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I have a shed which is a little less than 100 feet from my home breaker box. I currently am poor-manning it running a 100 foot extension cord out there. I have already purchased a branch electrical box and wired the shed. I am about to rent a trencher to trench for the electrical cable.

I would like to run 240v out there just in case I ever need it so I do not have to redo the job in the future. However, at this point I want 2x 20 amp circuits running out there for saws and things on one breaker & lights, tv, receiver etc on the other. I am going 20 amp vs 15 amp just to have some extra capability. If your opinion on this differs, I would like to know more.

So here is my main question. With a 3% voltage drop in mind, what wire do I need to run out there given the distance (100 ft) to obtain a max 20 amp 240v capability and/or 2x 20 amp 120v capability? Can I go with a smaller gauge electrical wire to achieve this or is 6/3 what I need to go with? I do realize that the most important part of the equation are the breakers installed on the home side. Those must be at or below whatever electrical cable I run out there. Would it be possible to run 1x 10AWG/3 UF-B electrical cable out there with 2x 20 amp breakers in the house?

The online electrical wire calculators are confusing me... I guess what I do not get on the 3 conductor cable is if the cable is capable of supporting 20 amp per hot wire or a total of 20 amp across all 3 conductors.

Is it reasonable to assume that a 100 foot 10AWG/3 Conductor wire is capable of carrying 2x 15 amp loads @ 120v or 1x 30 amp load @ 240v?

I do not plan on welding (due to wood shed/wood shed floor) or anything like that. I can also probably step down to two 120v 15 amp circuits as I can only operate a tool at once. i may want to install an a/c unit out there though at some point. probably 120v. the shed size is 12x20.

This install location is in east north Dallas. Lots of clay and rocks. If i can avoid digging an 18" ditch, I would definitely prefer it. I will not permit the install but do want to follow code and do so with the cheapest trenching method and electrical cable possible.

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    Are you OK with running conduit instead of a direct bury cable? – ThreePhaseEel Apr 30 '18 at 4:38
  • whatever is the cheaper way to go. especially on the digging side. does the plastic grey conduit work? i already installed a 90 degree bend before pouring the slab for the shed of 1 1/4" – ThaKidd KG5ORD Apr 30 '18 at 4:39
  • oh some additional info. this install location is in east north dallas. lots of clay and rocks. if i can avoid digging an 18" ditch, I would definitely prefer it. i will not permit the install but do want to follow code and do so with the cheapest trenching method and electrical cable possible. – ThaKidd KG5ORD Apr 30 '18 at 4:54
  • Please add new information to your post, not in a comment. – isherwood Apr 30 '18 at 14:55
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Sounds like cheap is a motivation, but there's a relationship between depth of dig and cost of materials.

  • At 24" of cover you can use plain cable such as UF.
  • At 18" of cover you can use cheapie PVC conduit or EMT.
  • At 6" of cover you can use Rigid metal conduit.

So as expensive as Rigid might be, it might be significantly cheaper than renting a trencher if you can hand-dig or hydro-dig with a hose to 6".

If you run conduit, you can use THWN-2 single wires instead of expensive, balky cable, and you can go cheap now and add more or larger wires later. If you use metal conduit, you don't need a ground wire.

Also if you are cheap, the first thing to shortcut is the 3% voltage drop thing - that was always an old wives tale, is usually computed wrong anyway, and is simply not needed. Also, 100' distances are within 3% even at statutory max current!

  • For 20A, use 12 AWG UF or THWN in 1/2" conduit.
  • For 30A, 10 AWG " " " 1/2" conduit.
  • For 40A, 8 AWG " " " 3/4" conduit
  • For 60A, 6 AWG copper ... ... 3/4" conduit or 4 AWG Aluminum in 1" conduit.
  • i am guessing the metal conduit needs to be connected to the branch electrical box. does it also need to attach to the main breaker box? that will be hard to do. – ThaKidd KG5ORD May 1 '18 at 0:03
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    Actually, you can transition between wiring methods in any junction box. Provided the junction box has enough cubic inches for the splices, and it is located where both wiring methods are allowed. For instance NM cable is not allowed Outdoors, so you would need either an indoor junction box or UF cable instead. You can do this on both ends if you like. – Harper May 1 '18 at 1:25
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    @ThaKiddKG5ORD Harper has given some great advice here. To add to it, I'd advise installing 3/4"-1" conduit in the trench. While you may not need/want it now, it allows you to pull larger wires later, if you ever decide you need more power. The extra few dollars you spend now, could save you a lot more in the future, if you ever decide to upgrade. – Tester101 May 1 '18 at 11:46

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