I have been trying to use the voltage drop calculators but it seems like they give a different answer than the last one. Distance from breaker box in the house to the sub panel in the detached garage is going to be approximately 130 feet, 100 amps, 220 volt. Trying to figure out if I should use 2/0 or 4/0 aluminum wire. Any advice would be appreciated.

1 Answer 1


Going only 130 feet, you do not need to up-size wire for voltage drop. The usual #1 Al will suffice for 100A breaker.

Any plan requires a 125% derating of the expected load. For instance, if you actually intended to pull 100A as a regular, frequent load, you would need 125A to be provisioned, i.e. A 125A breaker and wire. By extension, the 100A breaker is sized to support 80A of frequent load.

Now there are a couple ways those voltage drop calc's get you. #1 is they prod you for the breaker value, not the practical load - 100A not 80A. You should enter 80A. #2 is that they are rather obsessed with 3% as a "very hard maximum" possible voltage drop. That's not supported by Code; Code wants to see 8% among all voltage drops. Definitely use a number larger than 3%, like 5-6%, and see what the result is. Very often, you find the actual number works out to 3.34% or something, and it's just not worth paying a lot more over 0.34%.

By my math, 4/0 aluminum would support a 200A feed breaker. However, it will not fit on a 100A feed breaker.


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