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Building Matters

Does Your Electrification Project Require a Service Upgrade?

Step-by-step calculations for determining the capacity of your current service

Photo by Matthew Millham

Electrification involves replacing fossil-fuel devices, from furnaces and boilers to cars and lawnmowers, with their electric equivalents. The goal, as David Roberts stated in his widely-read Vox essay, is to “electrify everything.” Electrification has emerged as a major tool for fighting climate change. It results in immediate drops in CO2 emissions, even in regions where electricity is generated largely by fossil fuels. It provides a clear path to carbon-neutrality; as the share of wind- and solar-generated electricity continues to grow, emissions from electric generation will drop and ultimately approach zero. By eliminating pollutants like ozone and carbon monoxide from tailpipes, gas cookstoves, and malfunctioning heating equipment, electrification also improves air quality and human health.

Many new homes are being designed as all-electric from the start, and some municipalities are banning gas in new construction. In these houses, the electric service is typically at least 200A and is sized to accommodate electric heating, air conditioning, hot water, cooking, clothes drying, and possibly a car-charging station. The challenge occurs in older homes, many of which have only 100A or 150A service. Full electrification of these houses will usually require a service upgrade. But homeowners can often get started on a stepwise path—for example, replacing a natural gas water heater or installing a small ductless heat pump—without upgrading their electric service.

Determining capacity of current service

Practitioners of electrification—architects, engineers, energy auditors, and technicians—need to be able to assess the adequacy of the existing electric service. Overtaxing a service with new loads can create a safety hazard; it can also lead to nuisances like dimming lights, tripped breakers, and extra wear and tear on motors and electronics. But specifying a service upgrade when one isn’t needed also has its downside. The extra cost, which typically runs in the $1500−$3000 range, may…

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  1. charlie_sullivan | | #1

    For people facing the possible need to upgrade service, there are a few more options to consider beyond what's discussed here: monitoring for actual peak load in advance of installing new equipment, and active load management to prevent major loads from operating simultaneously. I'll describe each below, but first, thanks for this excellent and detailed article anticipating an issue that will become more and more important!

    NEC 220.87 offers the option of using the actual measured peak demand rather than the calculations laid out in this article. That requires monitoring over a 1-year period, or monitoring over 30 days and adjusting for seasonally varying loads. For my own all-electric house with 100-A service, I initially expected that doing this would indicate that we had plenty of capacity to add a 30 A, 240 V EV charging setup (EVSE), but after many months of staying below 46 A, we happened to run the electric dryer (which we almost never use) at the same time as the oven, the heat pump, and a bunch of other stuff, and got up above 80 A, leaving no room to add anything. The actual peak was remarkably similar that what I calculated as outlined above, indicating that this method works pretty well. So in my case, doing this only confirmed the result from the calculation, but that might not always be the case--monitoring for a month or a year might help some people avoid the need for an upgrade, or might even point to the need to upgrade service even though the calculations didn't call for it. Some people may want to start monitoring in advance of taking steps to electrify everything, to get that full year of data and be prepared to move forward.

    The other opportunity is active load management: smart controls that will prevent low-priority loads from operating when there isn't sufficient capacity to serve them. For example, if you are running the oven and the dryer, such a system would tell the EV charging system to wait and charge after those are finished. A water heater is another classic example of a load that could be delayed. This type of control system is not yet widely available or used, but the Elmec "EVDuty" smart EVSE has an option for a current sensor. With that, the system then operates even more intelligently than I described: it can adjust the EV charge rate to just take the amount of current that is available and never overload the panel, but rarely need to shut off charging completely.

    There are also more general purpose "load shedding controllers" that could be set up to control EV charging and/or other loads such as a water heater. The disadvantage of such a controller is that it is less of a turn-key system than the Elmec EVDuty setup, and it's intended to switch loads on and off, rather than smoothly adjust their current level. It can be interfaced with control wires on EVSEs and water heaters that are set up to be controlled that way, or can be interfaced with contactors to cut off power to whatever loads are deemed non-essential. But it requires an engineer or electrician willing to figure out those details.

    In addition to the cost savings one gets from avoiding a service upgrade, another advantage of the load management approach is that it reduces the peak load that you are putting on the grid. As more people electrify everything, we will have increasing issues with peak load, both in distribution networks and in generation. How that will be resolved is somewhat of an open question, but it's possible that we will see residential demand charges start to be applied, or perhaps discounts for customers who control their peak load. So taking the load management approach could turn out to be even more economically advantageous if a rate structure along those lines kicks in, but even now, it can be a less expensive and more civic-minded approach to take.

    1. Jon_Harrod | | #5

      Thanks, Charlie. I really like the idea of using measured demand, like you're describing, rather than a prescriptive calculation when it's possible to do so. I've often thought of that when sizing heating equipment, but the same applies to electrical service. And I agree, demand management and load shaping are going to be critical to making electrification go well.

    2. charlie_sullivan | | #6

      Update: It turns out the 15+ year old energy monitor I wasn't playing well with our new EV. It uses powerline communications that were disrupted when the EV was charging. So my data saying we were up against the limit of our service was flawed. I replaced it with an IotaWatt monitor and so far our peaks are below 40 A. I'll see what happens over the next month or two.

      So while everything I said about shedding controllers is true, and some people might need them, we probably won't.

      I recommend IotaWatt as an economical choice for monitoring, including logging individual phase currents, and the ability to monitor many branch circuits by adding inexpensive sensors. It would make sense to install one if you are even thinking about electrification, so you have the data to make decisions when the time comes. Also, if you put a sensor on the branch circuit for the heating system, you can also use that data to find the heating system run time, valuable data for sizing a new heat pump when you take that electrification step.

      1. user-2310254 | | #7


        Thanks for posting the info on IotaWatt. I like the idea of using an open platform rather than committing to a proprietary technology (smart breakers) that may not make it over the long term.

  2. paul_wiedefeld | | #2

    Jon, such a helpful article for me as I hope to electrify my house. Something I've seen on other online resources, but not in this article, is a buffer of 20% of the panel rating (so a 100 amp panel should have a max of 80 amps calculated as you show). Is this a best practice or being too conservative? If too conservative, is there a more appropriate buffer?

    1. Jon_Harrod | | #4

      Thanks, Paul. My understanding of the 80% rule is that it applies to sizing conductors for continuous loads (e.g. lighting or motors that are on continuously for 3 hours or more, including typical HVAC circuits). The conductors need to be sized at 125% of the rated loads, hence the 80%. Electrical service can be sized to meet the calculated load per the NEC 220.83 method without additional safety factors. However, it should be noted that the adjustments to general loads are based certain assumptions about behavior which work for most homes but don't cover every possible scenario.

      1. Expert Member
        BILL WICHERS | | #9

        I'm a little late to this article, but seeing as how I work with the issue of "continuous or not" loading commerically all the time I thought I'd comment.

        I'll start by saying this doesn't come up nearly as often in residential projects as it does in commerical projects. There are a few key things to think about though. It's the BIG loads that you need to pay particular attention to. Typical feeders that run things like receptacles in a room aren't usually an issue. Things to watch for:
        Water heaters
        Certain large tools that might run for long periods (think dust collectors in a large shop)

        Loads that are expected to run for 3 hours or more are considered "continous", and the 80% rule applies. This means you aren't supposed to load a circuit to more than 80% of rated capacity, and this includes most fuses and circuit breakers, and wiring. An example is a 20A circuit, run with 12 gauge wire and a 20A breaker, is only good for 16A of continous load. Can you upgrade the wire to 10 gauge to get a 20A continous capacity? Nope, you can't -- because the breaker is only 80% rated too. Specialty 100% rated breakers are available, but they're expensive, nearly always special order, and I've never seen one for residential applications. Even commerically, I find it's often cheaper to upgrade to a 600A switch than to get a 400A switch with a 100% rating, for example.

        Electric car chargers are going to be a big new thing, and they will typically be running for 6-8 hours or more. My own car charger typically runs for around 8 hours. These should be served by suitable circuits. The car charger manual will talk about this -- don't just oversize the supply circuit because the charger may specify a maximum current rating for the OCPD (OverCurrent Protection Device), the circuit breaker, and you can't exceed that. You can oversize the wire if you want a little extra insurance, since most of the heating issues tend to be with the wiring. Chances are the car charger instructions have allowed for this, and may specify a 40A circuit with a 40A breaker, but will only charge at 32A -- 80% of the supply circuit rating.

        The need for continous ratings doesn't apply to service entrances, since it's already expected that not everything will be on all the time at the same time. You would only have to worry about the 80% rule here if you had enough portion of the total load classed as continuous to exceed 80% of the rating of the service entrance, and I don't see that happening in a residential system unless you have a relatively small 100A service with several car chargers connected along with a typical home load. Even commerically this rarely comes up in normal systems.

        If you want some extra insurance on the service entrance conductors, don't size them from the special table in the code book for residential service conductors -- size them from the "real" ampactity chart 310.15. If you compare the two, you'll find that residential service entrances are allowed to be one size smaller than the "regular" ampacity chart allows, for example a 200A service can use a 2/0 copper conductor, which would be rated for only 175A in the 75*C column (where conductors need to be sized from for most applications). A 3/0 copper conductor would otherwise be required. The smaller conductor is allowed since the code understands that a typical residential service will normally run far below rated capacity, even over extended periods.

        It's worth mentioning that if a home is built as an all-electric home, with electric water heating (especially tankless electric water heaters), and multiple car chargers (which can be expected to run at the same time at night), it's probably a good idea to NOT follow the residential service conductor ampacity table and go with the normal conductor ampacity rating instead. Larger wire will run cooler for the same electric load.


        1. charlie_sullivan | | #10

          When you are considering conductor sizing for a continuous, frequently used load, it's also worth thinking about the fact that oversizing to reduce heat also improves efficiency. For example, a 50 foot run of 8 AWG wire has 2 V drop at 32 A. That's a small percentage of of the 240 V nominal voltage, but it's 64 W of loss, so it's the equivalent of running an incandescent light bulb whenever the charger is running, and would be nearly 100 kWh/year if it was charging at that rate 4 hours every night. Upsizing to 6 AWG could pay back in a year or two. Standards allow as much as a 5% voltage drop, but designing or much less than that is a good idea for loads that are on for enough time that the cost of the loss adds up.

          Keeping wire runs short is a less expensive way to reduce voltage drop and loss.

        2. user-2310254 | | #11


          How would you cover your bases as far as spec-ing a car charger for a new home, assuming you don't know what type of charger would go in? I'm pretty sure my next car will be electric (or at least hybrid). But I won't narrow my choices for a year or two.

    2. charlie_sullivan | | #14

      For completeness, I want to leave here a note on how the 80%/125% stuff works if you take the NEC 220.87 option of using the actual measured peak demand. The proper way to do that is to look at the data in terms of average current over 15 minute intervals. The highest 15 minute interval number should be multiplied by 1.25. Then you add the new load. If the new load is EV charging or a heat pump that might run continuously on the winter night, you multiply that number by 1.25 as well. And that sum must fit within the service rating.

      That works out exactly the same as if you starting by multiplying the service capacity by 0.8, and then saying that the unadjusted numbers, maximum 15 minute average measurement and new continuous load, have to fit in that 80%.

      1. user-626934 | | #26

        Do you know what the procedure is for when you want to add *multiple* new loads, all at the same time, and you're using the 220.87 actual measurement option?

        For instance:

        100-A panel service rating
        Measured 15-min peak Amps (30 days, winter) * 1.25 = 46A

        Want to add:

        9,600VA kitchen range
        5,000VA water heater
        7,680VA EV charger

        Seems to me that it would be appropriate to apply a diversity factor to the kitchen range and the water heater, but not the EV charger...but NEC 220.87 doesn't spell it out.

        1. user-626934 | | #27

          Also, interesting to note that 2020 NEC 220.87 forbids the peak measurement option when there's solar PV (or wind) already installed.

          Would be curious to hear if anyone has gone through trying to use the peak measurement option, add multiple loads, *and* add PV at the same time...

        2. Expert Member
          BILL WICHERS | | #28

          You can use "demand factors" to de-rate loads when you have a bunch of the same type of load that aren't expected to run all the time. The usual example given is for an appliance like an electric range that is installed in 50 units in an apartment building. The code gives demand factors that can be used to approximate the expected average load across all of those ranges, since it is reasonable to expect that they won't all be running at full power all the time.

          You don't typically apply demand factors to several devices that are all different types. In your example, I wouldn't try to go with less than full load, because it is not unreasonable to assume that the range is in use while the car is charging and the water heater kicks in. Imagine you get home, plug your car in to start charging (which will take many hours to complete), then you start dinner and run the dishwasher, which will call for hot water. You now have your car charging, some burners running on the stove, and the water heater will probably be on to keep the dishwasher happy. Everything is no running together, and might continue to do so for 30+ minutes or so, maybe even an hour.

          A 100A panel isn't a lot for what you're looking to do. Your application would be 92.83A of load on a 240V service. All those loads would be 240V loads, so you have perfect balance between legs in the panel, but you have very little capacity left for some lights in the kitchen where you're running the stove and dishwasher, maybe the TV where you're watching some evening news... Maybe this is the summer too, and your air conditioning wants to come on. Now all of a sudden CLICK! You find yourself no longer cooking dinner, no longer washing the dishes, your car isn't charging.... Because your 100A main breaker just tripped.

          It's always a good idea to not push right up to the maximum with electrical things. Try to keep some extra margin.


  3. user-2310254 | | #3

    When updating our newly purchased townhouse, we decided to eliminate the gas appliances. Before we could move forward, we had to update the 10-year-old, 150-amp service panel. Getting the panel reorganized, adding a subpanel, and running a few new circuits was relatively inexpensive. But as Jon suggests, you do have to have a plan.

  4. EudoraAquino | | #8

    I don’t have time to read it all at the minute but I have bookmarked it
    and also added your RSS feeds, so when I have time I will be
    back to read a great deal more.

    Eudora Aquino @ Homeadvisor

  5. GaryGech | | #12

    I just installed solar, and the entire key was the service upgrade. The answer is always yes, if you are upgrading anything, go with a service upgrade. Apple would not build a computer with yesterday's memory and power supply, they regularly upgrade. My service panel was from 1978. The young electrician was like, "it still looks good". Come on, pull it out and do what is right. In the field of electrical service panels, one phrase everyone trusts, "Square D". Behind every major innovation is a patent. If you refuse to validate that, you fall behind. There is a great deal at any point in time we do not understand, the way construction was done in the past. At one point, perhaps the microwave was built like a tank but the old windows won't hold up. In the next ten years, expect less electrical use as appliances and lights become more efficient. But also expect solar, and battery backup, and that requires an upgrade, and it should be subsidized.

  6. charlie_sullivan | | #13

    Certainly, old panels that are either obsolete technology or worn out should be replaced. Googling the brand of your panel can help you figure out whether it's one of several out there that are truly inferior, which would bump up the priority of replacing it. If you do that, you should consider whether or not you need a service capacity increase, or will in the near future, because it will be a good idea to do both at the same time if you do.

    Whether the panel and/or the service need to be up-sized to accommodate solar is a different question. For PV, it's often the panel current rating that is the limit, not the service capacity. Sometimes you want a panel rated for higher capacity than your service. That's a complicated topic that might be a good topic for a future blog.

  7. masincalgary | | #15

    Hi. The answer for my house is yes, I need an electric upgrade. The city wants me to spend $15,000 to $20,000 to dig 10 linear ft under a sidewalk (from my property to the box) for them to upgrade the service from 100 amps to 200 to my house. This to accommodate the back up heat which is a code requirement for my climate when I install an ASHP. I won't do that. There are no alternatives to the cost of that work, and anyone can see the value/$ is insane.
    My contractor suggests two alternatives and a new electric panel. One is a switch (the word I'll use). He used "load master breaker" and the other is a smart panel (this option costs 4x as much as the master breaker).
    Two general questions for the blog I have are
    1) In doing this essentially deep retrofit of my home, doesn't it make sense to size the systems for average to a reasonable max capacity of my home. (Why invest in infrastructure that couldn't handle Christmas visitors for e.g. or its eventual sale to a possibly larger family?)
    2) Will the master breaker or smart panel need upgrading as more tech rolls out (e.g. my 10 yr old laptop hardware won't accommodate the software upgrades)?
    3) and both the solutions (which would add on to a new panel, which I consider to be a sunk cost) cost more than my laptop! How can that be?

    One thing that helps me move forward on this dealing with my questions, so thank you VERY much!!

  8. charlie_sullivan | | #16

    What calculations and or measurements have you done that say you need 200 A? What is the power of the electric backup heat?

    What is that plan for the "load master breaker"? What loads will be controlled by that?

    I like low-tech solutions that won't become obsolete. Building a load management system out of generic components rather than a proprietary system seems like a better plan to me. I think of the unit in my first comment,, in that category.

    You might post this to the Q&A section rather than as followup questions on plot posts, to get more specific help for your situation.

  9. masincalgary | | #17

    Thank you, Charlie. Every electrician (3) have said that I need a new panel (not enough room on it). One contractor said I wouldn't need back up service, but he would install a large ASHP and he did not get an electrician's advice. The contractor who has done this before says that the city will not approve a permit for a (right-sized) ASHP without back up heat, and during the coldest days my service would not cover the stove, ASHP and back up all at once. So we are looking for solutions to keep the 100 A limit by swtiching something off. I do plan on getting solar. I don't want to get a simple but costly switch, if next year I need to get something else to handle the solar. My car is 20 yrs old. Similarly, I'm hanging on to it until I can buy an EV.
    Thank you for suggestion a post in the general Q&A. I didn't know I could do that.
    What helps me make decisions is understanding, so I really appreciate every single thing that I learn here. Learning from different angles also covers more ground.
    In answer to your questions:
    - The Mitsubishi lists Resistance Heaters 8/10/15/ kW (i.e. a range, presumably I'd need the midsize, 10 kW) and the 3 ton ASHP needs a breaker size 30. Daiken data is hard to find.
    - The electrician has looked at all my loads on the panel. (And I sent a list of all my appliances and loads before he came).
    - I imagine the load breaker would control the stove. (It's easy to cook without it).
    - This is the smart panel, which looks the same as what is written about here

    1. Expert Member
      Akos | | #18

      The 99% outdoor design temperature in Calgary is -15F. At that temperature most cold climate heat pumps can supply sufficient heat without electric backup. Some are rated down -22F and can supply heat even bellow that. Our code (Ontario) requires a heating system that can supply the building load at design conditions, it does not care how that is supplied. I doubt your code is any different, requiring backup of any kind makes no sense.

      Even if you do need to add in resistance backup, as long as you keep it reasonable, you might be able to squeak by with a 100A service. For example a 100A service for an all electric 2000sqft house with finished basement has room for about 7kW of resistance backup (4kW if it can run at the same time as the heat pump).

      Demand calculations are very conservative, so even if you are close to the limit on paper, it is extremely unlikely you'll ever see load high enough to trip your main circuit breaker.

      I would take that $15k-$20k for the service upgrade and use it improve your building envelope. Increasing building efficiency means smaller heat pump, smaller or no electric backup and lower running cost. Much better use of your dollars in the long run.

      1. masincalgary | | #20

        That's really interesting, Akos.
        So you're telling me that I don't need a back up resistance heater according to the code, as long as it's a rated heat pump for cold (as in -22F). I thought they were rated down to -15C but my Mitsubishi Zuba brochure does read "Heats even at -30C and beyond." I need to check that (is the code accessible to me?)
        I have improved my building envelope. That is the first thing I did, which is why I'm ready for a heat pump now that my furnace died., and why I managed all winter using a few space heaters last winter. I agree. I am not going to spend that sum of money for the utility to dig and replace their 50 yr old wiring (they may need to do something themselves). I'd much rather save those dollars for the retrofit itself.
        My contractor says that he never uses his back up heater. I don't anticipate using it either.
        Thank you for your info. I guess I'll need to talk to the electrician.

      2. masincalgary | | #21

        Also, Akos, can you give me a couple examples of heat pumps and how I can know their rating? The two I've been considering are Mitsubishi and Daikin. Thank you.

        1. Expert Member
          Akos | | #23

          I would read through this thread as it is similar climate as yours:

          There are links to some manufacturer specs and part numbers to start your search. The Carrier (which are rebadged Midea units) seem to have better extreme cold performance.

          You can also look at Midea high static units, which come in 24/36/48, for the money they are excellent value:

          You'll have to do some digging to get engineering data on it, I could only quickly find for the 9-24 units:

          If you don't mind DIY, there is also MR Cool DIY air handler. These come with quick connect line sets, no specialty equipment required, for a simple install. They are also speced for -22F. The MDU18036 might be in the ballpark.

          Whichever brand you go with, make sure there is local support.

          Building code should be available on-line, but not the easiest to read through. I would talk to your building department and see what they say first. They can also point you to the correct section of the code.

    2. charlie_sullivan | | #19

      For solar, you need a panel with a high enough current rating for the power supplied from the utility plus the power supplied by the solar. (Conceptually--details are complicated.) So upgrading the panel without upgrading the solar actually makes it easier to accommodate lots of solar, compared to if you upgraded the service as well.

      As Akos says, the capacity rules described in the article here are quite conservative. If you size things based on NEC 220.87 instead, using measured consumption, you will likely find you have more room than you thought.

      As far as how to structure your load management, I would think to first turn off the EV charging, as you can do that overnight instead, then turn off the domestic hot water, as you have a tank backing that up, and finally turn off the HVAC. That way, if you have oven warming up while cooking on the stove top, you can go ahead and do that--it will just pause the other stuff, perhaps including the HVAC. Otherwise you'd need to go turn down the thermostat to use the stove. That's more trouble. You'd probably never activate the system, but you might need it to make the inspector and electrician comfortable.

      I wouldn't worry about the solar requiring new controls. As long has your panel has capacity, the solar will only help capacity issues as far as your 100 A service capacity constraint.

      The PSP load shedding controller I linked before would work great for that. The small one has four channels that could control the EV, the hot water and the HVAC, and still have a spare channel. And those can each be set up to use a control signal from that controller rather than having a large and expensive relay physically cutting the power to them. The only problem is that somebody has to figure out how to connect all of that and set it up.

      But you might not even need that. I'm in a 2200 sq. foot all electric house with EV and am doing fine with 100 A service--my highest 15 minute draw so far has been 57.5 A.

  10. masincalgary | | #22

    That makes me feel much more hopeful! My house is 1800 sq ft. Is the load shedding controller really complicated for most electricians? Is it a certain type of electrician that knows how to set them up?
    And where are they purchased? One would think that they'd be associated with the heat pump distributors, but it's often feast or famine in Calgary regarding trades.

    1. charlie_sullivan | | #24

      I don't know about canadian sources specifically but here's one distributor selling those online:

      I don't really know how to find an electrician who would be up for figuring that out and setting it up. Somebody who does industrial controls would do similar things regularly, but would they want to do a little residential job? I think you just need to ask around.

      For simply pausing the HVAC while it's running, an HVAC tech would easily be able to figure out wiring the thermostat wiring controls through the controller. For a water heater, it might turn out to be trickier. The new ones have a socket defined by a standard, CTA2045, where you plug in a simple, cheap control module. But I don't know of anyone making a dead-simple on/off control module you can plug in there, even though it should be possible. So you might end up needing to use a relay box to hard switch the power to the water heater. But maybe you can use one of the 15 A Rheem models and not have to switch it off, and only control the HVAC, and later the EV, with the load management system.

      1. masincalgary | | #25

        I did find an electrician and the work is due to happen next week! It's saving me a costly dig. I'll post back here once all is said and done. :)

  11. AdamPNW | | #29

    Picking up an old thread here;
    What if you want to add a new structure (in my case a primary residence) to our existing 200 amp service, currently serving our manufactured home?
    We plan to move into the new house when completed and keep the existing manufactured home as a rental, on the same meter.
    To calculate the extra load, do we simply add the 2 structures together, treating them as one big house? Or is there a demand factor that comes into play when calculating separate structures.

    Some load info: both structures will be all electric, heat pump HVAC, Heat pump water heater, high performance envelope, 10kw PV array, and a single level 2 car charger. Mild climate (4c)

  12. MikeM88 | | #30

    Sometimes this question comes my way when I have to install new modern electric appliances in an old house that originally had a gas stove. Now, after reading this, I finally have somewhat of an answer, or at least somewhere to point them to. Thanks for posting!

    Kind regards,
    Mr. McLean @ Speedy Appliance Repair LTD

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