We had our Washington home built in 1997 and want to add at least one EV charger. Electricians have said the 200A CB panel is full (4850ft home so ok) and doubt we have adequate service; we need to upgrade to 320A service. Contacting the local utility Puget Sound Electric, they confirmed the transformer is capable and has a 4/0 underground run to the stub at my property line. But the 100' underground (and under driveway!) feeder from there to my meter is 1/0 - PSE says that needs upgraded to 4/0. Total cost for that upgrade + 320A panels is ~$15k. Wow. Based on 1997 codes, was it errant for PSE to install the 100' 1/0 feeder run in the first place for this large of home? It was equipped with electric everything except gas furnace, which has since been updated to the multi-stage heatpump w/ gas backup.

  • A load calculation should be the first thing done to see if there is any extra power to be had from the panel. Next is the size of the the charger needed. Most people are happy with 20 or 30 amp charging, which your panel might handle, instead of the big 50/60 amp chargers that usually only boost ego.
    – crip659
    Mar 20 at 16:22
  • The power companies do not follow the codes and have their own rules. Are you sure they installed that run? A 1/0 service is normal for a 200 amp panel. It's probably aluminum.
    – JACK
    Mar 20 at 16:36
  • Well you probably cheaped out in 1997 by only installing a 200 amp service in a huge home, which was probably acceptable to a load calc but not to how houses actually get built. There are no codes that say to install larger wires than the box will serve. But a gas hot water heater or dryer will probably be cheaper than a panel extension.
    – Tiger Guy
    Mar 20 at 18:21
  • The amount of people in the house will matter also. 2 or 3 people in a big house will not use that much electricity, compared to a big family of ten plus and a upgrade might make sense, if you want to use more. Without more information, a full panel might just mean a need to add a cheap(compared to 15,000) sub panel or an 15,000 upgrade is wanted.
    – crip659
    Mar 20 at 18:42
  • You have gotten a good answer below - and I put in a Tl;dr to make it easier for you to digest. This is very solvable without a panel upgrade.
    – KMJ
    Mar 20 at 22:12

1 Answer 1


This was absolutely fine in 1997, assuming a proper Load Calculation was done at the time. Home EV charging was not an expected load in 1997, and 200A is quite standard.

There are times when a heavy-up makes sense. I don't think this is one of them. It comes down to cost/benefit, and $15,000 is a pretty steep cost. The alternatives are not free but are much lower cost, and the benefit from the full service upgrade is actually minimal, as you will see.

Why is the upgrade benefit minimal? Because most of the time you have enough power. EV charging can be done reasonably effectively with 20A, and 30A is more than enough for nearly everyone. The key is that you only run the big loads part of the time.

  • Does your HVAC system run 24/7?

It might on the coldest or hottest days of the year. But the rest of the time it is either cycling between 100% and not running at all or, if you have a variable speed heat pump, running at far below the full capacity.

  • Is your electric oven on all the time?

Unless you're a commercial baker, it does not. Probably just a couple of hours a day, and most of the time those hours are around meal time. When do you generally need to charge your EV? Between dinner and breakfast so that you can drive away in the morning - the oven is usually off that entire time.

  • Does your water heater run all the time?

No, it does not. Most of the time it just sits, full of hot water. In fact, even when you are actually using hot water the tank can provide water for a while without heating more water. (Tankless is different, but you don't use tankless for a big home on 200A service, so I know you don't have tankless.)

Load Sharing or Shedding to the Rescue!

There are a number of different load shedding/sharing devices available. Of particular use for EV charging is EVSE (Electric Vehicle Service Equipment, a.k.a., "charger") that connects to a power monitor in your main panel. It checks how much power is being used for everything else and then uses the rest of the available power, up to the amount allowed for your EVSE (based on device design and breaker and wire size). For example, you could have your EVSE provisioned for up to 50A (which would actually be 40A of true load because of the 80% continuous use derate). When the monitoring system detects that all other usage is under 150A, which will be most of the time, then it lets you use a full 40A on the 50A circuit. If it detects usage of 180A then it will allow 16A (80% of 20A), and so on. The exact numbers may vary a bit depending on how other continuous loads/80% derate are included, but this is the basic concept.

You can't "roll your own" on this type of thing, but there are readily available systems that can do this at an affordable price - saving you most of that $15k while still providing "full" EV charge rate most of the time, partial some of the time and none at all (unless you manually or automatically turn off other loads) a tiny fraction of the time.

Perhaps the most extreme situation you will have is Thanksgiving (or another big family and cooking day). But even then, when the HVAC cycles off you'll have some power for the EV.

  • 2
    The tl;dr version of this is: install an EVSE with load management like the Emporia. Then you only need 6A in your load calculation, but most of the time it will charge at full rate because it will see the available capacity and use it.
    – KMJ
    Mar 20 at 22:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.