground is for safety, not for current flow. The purpose of the connection to Earth is to establish "ground" at the location of the electric service, to make sure that the "earth" is at "ground" potential electrically, and the reason that's important is that if you stand on the Earth, and you don't want current to flow THROUGH you if you touch something. Without that connection to Earth, the "ground" will float to whatever the electrical system wants it to be, and "wants it to be" is based on things like "sneak paths", "leakage currents", basically anything else that can carry current that you never expected was part of the circuit. The dangerous thing is that YOU now don't know what GROUND is in terms of electrical potential, so ANYTHING you touch could potentially be energized with respect to ANYTHING else you touch. The purpose of "grounding to earth" is to make sure that ONE side of the circuit is at KNOWN electrical potential with respect to Earth, so that YOU KNOW what is safe to touch. Hopefully that makes sense.

    The -48v I mentioned earlier is a good example of ground not being what people think. Everyone thinks "+12v", because in a car, the negative is connected to the frame of the car, so that is "ground", electrically. This is what is known as a "negative ground system". In the telecom world, we use the opposite -- a "positive ground system", where the POSITIVE is connected to ground. That means the red wire is the NEGATIVE in the telecom world. The other wire is called the "RETURN" (not "ground", and not "positive"), so we end up with two wires, a -48v and a RETURN at each device (and typically two circuits like that, an A and a B, for redundancy). The reason for this is that with a positive ground system, galvanic corrosion from leakeage currents (which are unavoidable in large, outdoor systems), doesn't rot away the metallic sheild of communications cables.


    1. charlie_sullivan | | #33

      Your explanation of the safety function of earthing is one of the best I've read. Nicely done.

      1. Expert Member
        BILL WICHERS | | #34


        I did notice I've been making a lot of typoes today, and not just here. Hopefully I've corrected all the typoes in my last post. Typoes when trying to explain technical concepts are receipes for disaster! I think that the better everyone understands this stuff, the safer everyone can be. I have also always liked the philisophy of Richard Feynman, who had said, basically (paraphrasing here) "any advanced concept should be able to be explained to any regular person without any special knowledge provided someone who understands the concept is willing to take the time to explain it properly".


    2. maine_tyler | | #35

      What's amazing is that if you do some lazy googling, you'll be bombarded by numerous sources that get this totally wrong. Here's one example, from

      "Electricity always looks for the shortest path to the earth, therefore if there is any problem where the neutral wire is broken or interrupted, it is the grounding wire that provides a direct path to the ground. This direct physical connection allows the earth to act as a path of least resistance and prevent an appliance or a person from becoming the shortest path."

      This is wrong in a number of ways, from not understanding Ohm's Law, to thinking electricity is trying to get 'to earth.' It also doesn't seem to understand that the actual reason for the equipment grounding conductor is to trip the breaker via the neutral bond when there is a hot-to-chassis short.

      By the way, I haven't had time to confirm, but I think the bit of light flickering I still noticed after the utility came was just on the local branch circuit (yes I have a kitchen outlet on a circuit with kitchen lighting, bummer), whereas before it was system wide.

      1. Expert Member
        BILL WICHERS | | #38

        Yeah, I've seen that kind of thing before. The earth is actually NOT a particularly good conductor of electricity, and how good it is varies based on the composition of the soil in your area. There are some places the Earth IS used as a return path (Single Wire Earth Return (SWER) primary systems are a notable example, as are HVDC links when operating in monopole mode in some cases), but even then, the reason the "Earth" is the return, is because the other "end" of the loop that forms the electrical circuit is ALSO using the Earth. Electricity wants to basically go back to where it came from, it DOES NOT want to "go to Earth". Earth is the "other end" of a circuit only for lightning, in which case the Earth is the "other" charged object that the electric charge is jumping to.

        If your neutral opens, your ground system will likely carry some of the current that was previously carried by the neutral conductor, but it will be from your ground rod to the ground of the utility pole. This isn't usually a particularly low resistance path, so you'll see much higher than normal apparent volt drop on the "neutral" if this occurs.

        There are a LOT of misunderstandings about electricity, and it's unfortunate when people publish information that is incorrect and spread misunderstanding to others.


  15. maine_tyler | | #36

    "you don't want current to flow THROUGH you if you touch something. Without that connection to Earth, the "ground" will float to whatever the electrical system wants it to be, and "wants it to be" is based on things like "sneak paths", "leakage currents", basically anything else that can carry current that you never expected was part of the circuit. The dangerous thing is that YOU now don't know what GROUND is in terms of electrical potential, so ANYTHING you touch could potentially be energized with respect to ANYTHING else you touch."

    And it is not current 'from the circuit' that would be going through you in this (ungrounded) scenario. It's the 'more out there' stuff that could cause trouble: things you know more about than I Bill, but things like induced voltages, distributed capacitance, and other 'stray' things.

    A simple circuit analysis of a floating system actually could lead one to think that a floating system would be a *safer* system than a grounded one because there would be no 'hot.' But these stray dangers exist. Also, a double ground fault of L1 and L2 on an ungrounded system could present 240V to ground instead of being limited to 120 when N is grounded.

    1. Expert Member
      BILL WICHERS | | #39

      The problem with floating systems is that it is very easy for a failure or aging problem to create an unknown path to ground, which then "unfloats" the system. Now you have a system that you THINK is safe, that isn't. The biggest problem is that these kinds of issues usually go undetected until they break something or hurt someone. The entire point of the ground wire is to provide a current path between things you might touch and the neutral return, to cause the overcurrent protection device (OCPD, a breaker or fuse) to trip, clearing the unsafe condition. The ground rod, or other "grounding electrode" is there to estabish the local ground reference, to ensure that "stuff" that you might touch is at the same electrical potential as the groundED gizmos so that no current will flow through YOU.

      A double ground fault wouldn't be able to maek 240v appear on a chassis. You'd need an open neutral/ground bond AND a ground fault on one of the two lines to creat that condition. Even then, it's iffy if you'd actually see 240v above ground, because the utility's side of your service should also have the neutral grounded. In this situation, the apparent voltage you'd see above ground would also depend on the resistance of the ground path through the Earth between the utility's ground and your own in the area of the fault. The lower that resistance, the lower the apparent voltage would be. If L1 and L2 both went to ground, but neutral wasn't open, you'd have current running through the ground between the two, but neither would be any more than 120v above ground, because the voltages on L1 and L2 are not additive -- only the DIFFERENTIAL VOLTAGE between the two is 240v. Either to ground is only 120v. For the "real" reason this is so, you need to think about the AC sinewave. The sinewave on L2 is 180 degrees out of phase with the sinewave on L1, so the instantaneus voltage differential between the two is double that from either one to ground.


      1. maine_tyler | | #40

        "A double ground fault wouldn't be able to maek 240v appear on a chassis."

        Bill, what I wrote was in regard to a floating system (so not a system we typically see). If floating, and L1 'grounded itself,' then L2 becomes (theoretically) 240V to ground and N becomes 120. I'm essentially pointing out an example of your sentiment that, "it is very easy for a failure or aging problem to create an unknown path to ground, which then "unfloats" the system."
        If it "unfloats" to the outside of the winding, we potentially end up with more than 120V to ground on one of the lines-- up to the full 240V. And like you say, it's likely to be unknown.

        Not that it changes anyones point here, but L1 is 180 degrees out of phase to L2 (or 'inverted') *with respect to neutral.* The neutral reference is often assumed/implied so OK not a big deal... but for learners, it's important to understand that the center reference is necessary to see the 2 inverse sine waves. A center tapped coil is essentially two series aiding sources, like two batteries in series with polarity -+,-+
        End to end its just a single 240V waveform.

        1. Expert Member
          BILL WICHERS | | #41

          They are out of phase with respect to each other. is another way to visualize it, same basic idea -- it's two halves of the same phase. The neutral is just the midpoint.

          I see what you're saying regarding a floating system, which is you would end up with if the ground/neutral bond were to open. I did actually see this happen once, in a way, on a commercial 120/208 three phase system. One of the phases was shorted to ground (by a screw at a receptacle as it turned out), and the ground/neutral bond was open at the transformer. This resulted in the neutral showing 120v to one phase, and the other two phases showing 208v to ground. It actually took me a bit to figure out what was going on simple because it was unexpected. Once we fixed the ground bond at the transformer, the circuit breaker for the outlet with the shorting screw tripped and we found that problem too.


          1. maine_tyler | | #42

            Yeah i guess I neglected to mention that when I said 'chassis' I meant to say assuming it wasn't bonded to N, so that if contacted by L2, it would float to L2 potential, and if L1 was grounded, then you have the 240.

            Basically, [with a floating system] if L1 becomes earth potential via a fault, L2 becomes 240 to earth, and vice versa.

          2. maine_tyler | | #43

            >"They are out of phase with respect to each other."

            I was gonna leave this alone because we're pretty far into the weeds at this point, and it's now mostly academic (at least as far as residential electricians would be concerned), but, I have no self control...
            The above statement is not strictly accurate, imo, if by 'they' we mean L1 and L2.

            Saying they are out of phase 'to each other' is to imply L1 alone IS a phase. L1 alone and L2 alone are not 'phases'; they are lines or wires, and can be thought of as point nodes as far as phasing. L1 TO L2 IS a phase, but only a single one— a 240V volt one, so decidedly not '180 degrees out of phase' because there is no comparison being made (yes it's 120 degrees out of phase with the other two distribution legs, but that's not the comparison of interest to this story).

            Where we get 180 degrees out of phase is when the neutral is the common reference. "Neutral to L1" is 180 degrees out-of-phase with "Neutral to L2". Neutral being the common reference and, visually speaking, the x-axis of the sinewave graph.

            If we allow ourselves some DC anology:
            Two ends of a battery are not 'out of phase'. For the purposes of the DC analogy, phase will mean whether the polarity reading is *going from* + to - OR *going from* - to +. From one end of a battery to the other, we *go from* - to + left to right. [-+][-+][-+] (3 batteries series aiding).

            But, if we make one of the points where two batteries join the reference—instead of one end of the string—now one side is *going from* + to - while the other side is *going from* - to +.
            [-+] REFERENCE [-+]

            This is like a 240V center tapped coil. But for AC, polarity notation is more just to note reference points since polarity is always changing, so the polarity markings might look like this:
            [+-] Neutral Reference [-+]

            Which shows the same polarity (-) at neutral reference, and the same polarity (+) at each end of the winding. And because the measurement polarities are noted as being the same, they must be described as being 180 degrees out of phase in time.

            [+-] Neutral [-+] that is zero degrees out of phase (as a DC source more accurately is) would be like 2 batteries in series that are bucking voltages. Like: L1 to N=120V, N to L2=120V, L1 to L2=0V

            Does this matter? Probably not. It depends. I know there is some confusion out there. Some people definitely think L1 as a WIRE is itself somehow out of phase with L2 as a WIRE. That's incorrect or really just incomplete.

          3. Expert Member
            BILL WICHERS | | #45

            OK, I see what you're saying. I'm going to have to agree with you there, my description isn't entirely accurate. They way I usually explain it for visualization purposes is that L1 is a sinewave, L2 is 180 degrees out of phase with L1, but it's really with respect to ground, or neutral, in this case. You can then visualize the 240v between L1 and L2 by seeing how the difference between two vertically oriented (really need a drawing here) points on the two sinewaves is twice that from ground to either of the two sinewaves. This makes a lot more sense when explained using a drawing, which is how I usually do it -- and then the midpoint reference is obvious since I draw a horizontal line there. Sorry for any confusion.

            I've seen two many people think that residential electrical service is "two phase", which isn't correct. There actually is (was, anyway) a 2 phase electrical system, but it used a 90 degree phase difference. Standard three phase uses three phases, each 120 degrees out of phase with the others. I usually refer to residential single phase service as "split phase" just to make a point that it's still single phase, just with a center tap in the middle of the one phase. I've had people tell me "but "split phase" is a type of motor!", to which I usually say "yeah, but I need SOME way to make a point of residential service being single phase with an extra thing in the middle!" :-)


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |