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Understanding Voltage Drop to a Sub Panel

16657 Views 6 Replies 5 Participants Last post by  Stevecook3dw
Hello everyone, I'm a complete newbie when it comes to electrical, but I've been reading as much as I can and trying to build my knowledge. So please bear with me.

I'm working on running power out to my shed. It is a 60 amp subpanel, that's approximately 210 feet from the main panel. Now I know at this distance I need to be worried about voltage drop and that I should try to stay at less than 3%. The shed will be used as a small workshop, with small power tools(no 220 runs). Shed size, 16x24.

My questions:

1. Since the subpanel is 240, and all of my runs off the panel are 120, should I only be worried about the voltage drop on the downstream side of the panel(to outlets, lights, etc)? Or should I be worried about both?

2. Does code require that you remain at less than 3%, or is that a recommendation?

3. I had planned to run 4-4-4-4 aluminum urd, and 4-4-4-6 SER(inside), but it looks like this might not be acceptable according to some online calculators? But even with the online calculators, on various sites I get different results. If I stayed at less than 5%drop, would that be ok? Or do I need to go up to 2-2-2-4?

Thanks for your time!
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I think you are confused about your subpanel. It is possible to run a 240 volt only subpanel from a main panel in the United States. That would require two hot leads plus a ground. If you wired a subpanel as 240 volt only, you would not be able to get 120 volt circuits from such a panel.

You are wiring a 120/240 volts panel, which requires two hots, a neutral, plus a ground. The voltage drop is a function of the number of amps you draw, the type of wire (copper or aluminum), the size of wire, and the total distance (to and from the subpanel). In your case, the total distance is 420 feet, however the calculator I used requires one way distance (it doubles the distance internally). If you draw the maximum amperage (60 amps in your case), and you use Al #4 wire, your voltage drop is going to be about 10 volts, which is a little over 4 percent, which is probably fine. To verify the drop, check out this website

You should get the same result no matter what voltage calculator you use, since all of the calculators use the same formula, namely voltage drop =
I * R, where I is the current and R is the resistance of the wire. What can be confusing is that the voltage loss requires you to use the TOTAL round trip distance, since you lose voltage in both directions. Some calculators do not make it clear whether you should use the one way distance (the calculator doubles the distance internally) or the round trip distance.

If you want to verify, use the actual resistance per foot for aluminum wire. This website gives you good information on wire resistance per foot for a variety of materials Note that for #4 Al wire, 60 Hertz, 420 feet, total resistance is 0.171 ohms, so the loss is 60 * .171 = 10.3 volts, which is the same result as the on line voltage drop calculator.

Your percentage drop is calculated based on the nominal voltage of the run, in your case 240 volts. The actual voltage drop is the same no matter what the voltage, but of course the higher the nominal voltage the lower the percentage drop.
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Thanks Dan! Lots of good information. I appreciate you taking the time to answer my question.
2. Does code require that you remain at less than 3%, or is that a recommendation?
The Code does not mandate that you maintain a certain voltage drop - it's only a recommendation (i.e. not enforceable). It's from 210.19(A):

Informational Note No. 4: Conductors for branch circuits as
defined in Article 100, sized to prevent a voltage drop exceeding
3 percent at the farthest outlet of power, heating, and lighting
loads, or combinations of such loads, and where the maximum
total voltage drop on both feeders and branch circuits to the farthest
outlet does not exceed 5 percent, provide reasonable efficiency of
operation. See Informational Note No. 2 of 215.2(A)(1) for voltage
drop on feeder conductors.
So in a nutshell, it's good practice to not exceed 3% for branch circuits (i.e. from panel to last outlet), or 5% total (including branch circuits and all feeders back to the service disconnect).
You are wiring a 120/240 volts panel, which requires two hots, a neutral, plus a ground.
This post was very helpful. Curious though, could you just have a grounding rod at the destination and not have to run the ground wire you mention?
Stevecook3dw your ABSOLUTELY incorrect about that. 2 grounds creates a ground loop which is one of the reasons cattle and horses are found dead after lightning, Also called the step potential. Read the Soares Book on Grounding. Explains the reasoning in simple terms and with lots of pictures.
thanks. That could be GROUNDS for a legal action! :~]
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