350 ft voltage drop problem

Not open for further replies.

I have a 120 V / 30 A, LED sign that i need to run power to. It is located 350 feet from the panel.

My calculations tell me that I need to run 1 AWG to that sign, for proper voltage drop.

Vd = (2 x K x D x I) / CM

Just to me that seems outrageous! Myself and fellow electricians I'ved worked with say the same thing. We believe that it should be more like 6 or 4 AWG. But my calculation says differently?

Am I doing something wrong with my calculation?

Any help is greatly appreciated and if I left anything out just ask.


Staff member
If the load is actually 30 amps, 350' one direction and 120 volts it will take 1 AWG copper to stay under 3% drop.

1/0 AL would keep you at 3% and cost less.

Is the load 30 amps or is that just the circuit size?

By the way, don't forget the EGC will have to be the same size as the circuit conductors {250.122(B)}


Senior Member
London, England
Remember that whilst 3% voltage drop is a widely accepted figure, it is not actually a code requirement.
A larger voltage drop might be acceptable, depending on the specification of the sign.

Does it have to be 120 volt ? IME most LED signs use switched mode power supplies that also accept 240 volts, could be worth checking.

If the load is 120 volt only, it may well consist of a number of modules or sections. I doubt that a sign that big is a single unit.
In that case it should be possible to run a 120/240 volt MWBC and divide the load between both legs.

Smart $

Esteemed Member
Yes it is only 120 V. I had asked the sign makers that question.

The LED sign is 3' x 8' and has over 9,200 LED's in it.
OK, but the question remains how many power supply units?

For example, if the sign has 2 PSU's, you could power with a 2P 15A breaker and run a 3W MWBC. Effectively this would power the sign with 240 (though each PSU at 120V). Even at full 15A on circuit, you would only have to run 3/C #8 Cu. (1050') Compare with 30A and 2/C#1 Cu (700').

If source panel is 3?, let us know that along with number of PSU's...
Last edited:


Senior Member
Ona 30A CB the most amps you are going to have is 24A.

24A @ 350' with 3% drop using stranded uncoated copper is a #2
Let go to 5% drop and you can use a #4.

From a cost standpoint you might want to consider using aluminum as an alternative.

I wouldn't do anything until you have the actual cut sheets from the supplier.


Senior Member
Northern illinois
There are other options such as running a higher voltage out to the sign and installing a transformer there to get the required 120V.

If the load is consistent, buy a cheap buck/boost transformer and send 140V out there. it will be 115 by the time it gets there.

It might also be that the sign itself won't care if the voltage is low. read the specs on the sign. It may work fine as low as 90V.

I would crunch the numbers and show the end user what his various options are.


Senior Member
Considering the Cost of the large size of the cable Have you considered stepping up to 480v, possibly using a 1ph 240v L-L source and then 480 t0 120v? It would reduce the amps that you would need to size the cable for to 25%. I've suppied sets of transformers all of the time for applications similar to this as is done all of the time in long runs at airports for their avionics.
Not open for further replies.