We are thinking of building an ADU (Auxiliary Dwelling Unit, or "in-law suite") in our backyard.

The planned ADU's square footage will be 852 square feet. It will be 70 ft away from the main house, which is 1045 sq. ft.

We submitted our plan, but it didn't pass. This is because of the main service, which is 100 amps. The inspector told us to install a 400 amp main, to service the back house.

We would like to know which rule in the NEC the inspector is referring to. Can you tell us?

  • This is an Article 220 load calculation problem. The square footages are a good start, but can you post photos of your existing breaker panel(s)? Also, are your hot water heater, dryer, and range gas or electric, how is your existing house heated/cooled, and how many circuits do you have to turn off to turn off all the kitchen counter receptacles? May 25 '20 at 21:58
  • Also, is your existing service 100A or 200A? May 25 '20 at 22:00
  • Do you want the ADU to be separately metered? If so, it could have it's own service and you wouldn't have to mess with the one in your house. May 25 '20 at 23:33
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    There are also sometimes local code requirements concerning ADUs that will guide your design. I have seen jurisdictions that require the ADU have separate utility services in case it is ever split from the main residence and sold as a separate property.
    – mfarver
    May 26 '20 at 0:52
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    @Bukol -- I take it the current box is outdoors? Also, I take it the main house water heater, range, and dryer are all gas? And can you please tell us how many breakers you have to turn off to turn off the kitchen receptacles, or post a photo of your existing breaker panel for that matter? May 28 '20 at 1:36

Maybe some local code ,but 400 A is excessive power unless you have something like a weld shop. I have 200 A in 2500' plus 1000' garage+room plus a 96' out building. Includes 5 ton AC. circuits for an electric stove and and an electric dryer have never been used , but are dedicated . Also a 20 A 220 in the garage , I told the electrician I might want to weld one day, I never have but I bet I could if I wanted to. Looked in the box for curiosity : 28 breakers used, 2 -50 A, 7 -30 A , 19 -20 A . How in the world would one need twice as much power ( 400 A) in a home ? I assume the licensed electricians hooked it up correctly.

  • Mostly its because of two things: 1) giant oversized toasters (heat pump strip heat, electric tankless hot water heaters) and 2) folks who don't grasp Art. 220 load calculation rules. I personally wouldn't do 400A service unless I was dealing with a very generous ADU situation May 26 '20 at 2:45
  • our adus electricity is only 60amps in total of appliances electrical calc the higher amp we are planning to use is microwave which is 1100 amps no toaster nor electric water heater no cooling only heating and its gas powered
    – Bukol
    May 26 '20 at 12:58
  • @Bukol A microwave only provisions for a couple of amps; that number is watts, different thing. It sounds like your actual electrical needs are very modest. 400A service simply makes no sense unless the ADU is going to be electrically heated with on-demand water. My own unit is gas everything, and we only have 30A service. May 26 '20 at 18:21
  • @Bukol -- can you post photos of your existing breaker panel, at the very least? May 29 '20 at 3:18

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