I currently have a sub panel in my detached garage, fed from a 30 amp breaker in my main panel in the house. The wire run is through conduit on the exterior of my home, I would estimate it at 30-50 feet long just to shoot high. I am wanting to upgrade the garage to 100A service and am having some trouble determining what gauge and style of conductor would be most suitable for my application. I am planning to install an electric heater and potentially also want the room to power a welder later on, as well as some machine shop tools I already have. I used a voltage drop calculator from Southwire that suggested 1 AWG copper, but have also seen suggestions online that 3 or 4 gauge copper may be acceptable. I am open to copper or aluminum options. Is there any NEC guidance on this I could refer to? Otherwise I'll take friendly advice. Thanks in advance!

  • The saving with using AL will probably pay for the panel or more. The big question is have you done a load calculation to see if you have an extra 70 amps to add/use. Probably better heat choices than a big toaster for heating, unless you meant something like a mini split/heat pump.
    – crip659
    Oct 4, 2023 at 18:02

1 Answer 1


Something is wrong with your parameters used for Southwire, 50' at minimum legal size #3 CU @100A is less than 1% voltage drop, no need to upsize. Minimum allowed size is determined by NEC Table 310.16 using the column that aligns with the max temperature rating of your panel and breakers, which in all likelihood is 75°C.

#1 AL or #3 CU would be minimum size for 100A but market forces make 90A #2 AL "mobile home feeder" fed by 90A breaker to a 100A or bigger panel is a very good budget option and certainly large enough for the loads you presented. Your new subpanel must have an amperage rating at least as large as feeder, but can be higher. Large panels are always good.

A "Load Calc" as prescribed by the NEC will to determine if your existing panel/service is large enough to support the added loads will likely be required to get approval by your local Authority.

  • This is the exact info I was looking for! Thank you. Yes, I do still need to do a load calc before I set anything in motion. My main panel is only 100A so sizing for 90-100A service to the garage is overkill anyway, but I don't want to do this job again down the road if I can help it. Any suggestions on a good resource to do a load calc? Oct 4, 2023 at 19:01
  • When I need a calc I drag my feet through Section 220 of the NEC which is a bit clunky especially for a rookie. I have seen calculators online, but I can't give endorsement since I haven't used them. Oct 4, 2023 at 19:09
  • For general concept look here ask-the-electrician.com/… Oct 4, 2023 at 19:15

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