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Community and Q&A

400 Amp Service-Really?

nynick | Posted in General Questions on

I’m pushing my GC to drill down on the estimates for our project and renovation. Trying to be brief, we’ll have a small, all electric house (2500sf)  with 2 Heat pumps, condensing dryer, induction stove, microwave, ERV and Heat Pump hot water heater.

The detached 3 car garage with an upstairs apartment will have almost exactly the same, with the Heat Pumps most likely being smaller.

Their electrician of choice is telling us that we’re going to need to upgrade to 400 Amp service or run a second 200 Amp service. I find this hard to believe, but these guys usually know what they’re doing.

Running new utility wire from the street will be costly I’m sure, not to mention trenching or a new pole etc.

Does this sound right? I’ve never heard of 400 Amps for a home.



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  1. paul_wiedefeld | | #1

    Seems high as described -but do you plan on electric car chargers?

  2. mgensler | | #2

    We have a 4500 sq foot house that has 4 mini splits, induction cooktop, double electric ovens, hot tub, pool, resistance hot water backup, one on demand electric water heater for hand washing, two electric vehicles, two condensing dryers. I checked our sense monitor and we've hit 25,000 watts twice in the last 3 months. Both times it was a very brief spike for maybe a minute. We do have the EVs set to charge at different times of the day.

  3. Expert Member
    Akos | | #3

    If this is all running off a single meter, I can't see the load needing more than a 200A service. A typical all electric house doesn't use all that much more electricity, things like big EV charger ports, hot tubs or pools are the ones that can really crank up demand.

    If the ADU is on its own meter, than you need the electrician to do a demand calculation for both the house and the ADU. Even in that case I can't see you needing more than a 200A main feed with a 200A panel for the house and 100A panel for the ADU. Usual demand calculation for a small all electric apartment is around 75A to 85A. The house would be a bit more if you have an EV port.

  4. Expert Member
    BILL WICHERS | | #4

    Sometimes load calcs will dictate larger services than you may expect. That might be the reason, and if it is, your electrician should be able to tell you immediately.

    I doubt you will really "need" more than 200A here though. My first thought for your design (without knowning all the details, so keep that in mind :-) is to put a 200A service in the house, then feed a 100A subpanel in the garage from the main house. If you have a large solar system, or you plan to have several smaller (or one large) EV charger in the garage, that may change things.

    If you do have to upgrade your service, my preference here would be to run the 400A service to the main house, then put a "large" panel after the meter that would feed the existing 200A panel in the main house and the 200A panel in the garage using seperate breakers. When I say "large" panel, I mean a panel that takes what are known as larger "frame size" circuit breakers. If you look at a normal residential panel, you'll see that you can only get 100A, or sometimes 125A, breakers for the branch circuits -- no larger breakers are available. Commercial panels can handle bigger breakers. As an example, I specified some 600A breakers for a project at work just today, and there will be several of those in one of the large panels. You can get larger panels that can be used in residential systems too, you would need something with a 400A or larger bus, then appropriately sized breakers to serve the subpanels in the house and garage. I would leave at least one spare space in the large panel for a potential third panel somewhere too if you ever need to upgrade.

    Note that "400A" service from a utility is only actually 320A service, so keep that in mind if you really do have very heavy loads.


  5. nynick | | #5

    This is off a single meter already. The existing house already has a 200 Amp service. We're renovating the house and building the garage and ADU from scratch. I think the ADU is throwing the monkey wrench into the works.

    My thought was to run a 100 Amp panel to the garage off the 200 Amp service that's already in the house. I was going to ask them to put a car charger in the garage as well in case we buy an EV.

    Yes, I am considering a 8.4KW solar system also.

    1. Expert Member
      BILL WICHERS | | #6

      A typical Level 2 EV charger uses a 40A circuit, and won't actually draw more than about 32 amps when in operation, so you could run that off of a 100A feed to the garage, assuming you don't have another large load out there (like a second charger, or a big electric range in the living space). If you are planning for multiple chargers, something more than 100A would probably be a good idea. There are Level 2 chargers that take larger feeds too, but they are much less common than the usual "40A" type.

      There are some special rules for solar systems, so watch out for that. You can often get around those issues by using a dedicated feed for the solar system off of that "large panel" I mentioned earlier, so you'd have a large distribution panel that would feed 200A to the main house, 100-200A to the garage, and a third breaker for the solar system. Since that "large panel" would have at least a 400A bus, you have more wiggle room for the solar system, which would otherwise effectively derate one of your other panels due to the way the code works for those systems.


      1. Expert Member
        Akos | | #7

        Hardwired EV chargers can be connected to smaller circuits. Most have the option to go down to 15A circuit so 12A charge.

        Even the EVs that have a 7kW or 11kW charger will charge at reduced power from a smaller base, so the only thing you give up is charging speed. Even a small 20A base adds about 25mil of charge per hour so that is a LOT of charging over night over 10 to 12 hours.

        If you are doing mostly local driving a plug in charger cable for on a dedicated 120V outlet is good enough (I would go for more though), there is no need to go overboard on a charger base unless you are driving a lot of distance each day.

        For example, in case of the OP, assuming the ADU comes out to around 70A demand, you can go with a 20A or 30A hard wired charger and still be bellow the 100A limit.

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