I have 400 amp service in a 3800 square-foot home. I ran 100 amp to a garage. I want to build a mini-home that will require approximately 130amps in appliances, etc. What size sub-panel do I need?
The mini house will have:
- stove 40 amp
- water heater 18.75 amp
- stackable washer dryer 30 amp
- heat pump 8.87 amp
- dishwasher 20 amp
- fridge 8.5 amp
- microwave/hood 15 amp
- lights & outlets
That's why I figured 130 amps. The load in the main house should be able to handle a 100 to 150 amp sub panel. Need help deciding what size I should use.
Best Answer
I ran your loads through a calculator and it spit out a main panel load of 66 amps. That tells me you are probably alright. But I am concerned you are not talking about heat pump emergency heat. Heat pumps do not work in very cold temperatures, that's where emergency heat comes in. I note this building doesn't have any gas.
So while I think it'll work at 100A, I'd feel more comfortable seeing 150A because I worry that there are things you've left out. So that's where I am with feeder.
As far as your subpanel size, there are 2 things we're concerned with.
Number of spaces in the panel
The usual mistake is to consider only panel ampacity and get a panel that is much too small (in spaces). Remember, you have an all-electric house, which means a lot of 240V (2-pole, double-sized) breakers. You need the same number of electric appliances as a bigger house, they just have smaller numbers on the breaker. So even a 30-space panel would be scary-small. I would aim for 40-space.
There's a scam out there called "Circuits" - such as a panel that is 20-space/40-circuit. That relies on breakers that are now illegal - panel makers shouldn't be making these claims anymore, but they are. By "Illegal" I mean a 40-"circuit" panel relies on very few of them being AFCI/GFCI or common-trip -- and in fact, most of them must be!
Ampacity of the panel
The amp rating of the subpanel itself is flexible. It only needs to be greater or equal to the supply breaker. So if your supply breaker is 100A, then you can use a 100A, 125, 150, 200 or 225A subpanel.
Because it is an outbuilding not connected by a breezeway, your house will need its own main disconnect switch. The easiest/cheapest way to get that is select a panel with a main breaker. Again, any size will do >= the feed breaker size.
If you think "I'll get them the same size so the nearby breaker will trip first", unfortunately it does not work that way. It's a coin-flip which one will trip first, unless it is raining and you are dressed for bed, then it will be the far one obviously.