Previous owner of house ran 10/2 to his outbuildings which are 125 feet. He has lights and would run power tools like a mitre or smaller air compressor. At the breaker is a 20 amp for each run.

Is this ok? I won't be running anything other than what he had and I don't plan on doing anything more intense like awelder or IR heating. Just smaller power tools and lighting.

Or should I re run with larger guage?

  • 1
    Your voltage drop may be a bit on the high side if you pull close to 20amp at 120v but the voltage drop is not a code enforced value just a suggestion 3% for the feeder and 5% at the end of the run. If within 10% at full load you will be fine for most devices. Make sure to allow your saw or motor driven device to spin up prior to starting the cut, when the motor is spinning up it will draw 3-5x Full Load Amperage (FLA) so giving it a few extra seconds will reduce the heating on the motor windings.
    – Ed Beal
    Aug 13, 2019 at 21:28

3 Answers 3


Voltage drop is caused by the current you are actually pulling, not breaker trip rating. Many people compute voltage drop based on trip, but that's silly - if you're actually pulling breaker trip current, you've got other problems. And many people consider 3% a hard limit (a bit silly since the only number Code speaks of is 8%, but I think they are worried about installations where there might be three such drops consecutively).

Anyway, if one were being silly like that, you are just within limits on that cable.

Otherwise, you have loads of headroom and shouldn't give it any further thoght.

You cannot up-breaker to 30A because you have 15A or 20A outlets on that cable, and those need 20A breaker protection. However, if you fit a subpanel in this location with 15-20A breakers for the outlets, you can then re-breaker the supply to 30A. Assuming 24A draw (the sensible max), you would have 0-5.25% voltage drop, which is concerning, but not serious.

If you are willing to fit a 10KVA transformer at the outbuilding, the circuit can deliver 7200W of 120/240V split-phase at 0-3.25% drop. That is 60A@120V or 30A@240V. Nobody's gonna worry about 3.25%.

If you fit two commonly available 15KVA transformers, you could deliver 14,400 watts, or double the above, at 0-1.64% drop With two less-commonly-available 20KVA transformers, you could deliver 18,000 watts at 0-1.31% drop. This is most of a standard 100A (24KW) house service.

  • 1
    I ran a skill saw with lights and nothing tripped. I'll consider the additional serivce. Possibly might need to put a chest freezer out there.
    – Robb
    Aug 13, 2019 at 21:57
  • 1
    I'm not surprised, assuming the light isn't one of those brutal 500W halogens. LEDs are your friend. The freezer will have a big startup surge, but don't be surprised if it's less than 1 amp while operating. Aug 13, 2019 at 22:01
  • Nah its all led and there's a couple 8 foot flourescents but I'll replace those. Everything seems fine so I figure as long as I don't run too much I'm good. I'm just a weekend warrior anyways.
    – Robb
    Aug 13, 2019 at 22:40
  • 8 foot fluorescents can be beasts if they are VHO - I've measured one VHO at 440 watts. I have been converting my VHOs to HO electronic ballasts but you could also convert ballast and tube to T8 true-fluorescent. They also make 96" LED "tube replacements", just make sure to get direct-wire aka ballast-bypass, so the ballast goes away. Aug 13, 2019 at 23:49
  • I rewired the lighting so I can switch the flourescents off and just run the led track so that should help. The flourescenuts mostly handle when I need to search our junk. I'm sure it's drawing a lot. It's pretty old looking.
    – Robb
    Aug 14, 2019 at 0:56

The 10-2 with a 20AMP breaker is ok. The 10-2 could go to a 30 amp breaker but your standard outlets would limit that to the 20 AMP breaker. Just curious, are the runs in conduit or direct buried cable?

  • Buried until the outbuilding then there is some conduit that comes up the surface. Previous owner claims it is two feet and it is the type of wire you can bury.
    – Robb
    Aug 13, 2019 at 21:21
  • Well 2 feet is the rule for direct burial, so at least we know he knows the rule lol Aug 13, 2019 at 22:03

It depends. it sounds like each building gets its own 120v 20A circuit. As long as there's a ground wire run back to the panel, this sounds OK. 10 gauge is more than sufficient to combat the voltage drop.

The catch here is what kind of cable are we talking? If this is UF cable, you're good. If it's NM cable, not so much (even if it's in conduit).

Is it enough?

20A (especially with power tools) is just barely enough to run one 15A tool and lights. If you're looking to improve it, I would at least go to a 40A 240v subpanel and run 8 gauge THHN (3 runs + a ground, all in conduit) and then you have ample power for anything you wish to do out there.

  • I think it is UF. He said it was rated outdoor without conduit if it's buried 2 ft. So I'm guessing if I have an air compressor and a saw I might blow the circuit if the compressor kicks on when I'm sawing?
    – Robb
    Aug 13, 2019 at 23:28
  • 1
    Maybe. My dinky compressor is only 2A, so you're only at 17A there. Add in another 1A for lights and you're still below budget. Still, if you're close it might be worth adding more. 125ft is a long way to walk to reset a breaker
    – Machavity
    Aug 13, 2019 at 23:47

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