Have an existing underground circuit that goes from the house to the end of the back yard. It's about 40 years old, and I have no idea what shape the wire is in, but it still works. I'm thinking about build a shed at the end of the yard to use as a woodshop. I don't want to actually wire the building up, but I was thinking of running a new line underground. What would I need to run a 20 amp line around 100 feet underground.

  • 100ft underground? 30M? That is quiet the hole you are digging. You sure you don't mean 100cm? 100cm is doable (barely) with a trench digging shovel (in the right soil, if you are lucky). 100mm seems reasonable. 30m is some serious earth moving Commented Sep 15, 2016 at 6:04
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    @Oxinabox, I think it's safe to assume he means 100' linear feet, not 100' deep. Commented Sep 15, 2016 at 11:44
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    You say a "20A line", but what is the load? Commented Sep 15, 2016 at 11:44
  • @SpeedyPetey ooooooooooooooooooohh that makes more sense Commented Sep 15, 2016 at 12:17
  • Yes I mean underground, for 100 feet, not 100 feet underground .... and 110 volts not 220
    – user379468
    Commented Sep 15, 2016 at 18:57

1 Answer 1


Wire size

You can use a calculator like http://wiresizecalculator.net/ to figure this out.

Keep in mind that the distance is not just the distance underground, but the entire wire distance from the panel to the receptacle/switch/fixture.

For 140', you'll need #6 copper or #4 aluminum.

Burial depth

For a 120V, 20A circuit, protected by GFCI, in residential use, using direct-burial rated wire (UF), the minimum depth requirement is 12".

See also:

  • Do you know if I can run that wire inside conduit? would rather make it as weatherproof as possible if I'm going to the trouble of digging the trench
    – user379468
    Commented Sep 15, 2016 at 18:58
  • @user379468 Yes, so long as it's "wet-rated" cable (eg: THWN or UF). Regular NM (or THHN) is not allowed underground, even in waterproof conduit.
    – gregmac
    Commented Sep 19, 2016 at 15:02

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