figuring voltage drop for a single outlet.

[TerryBarnhill]

I have a customer wanting to add a 400W MH flood fixture to shine in an area of a dark parking lot. I have a 120 panel I can use that will require a 400' run to the pole. The only thing I will put on this circuit would be this one light -- roughly 3.5 amps. Should the VD be figured on this actual amperage, giving me a 4.67% drop using 12AWG? With the breaker sized at 15A, should the VD be figured at the full potential of the breaker, giving me a 4.9% drop using 6AWG?

If this was feeding a single receptacle, I would use 15A as my base, but with it just feeding, (for now) a single light, I'm unsure what would be the proper amperage base. I doubt anything would be added to this light circuit, but you never know. On the other hand, I suppose it's really not my concern what might happen in the future as long as I do what is correct in my present install.

Just wanted some thoughts from others. Thanks!

 

[Dennis Alwon]

VD only needs to be figured for the actual load. I would use minimum #10 wire.

 

[infinity]

If the connected load is only one light I would use that as the basis for my VD calculation. Just eyeballing it I would pull #10 AWG conductors.

 

[gar]

101124-1413 EST

Terry:

When you encounter a question like this again just use some logic to analyze the problem.

Your question has nothing to do with the breaker size other than the breaker will define the smallest wire that can be used.

The real question is what is the lowest voltage your load can tolerate and provide the desired result, and the lowest supply voltage that may occur under normal conditions. This determines maximum tolerable voltage drop. Other considerations might define a lesser drop.

Suppose the light could operate at 100 V and provide adequate light and life, and the lowest supply voltage under normal conditions is 110 V. Then you can tolerate about a 9 % voltage drop. Suppose the load is 4 A, then the maximum loop resistance would be R = 10/4 = 2.5 ohms. At 800 ft of loop wire, call it 1000 ft, I could roughly use #14 which is slightly over 2.5 ohms at 1000 ft and 20 deg C.

I will not suggest what you should use, but #10 is probably overkill.

.