I am installing a 100 A sub panel in my barn which is 170 feet from the main panel. 100 feet will be underground and 70 feet will be through the crawlspace underneath the house. My main panel is 200 A. I want to use aluminum wire.

The barn measures 28' x 50'. Within that, there is a 12' x 20' workshop. I'm planning on running lighting, and power tools. I will have gas heat so I don't need to heat the place electrically.

  • 2
    Key question is: what size actual load do you expect to use? Jun 27, 2019 at 16:36
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    What loads are you planning to run in this barn, and how many square feet is it? Jun 27, 2019 at 23:31
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  • @manassehkatz - I would have expected the wiring to the sub-panel to have to be adequate for the master breaker in the sub-panel (so a 100A sub-panel will need to be cabled with something that can cope with 100A - even if the OP only plans to stick an LED light and a USB charger in the barn.) Jul 1, 2019 at 16:13
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    @MartinBonner There have been many answers on DIY that boil down to: The breaker in the main panel that connects to the wire for the subpanel has to match the wire correctly, but the subpanel itself can have a main breaker as big as you want because it is only used as a shutoff switch, and therefore you can put in a really big subpanel (that won't take a small "main breaker") and there is no problem at all. This is actually a very good thing because often with GFCI & AFCI you need a lot of separate full-space circuits and the easiest way to do that is a big panel. Jul 1, 2019 at 16:21

1 Answer 1


I have a very strong sense of already having answered this question. If this is a dupe, I apologize.

For a 100A main breaker the smallest wire you can think of running is #1 aluminum.

If your breaker is 100A, your largest expected load ought to be 80A.

You do have a considerable distance there, and it's worth thinking about voltage drop. Wire salesmen will tell you to compute voltage drop based on breaker trip and 3% drop. You are much more correct to base it on actual expected loads and whatever drop will keep it less than 8% for all runs from main panel to final outlet. Since this will be the bulk of the run, we can give it 6% to play with, though less is nicer.

When we do a voltage drop calc on #1 aluminum, 80A (the largest expected load allowable on a 100A breaker), 240V and 170 feet, it computes out to 2.75%. *Well, then. We certainly don't need to bump wire size here**.

#1 aluminum is the right stuff.

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