Acceptable Wiring Size For Service To Detached Garage

Hello everyone. I recently had a new concrete driveway poured and during site prep the contractor broke and repaired (spliced) a #2 AWG wire that was used to service my detached garage. After a heavy rain the splice failed and as a remedy the contractor wants to run wire around the back side of my house through the front yard and then to the detached garage for a total distance of 280 feet. He wants to run aluminum wire. I have a 100 Amp service in the detached garage. I asked him about the additional distance that would be required to service the detached garage and he said that number 2 AWG was good for up to 500 feet. Does this sound correct? Thank you in advance for your replies. Andy

 

First question is why do you have a 100 amps to the garage? You could run a whole house on that. A 100 amp subpanel does not mean you need a 100 amp feed. Do you have a large welder, compressor, and electric heat, maybe an electric car charging station in the garage?

 

Voltage drop is dependent on the load as well as the distance and would factor heavily in determining the correct wire size.

 

It also depends on which code version your local area is following. The latest codes do not allow 100A on #2 aluminum for any distance. In prior versions, the distance as well as the load is important with a recommendation of not exceeding 3% voltage drop. For a full 100A load, that means you would need aluminum wire no smaller than #4/0 for a 280' distance. Whether or nor you need to support the full 100A is based on what machines you use in the garage and what you can negotiate with the contractor. In either case I think the 500' distance is absurd, and certainly is not based on the NEC.

 

Thanks for the replies all. I plan on brewing beer in the garage. I will never use all of the 100 amps but I also have a small apartment above the garage. The line is buried now so the point is probably mute at this time. I guess to know for sure I should measure the voltage drop at the garage breaker box????

 

What was actually installed? Number 2 aluminum is not going to carry very much current 560 feet. When the apartment is occupied you might not be able to brew any beer in the garage below.

You would need to do a load analysis (the NEC gives the rules) for the garage and for the apartment to figure out the amperes (or watts) needed out there. Then use wire resistance charts (available using Google) to come up with the minimum conductor size for the feed. Included in a load analysis would be the machines you plan to use in the garage.

 

That statement alone tells me you need an electrical contractor to do this work and not a concrete contractor.