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.
Electrical – Voltage drop for aluminum wire to detached garage
electrical
Related Solutions
It can only be 3 if this is the service drop - if it's a sub-panel it needs to be 4-wire.
3/0 (000) should do it. There are MANY voltage drop calculators on the web, as well as tables/charts. I happen to like the one linked, but they are mostly similar and should all give similar results. Shoot for 3% or less drop at rated (or calculated) load. I added 10 feet to account for getting the wire in and out of the ground, rather than just the horizontal distance - wire length, not "as a bird flies" is the number that matters for length.
I prefer to allow for the full load (100 amps) rather than attempting to finesse 30A 240V dryer + 120 volt loads unknown; I've lived with an undersized feed and I'm not fond of self-induced brown-outs.
To identify the wire gauge, the best way might be to take the cover off of the breaker box (advise to do this with power off for safety, but power can be left on - proceed at your own risk) and see if there are any indications on the wire sheathing. If there isn't, use a guide like to identify the gauge of wire.
As for copper vs. aluminum wire, aluminum might be cheaper in larger sizes. However, aluminum requires some special anti-oxident joint compound when you make connections with aluminum wire (see Ideal no-alox for an example of this stuff). If you don't use the compound, its possible that the connections will fail over time.
EDIT: Using the info you provided in the comments, and the wire voltage drop calculator at http://www.southwire.com/support/voltage-drop-calculator.htm, assuming:
- Single Phase 240V
- Direct Burial
- no more than 3% voltage drop
- 2 AWG aluminum wire
Your wire gauge will allow you to maximum run somewhere in the 75-80 amp range. I wouldn't go all the way to 100 amps, but 60 amp service should work. You'll need 1 AWG wire to run 100 amps safely that distance.
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Best Answer
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.