Electrical question
Hi.
It is not code, but is it dangerous……
My buddy has a 240v 3-phase with a stinger line in his shop. A local electrician ran a multi wire branch circuit (round robin) for him so he could have three 120v outlets.
Only two (A and C) of the three hot lines coming into the panel will make 120v with neutral as the middle B line stinger in makes 210 w/neutral
This three hots sharing one neutral is fine when on the three different lines on a 208/120 3 phase, but in this case, he has two of the 20 amp breakers on the C phase. Does this create a dangerous overload situation for the neutral? I know it is not up to code, but I want to KNOW if it is going to create a dangerous situation to man or machinery.
I am already prompting him to get his situation evaluated by a pro electrician and it might mean transforming to another box at 208/120 and run his single phase circuits off of that.
I called the company that wired it and he said it is fine, no problem…. but I want a second opinion.
Thank you for your advice.
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Replies
Forgive me, but have you mentioned what the overall feeder size is? I think it's a bit hard to say if the neutral has too much imbalance on it without understanding what it's sized for.
As an aside, there are a few things in your post that I'm having trouble reconciling. Maybe it's my lack of experience with high-leg delta 3 phase, but I don't know.
> It is not code, but is it dangerous……
If you know what is not to code, could you mention it specifically? (An NEC reference is helpful.)
> A local electrician ran
If "a local electrician ran" something "not to code", I think that tells you all you need to know about that electrician.
> a multi wire branch circuit (round robin)
Is a "round robin" a specific type of MWBC, or just another name for one?
> I am already prompting him to get his situation evaluated by a pro electrician
So was the first electrician an amateur electrician if the new one is going to be a "pro"?
> I called the company that wired it and he said it is fine, no problem…. but I want a second opinion.
What did the AHJ who inspected the installation say about it? Surely a permit was pulled...
A round robin is just a common name for a mwbc. A licence does not a pro make. It is all 12gauge wire. It is 3ea 20 amp breakers with one shared neutral. The standard is to place one breaker on each of the three hots. This is done simply by color coding the hot wires blue, red, black and placing them next to each other on one side of the breaker box. Since a hot wire at 60Hz is turning on and off 60 times a second and these waves are at 3 different times, thus 3 × 60 =180 pulses a second, the single neutral can handle 3 hots because it is only supporting one hot at a time because of phasesome being offset. However a 240v 3ph with a stinger does not permit a 120v on the center B hot. So, this guy.put 2 hots on the C line, thus the wave occurs at the same time for two 20 amp circuits on one shared neutral. Because I am not a pro, I can only work on my limited pea brain, but I fear it is an overloaded neutral if they operated two 15 amp machines simultaneously. However, I am not sure. I am also assuming it is not to code because standard practice with a round robin is to have the three breakers on three different lines A, B, and C respectively.
The only way I know of making the 120 from 240 3P is like this:
https://electrical-engineering-portal.com/wp-content/uploads/2015/05/delta-connected-three-phase-four-wire-secondary-transformer.gif
If this is what is set up on the other side, then the neutral only handles the current from the two hot phases it is crated from. As long as there are no loads between the phaseA in the diagram and the neutral then you are fine.
This means you can only run 3 phase delta loads in the shop.
Sound normal to me if the service is High-leg delta. Phases A and C can be used for 120 volts but not B
https://en.wikipedia.org/wiki/High-leg_delta
High leg 240 is somewhat common for industrial service very unlikely to be found in a residence.
If he must have more 120 volt power a transformer and a 208V subpanel is the best answer or change the service to 208 volts but he will need to change any motor that is not rated for both 208 and 240.
The other problem if he has a large 120 volt load it will unbalance his current draw. Many utilities do not like unbalanced loads and are allowed to charge for the unused power plus a penalty.
Walta
Some of the names you’re using aren’t what I’m used to, but I think you have single phase 120/240v service derived from a three phase “high leg delta” service with a grounded center tap. This used to be common, but isn’t anymore.
You CAN share a neutral between TWO circuits, with a “circuit” defined as one line from one circuit breaker.
You CANNOT share a neutral between THREE circuits, with two of the circuits fed from the same 120v line and the third from the other 120v line. The two circuits fed from the same 120v line will NOT see their current cancel on the neutral, it will be additive instead, so you can overload the neutral in this case.
In a wye derived circuit you can run three circuits on a shared neutral since the currents will cancel due to the 120 degree phase offset between all three circuits. Each circuit has to come from a different phase. With a delta derived service, since the high leg is not useable for 120v service, you effectively have single phase service between the 120v lines and the neutral, so you can’t do any of the three phase wiring stuff.
I’ve included a drawing to help clarify things.
If you have things wired as shown in my “NOT OK” drawing, you need to correct. The easiest way is to just tie the two “circuits” together AFTER the breaker so that they become a single 20A circuit.
Bill
> Does this create a dangerous overload situation for the neutral?
It depends on the size of the neutral wire and the worse case combination (accounting for phase difference) of amps through the breakers. Bill's "not OK" can be OK with the right wire and/or breakers. Specifically, either a neutral sized for 40A or 10A breakers on the two C hot wires.
> Does this create a dangerous overload situation for the neutral?
If the neutral wire is the same gage as the 3 phase wires the neutral wire can never see more current than any one of the phase wires as any current flowing on the others phases will out of phase and cancel out part of neutral current.
Walta
It really has to do with the size of the overcurrent protection devices (breakers or fuses) and not the gauge of the wire. Current cancellation applies for linear loads, but non-linear loads can cause issues (lookup “triplen harmonics”), but this is rarely an issue in normal circuits.
When I spec a “full boat” circuit, with three circuits sharing a common neutral, I like to specify the neutral for one size heavier gauge for a little extra safety. The systems I’m designing are usually running all circuits at near maximum capacity though, which isn’t the case in most other applications.
Note that newer codes want tiebars on the breakers feeding these circuits with shared neutrals.
Bill
>"If the neutral wire is the same gage as the 3 phase wires the neutral wire can never see more current than any one of the phase wires as any current flowing on the others phases will out of phase and cancel out part of neutral current."
For the main feeder line, this is true. But it sounds like the electrician ran two circuits off the same phase and shared a neutral. In that case, the branch neutral could see more current than the individual branch 'hots' —even though the actual service line itself would not, as you say.
As others have pointed it, it mostly comes down to the sizing of the neutral, (and what ever Bill said regarding non-linear loads:)) but since it sounds like the neutral is sized same as rest, it seems suspect. Worth getting someone truly qualified in? Very likely, given the unusual system.
Why not just run separate neutrals like a normal 120 v circuit? At least for one of the circuits coming off the C-phase, which is currently being double dipped. (edit: or Bill's suggested fix of putting them on the same breaker. That'll ensure you have the over-current protection needed for the neutral)
>"thus 3 × 60 =180 pulses a second, the single neutral can handle 3 hots because it is only supporting one hot at a time because of phasesome being offset."
This is a bit off. They're not hard 'pulses' like on/off. They're (typically) sine waves. This means that rather than 180 pulses a second, you essentially get a consistent presentation of voltage/current when referenced to a true neutral. A rotating magnetic field as far as a rotor is concerned when applied to a stator.
This also means that the neutral isn't 'supporting one hot at a time' but rather is carrying the 'unbalanced load.' In a non-high leg 3-phase system, if all 3 phases have even draws, no current would flow on the neutral at all.
The high-leg throws a wrench into this a bit since the ground is not in the true neutral position. The phase angles from any line (A, B, C) to another will still be 120, but in reference to ground, they are 180(A to C), 90(A to B), and 90(C to B). The diagram below shows this phase relationship.
The A and C phases balance same as any other split-phase system with respect to the center-tap ground (180— so a 1:1 cancel). All three phases together still balance when used in a 3-phase load, (360/120=3 hence you need all 3 for proper balancing).
At the end of the day, I don't see why one would treat the 120-volt loads anything like a 3 phase system in terms of balancing. They should be handled as—essentially—a typical split phase system. The stinger/high-leg is just a bonus, kicking around for your 3-phase (or rare 208v?) loads.
a²+b²=c²
120²+208²≈240²
--------------------------------
From: https://www.jadelearning.com/blog/multiwire-branch-circuits-nec-210-4/
"If both hot wires are hooked to the same phase by mistake, the current in the neutral will be the sum of the currents in each phase conductor, rather than the difference, which could be more than double its current carrying capacity."
Basically you could be dealing with an overloaded neutral when the two loads from the same phase are running simultaneously (and the third load is not). The whole discussion on 3-phase, high-leg (stinger) etc. is kind of irrelevant to the specific question since you're essentially dealing with a standard split-phase MWBC with two hots running off the same phase.
*all assuming we've correctly identified the service as delta high-leg 240V line-line.
Thanks all for your input. Just as Tyler said, if two breakers are on the same phase and have a shared neutral it can lead to the wire becoming a stove top. Reason: imaagine each of the two breakers at 19 amps load. The breakers will not trip, but the neutral will have a load of 38AMPS running through it. If that wire is 12gu as in my buddy's situation, that would be 18 amps more than the wire is rated for; yet, the breakers would not trip because they are on the hot leads which only have 19 amps load each. The sum on the shared neutral would be 38 Amps. BAD! As for the pros, I called the company and the owber thought the building had already been swapped over to 208/120 which would be three equal 120v loads. His guys installed the three lines and just doubled up two breakers on the C phase. When I asked him if it was daangerous, he said it would be fine. He is gambling. Of course, the six sewing machines on those lines are below the amperage; however, three things:
1. this is not best or even good praactice. It is bad.
2. the breaakers are there for protection, that protection now allows 20 amps over load on that neutral wire before tripping.
3. It turns out, my buddy is putting two more higher amp machines on that circuit, so the drama could have intensified.
4. Do you think this company was going to come back after the building is switched over and move the breaker onto the B phase?
On a good note, the building will be switched over to 208/120 very soon. I told my buddy to call me when done, and I will do a courtesy check after he has them come in and fix it.
also told him not to put his new machinery on those lines until it is fixed and the circuit and machinery are evaluated for rating. I also told him to plug the machines that are on the two breakers on the C phase into only one of the circuits, so the breaker will trip if 20 amps are exceeded until they get that 3 phase transformer changed very soon.
I included an image to show how shared neutral (multi wire branch circuit-MWBC, aka Round Robin) works. How it shares the neitral safely, when done properly. Notice how the three waves cycle on and off 60 times per second (60 Hz). This is done as the generator is turned and sends a pulse. This is the same as turning on and off 60 times per second. Your lights do this, but you do not notice because it happens so fast. Since the pulse happens at three different locations in the rotation of the generator, the neutral only has one load (think outlet, lightbulb, etc) at a time that is on, it never sees more than one of the three lines on at a time. The hot line (breaker cannot share a line, because it would become 180Hz and all items would be on at the same time, 60 amps on a neutral. I added this, just for the folks who never dealt with 3 phase. I am not a pro, but this is what I have learned investigating for my buddy.
Thanks everyone who chimed in!
-Mike
mikeysp,
Aside from the technical dangers, you might wish to point out to your friend that if a fire or other accident occurs, he will be in deep doodoo with lawyers for any claims that arise. Any insurance company will always look for reasons to avoid liability, so best not give them any avenues.
“ Mikeysp His guys installed the three lines and just doubled up two breakers on the C phase. When I asked him if it was daangerous, he said it would be fine. “
In order to share the neutral the handle of all the breakers are required to be ties together. It should not be possible to tie 2 c phase breaker together with a 3 breaker tie.
If they installed a multi wire branch circuit without ting the breakers you should invite the county inspector to give you a second opinion.
Walta
I don't know code, but "a multiwire branch circuit consists of two or more ungrounded conductors that have a voltage between them". Two wires from the C phase don't meet this definition, so I'd say you have a "miswired MWBC", which isn't allowed. Tying breakers (which typically means they are adjacent and on different phases) is a way to force the "voltage between them" requirement.
The tiebar doesn’t “force voltage between them”. You can install a 3 pole breaker in a single phase panel, for example. The purpose of the tiebar is to make sure that all energized conductors on a circuit are shut off if any one is shut off for maintenance. The idea behind that particular code (and it’s a relatively new addition to the code) was to protect people working on the wiring. Most of the electrical trade thinks it was a dumb idea to add that provision, but it’s in there now.
Bill
>"I'd say you have a "miswired MWBC"
I think that's the succinct answer here.
I'm also not all that versed with the code, but I believe it has a few other things to say regarding MWBC's; so might be worth ensuring those boxes get checked too. One such is 300.13b which says a device cannot be used to splice neutrals, rather pig-tails are needed. I believe it's so the removal of the device won't lead to an open neutral situation.