I am building a 24'X30' garage that will have a guest space above. The service to the main building is 200Amp. The garage will include small appliances heat and a/c. There will be an efficiency washer dryer as well. It's not an apartment meant for full time use. How do I know if the service needs to be upgraded? I was under the impression from one electrician that there was no need. However the contractor is saying it may need to be upgraded. Any input would be greatly appreciated.

  • 1
    How big is your house now, and what big electrical appliances do you have (stove, oven, water heater, furnace, dryer, EV, etc)?
    – mmathis
    May 3 '17 at 21:16
  • Will the heat be electric? Will you be putting on-demand hot water heat in? Will the garage be strictly for storing cars, or will there be a pretty respectable wood shop in there? May 3 '17 at 22:18
  • We'll need the square footage, small appliance circuit counts, and HVAC loads for both the existing house and the garage+guest space May 3 '17 at 23:14
  • Considered putting garage on separate meter?
    – Kris
    May 4 '17 at 11:11

Generally we would go over 200A for a single family dwelling unless the house is over 3000 sq/ft, and even then we would run a calculation to see if a larger one is needed. When we see an addition to an existing building the code allows us to take the peak load of the existing building then add 25% to it then add the additional power you are installing in the new addition.

Now, where can you get your peak loads? If you look through your power bill for a year (2016). Somewhere in that mess they will give you the peak winter and peak summer usage, use the highest peak. Or you can call your utility company and they usually have that information available. It will be given to you in KVA or KW so you take that figure and divide it by .24 and that will give you you peak amperage for that year. If it is somewhere around 140A you need to consider upgrading your service before adding your garage.

If what I just said is confusing. You need to at least get some electrical professional to assist you with it.

  • How does the power company know my peak power useage when all they see is the monthly kW-hr total? Or is that supplied by the new smart meters - I have one but didn't think they were universal yet.
    – Mark
    May 3 '17 at 22:03
  • It really depends on what you have now. If you have gas heating and water heater for the main house I doubt an upgrade would be needed. If an electrician said it wasn't required he/ she would know more than a general contractor and any advice from me or anyone else that has not looked at the current loads and the loads to be added.
    – Ed Beal
    May 4 '17 at 4:37
  • Exactly, what Ed Beal said. Peak power for residential is an extrapolation of the entire grid in you area, and then applied to your single residence by averaging you meter reading for that day. Thus the addition of the 25% also it might be a light usage year. New smart meters are more precise and can deliver real time individual usage. May 4 '17 at 12:07

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