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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.

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    @manassehkatz those are called "electric resistive heaters"... – Harper - Reinstate Monica Dec 5 '19 at 3:11
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    This sounds to me not like a true "tiny house" but rather more like a "small house next to a big house (3800 sqft!)" - i.e., "mini" is relative and not in the "tiny" sense. – manassehkatz-Moving 2 Codidact Dec 5 '19 at 3:17
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    @Tina Gregory don't get discouraged by the "sparkies" hashing it out, hang in there for a helpful answer (or reference the calculators mentioned in one of the comments...) If a comment asks for more info., try to provide it by editing it into your question, that helps with quality answers. – Jimmy Fix-it Dec 5 '19 at 4:59
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    @EdBeal -- the OP is really building a detached ADU, not a "tiny house" in the sense you're thinking of – ThreePhaseEel Dec 6 '19 at 4:11
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    @TinaGregory -- also, where are you on this planet? Heat pump emergency heat is second only to instantaneous electric hot water heaters at murdering service calculations... – ThreePhaseEel Dec 6 '19 at 4:12
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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.

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