Wiring question

[Shawn Siegrist]

I have a 60 amp subpanel in my barn, I want to run a 20 amp circuit from the subpanel to a pole barn for lighting. The total length of the run will be 160', the last 81' of the run needs to be above ground. What gauge wire should I use? What kind of wire do I need to use for the above ground run?

Thank you for all your help,
Shawn

[Craig Moulton]

http://www.electrician2.com/vd_calculator.htm is useful for calculating voltage drop. Less than 3% VD is your target.

You get about the same voltage drop across a wire for the same current, even at different voltages. 2.4V drop on a 240V wire is 1% voltage drop, but 2.4V drop on a 120V wire is 2% voltage drop. That's the reason that we use higher voltage lines to carry power over long distances, with transformers at the end. With this in mind:

at 240V you'll need 10 AWG

at 120V you'll need 6 AWG

Both of the above numbers are to stay at or below 3% voltage drop.

So if you run a 240V line to the pole barn with a small subpanel there to split it to 120V, that's probably going to be cheaper than the 6 AWG cable, but price it and see.

[Chris Friesen]

at 240V you'll need 10 AWG

at 120V you'll need 6 AWG

That makes no sense. He's not going to have a step-down transformer, so by pulling 240V really he just gets two 120V circuits instead of one...but he'll still need the #10 wire.

For the underground run you need waterproof wiring. For the above ground I'm not sure if the same wiring is sufficiently UV-resistant...might be worth further investigation.

[George Bregar]

That makes no sense. He's not going to have a step-down transformer, so by pulling 240V really he just gets two 120V circuits instead of one...but he'll still need the #10 wire.

For the underground run you need waterproof wiring. For the above ground I'm not sure if the same wiring is sufficiently UV-resistant...might be worth further investigation.

No he is correct. The issue is whether running #10 will be cheaper than #6 with a subpanel. Small subpanels are so cheap my guess is yes.

[Anthony Scira]

For the run you can use the smaller gauge wire if you buy a 208v light for your pole.

If you run a standard 120v light you will need the larger wire.

Are you coming out of the ground and running exposed or putting some SCH 40 PVC up the pole to protect the wire ? If that be the case THWN should be fine.

For protection I would use rigid factory 90's and 6' stubs comming out of the ground just in case something hits it. The rigid factory 90's keep your pull line from physically cutting your PVC underground pipe and makes for a easier pull.

Sounds like you have a fun project over there !

[Dan Friedrichs]

There are really three choices here, and I think people are confusing them:

1) Run 120V to the pole shed, which means 3 physical wires (hot, neutral, ground*), 6 AWG. Use 120V light fixtures.

2) Run 240V to the pole shed (assuming the OP has 240V at his 60A barn subpanel), which means 4 physical wires (hot1, hot2, neutral, ground*), 10 AWG. Use 120V light fixtures.

3) Run 240V to the pole shed and use 240V (or 208V) light fixtures. This could be done with 3 physical wires (hot1, hot2, and ground*), each 10AWG. That's ASSUMING the OP is CERTAIN he would never want a 120V outlet or something in the pole shed, otherwise, he should add a 4th 10AWG neutral (back to option #2).

The question is, which choice is more costly: 3 @ 6AWG, or 4 @ 10 AWG, or 3 @ 10 AWG. At this length, small differences in wire cost could add up quickly.

(*IIRC, detached buildings' ground must be common with the supply ground? Although everyone would just want to drive a ground rod at the pole shed, that's generally not allowed, correct?)

[Rod Sheridan]

There are really three choices here, and I think people are confusing them:

1) Run 120V to the pole shed, which means 3 physical wires (hot, neutral, ground*), 6 AWG. Use 120V light fixtures.

2) Run 240V to the pole shed (assuming the OP has 240V at his 60A barn subpanel), which means 4 physical wires (hot1, hot2, neutral, ground*), 10 AWG for the hots, and 6AWG for the neutral, and use 120V light fixtures (since no current is hot1-->hot2, the neutral needs to be sized for the full current).

3) Run 240V to the pole shed and use 240V (or 208V) light fixtures. This could be done with 3 physical wires (hot1, hot2, and ground*), each 10AWG. That's ASSUMING the OP is CERTAIN he would never want a 120V outlet or something in the pole shed, otherwise, he should add a 4th 10AWG neutral.

The question is, which choice is more costly: 3 @ 6AWG, or 3 @ 10 AWG plus 1 @ 6 AWG, or 3 or 4 @ 10 AWG. If this were a short run, the obvious choice would be choice #1 (for simplicity), but at this length, small differences in wire cost could add up quickly.

(*IIRC, detached buildings' ground must be common with the supply ground? Although everyone would just want to drive a ground rod at the pole shed, that's generally not allowed, correct?)

Dan, your #2 post is incorrect. If he has the same current in both hot leads, his neutral current would be zero.

The neutral only carries the unbalanced current, so worst case (all loads on one line) the neutral would only have to be the same size as the line (#10 in your example).

Regards, Rod.

[Dan Friedrichs]

Dan, your #2 post is incorrect. If he has the same current in both hot leads, his neutral current would be zero.

The neutral only carries the unbalanced current, so worst case (all loads on one line) the neutral would only have to be the same size as the line (#10 in your example).

Regards, Rod.

Ah, you're right, Rod. I have no idea what I was thinking when I wrote that...

[Tom Veatch]

Dan, your #2 post is incorrect. If he has the same current in both hot leads, his neutral current would be zero.

The neutral only carries the unbalanced current, so worst case (all loads on one line) the neutral would only have to be the same size as the line (#10 in your example).

Regards, Rod.

If he runs 240 to the subpanel, would he need to run a neutral? I know 120 needs a grounded conductor, but could a couple of ground rods at the pole barn location handle that?

[Rollie Meyers]

There are really three choices here, and I think people are confusing them:

1) Run 120V to the pole shed, which means 3 physical wires (hot, neutral, ground*), 6 AWG. Use 120V light fixtures.

2) Run 240V to the pole shed (assuming the OP has 240V at his 60A barn subpanel), which means 4 physical wires (hot1, hot2, neutral, ground*), 10 AWG for the hots, and 6AWG for the neutral, and use 120V light fixtures (since no current is hot1-->hot2, the neutral needs to be sized for the full current).

3) Run 240V to the pole shed and use 240V (or 208V) light fixtures. This could be done with 3 physical wires (hot1, hot2, and ground*), each 10AWG. That's ASSUMING the OP is CERTAIN he would never want a 120V outlet or something in the pole shed, otherwise, he should add a 4th 10AWG neutral.

The question is, which choice is more costly: 3 @ 6AWG, or 3 @ 10 AWG plus 1 @ 6 AWG, or 3 or 4 @ 10 AWG. If this were a short run, the obvious choice would be choice #1 (for simplicity), but at this length, small differences in wire cost could add up quickly.

(*IIRC, detached buildings' ground must be common with the supply ground? Although everyone would just want to drive a ground rod at the pole shed, that's generally not allowed, correct?)

240V only is a bad idea, if one is going to spend the money it needs to be done right, otherwise a tap light is in order (battery powered light ). If cost is a major issue then the use of aluminum conductors should be considered. Lastly, a 208 V lighting fixture is not suitible for a 240V circuit, besides being a NEC violation, short lamp & ballast life will be the result.

Other then a single circuit to a outbuilding, a main disconnect & and 2 ground rods are required.

[Scott T Smith]

Since you only want to operate lighting, another option is to use the smaller wiring (10 ga) and install a passive buck boost transformer on the supply side to boost up the voltage on the receiving end.

[Anthony Scira]

I may have it wrong but when I see 120/240 installs its usually in commercial applications. To this day I have not seen 240 in a residential installation.

But I am a commercial electrician and have done little residential aside from the service changes. And those were always 120/208.

So my assumption is the OP has a 120/208 system and was making a common mistake of thinking 2 x 120 makes 240. Well I guess it does but not in a Wye connected transformer.

And you can not add ground rods and make your own neutral in another panel. Neutrals are tied together in the main panel and any sub panel has the neutral buss isolated from the ground buss.

[Shawn Siegrist]

I was planning on running a 30A/220V from the 60amp subpanel in the barn to a 30amp subpanel in the pole barn, using 6 gauge 4 conductor wire.

Off of the 60amp subpanel in the barn I will have two 15A/120V circuits each having three 100 watt incadescent bulbs on them, two 20A/120V circuits each having 3 receptacles on them, and one 30A/220V circuit for the subpanel in the pole barn.

Off of the 30amp subpanel in the pole barn I will have one 15A/120V circuit for 2 100watt incadescent bulbs and one 20A/120V circuit for one GFCI receptacle.

We have to run the wire above ground, running below ground is impossible do to a stream running between the two buildings.

Is all of this ok?

[Rollie Meyers]

What kind of "4-conductor" wire? What your planning on using can make a real differance.

[Shawn Siegrist]

aluminum, would copper be better?