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?
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.
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?
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).
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.
