I am running a long 20 amp circuit out to a stock watering tank from the barn. Given the length of the run (280 ft), I will need to use 4AWG. Midway, I will place a junction box and have an outlet pigtailed from the junction box as well as an outlet at the end of circuit.

What size and type of junction box should I have? The wire will run in buried plastic conduit and the junction box will be mounted outdoors. Both conduits will enter the box from the bottom (one conduit to barn, one to the end of the circuit at the stock tank).

  • It looks like you did your voltage drop calculation correctly. The problem when you get to #4 wire is there is not a standard multiplier like with #6 wire. The box size depends on the size of your conduit at minimum you will need 1". You are going to do what is called an angle/ U pull Both conduits going out the bottom of the box. The minimum size is 6X the size of the conduit + the size of the 2nd conduit or 7" box minimum. This information can be found in NEC 314.28(2). – Ed Beal May 2 '16 at 19:45

Given the large size of the conductors, I would use aluminum for this, specifically the new AA-8000 series alloys. Yes, aluminum has a bad reputation as a wire. But that only applied to the faulty AA-1300 alloys installed during the postwar housing boom, and even then to the small-gauge stuff where it was used instead of 12-14 gauge copper. 1300 is now outlawed in any gauge. 8000 is fine, and about 1/3 the cost of copper. The wire-run chart says to use 2ga aluminum. Which will require larger conduit.

Second, it sounds like you also want 120V convenience outlets out there too. But if you didn't need that, I would ask your vendor if they offer a machine that is 240V instead of 120V, and wire a 240V 10A circuit. That would let you do this in 10ga copper. (In those smaller sizes, aluminum isn't much cheaper.)

Double the voltage, halve the current, quarter the losses.

  • Thanks for the suggestion Harper. I will check into using aluminium. I need 120 outlets (20A) for a water heater during winter in the stock tank so while I would like to use 10ga copper, it is not possible as far as I can tell. – Brad Smith May 4 '16 at 2:22

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