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Author Topic: Voltage drop question  (Read 4406 times)

Jacob Shaw

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Voltage drop question
« on: August 07, 2015, 06:19:01 pm »

I am having a little trouble with these online calculators.  Not sure if the aperage is both conductors combined or each conductor exc, so here is the senario.

Single phase run from main panel to another service panel.
Conductor will be 2-2-2-4 aluminum direct burial rated cable hung overhead
Run length is 420 feet.
Draw is 30a per leg (2 legs of course)
Single phase means that the original voltage is 120 per leg or 240 across both legs (duh but just in case someone asked)
What would the voltage drop be? Surely not 12.4 volts right?
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Stephen Swaffer

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Re: Voltage drop question
« Reply #1 on: August 07, 2015, 07:23:00 pm »

I get 8.03 volts dropped at 420 feet.   

#2 Al has a resistance of .319/1000 feet.

.319 X .840 (total distance travelled)=.26796 ohms

.26796 ohms X 30 amps=8.03 volts

How will this wire be supported if run overhead?
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Steve Swaffer

Jacob Shaw

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Re: Voltage drop question
« Reply #2 on: August 07, 2015, 08:35:05 pm »

They currently have a #4 service wire that is on prussics that are attached from trees, but I was thinking of replacing the prussics with kellums grips.
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Jacob Shaw

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Re: Voltage drop question
« Reply #3 on: August 07, 2015, 08:38:37 pm »

So you would have 111.7v at the other end?
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TJ (Tom) Cornish

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Re: Voltage drop question
« Reply #4 on: August 07, 2015, 10:06:33 pm »

So you would have 111.7v at the other end?
How much are you starting with?  Have you measured it?  Is your 30A per leg load continuous or is it an average draw with spikes higher than 30A like it would be with an audio load? 111v is pretty marginal.
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Stephen Swaffer

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Re: Voltage drop question
« Reply #5 on: August 07, 2015, 10:42:00 pm »

I agree that voltage is marginal-and the problem will be worse if you are trying to start any motors as ohms law always applies so at peak draw your voltage will be even lower.

URD (Underground direct burial) is not designed for overhead.  Overhead has a messenger (bare) wire that has a strand of steel in it for the strength needed.  The wire needs the mechanical strength to not only carry its weight, but also to withstand the stress caused by whipping around in 50+ MPH winds.  Of course, its your project so you can use what you want.  (One strand of steel doesn't seem like much -but I have seen it bend 2" rigid conduit quite handily)
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Steve Swaffer

Jacob Shaw

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Re: Voltage drop question
« Reply #6 on: August 08, 2015, 11:22:13 am »

The stuff is mobile home wire, and it is designed to go from A service pole to a home, but it is also approved for direct burial.  There may be some steel in it, but it is all insulated.  Service entrance wire is what is there now, and it has held up for years.  I would love for the voltage to be higher, but I simply cannot afford to donate a 4/0 wire.
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TJ (Tom) Cornish

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Re: Voltage drop question
« Reply #7 on: August 08, 2015, 07:09:31 pm »

The stuff is mobile home wire, and it is designed to go from A service pole to a home, but it is also approved for direct burial.  There may be some steel in it, but it is all insulated.  Service entrance wire is what is there now, and it has held up for years.  I would love for the voltage to be higher, but I simply cannot afford to donate a 4/0 wire.
Doing it poorly may be worse than nothing. How about one size up?
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Jacob Shaw

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Re: Voltage drop question
« Reply #8 on: August 09, 2015, 07:30:57 pm »

What do you mean by poorly?  I don't understand how replacing what is there with the same type of wire but bigger could possibly be worse than doing nothing.  Could you elaborate?  They don't seem to make device entry or triplex wire in #1, so I would have to go up to 1/0.
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Stephen Swaffer

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Re: Voltage drop question
« Reply #9 on: August 09, 2015, 11:50:59 pm »

 I can buy 2-4 ASCR (0verhead steel reinforced) for less than 2-2-2-4 URD direct burial.

1/0 4 wire overhead would be 62 cents a foot more than the 2-2-2-4 URD. 

I suspect that they do make #1-but likely a special order that would not make sense.  There can be a big difference in suppliers costs and pricing.

While "doing better" is a good thing and something I might find acceptable, it won't be a defense if something bad happens.  That said, the State inspectors in my area don't/can't enforce voltage drop recommendations in the code. They view it as a quality issue not a safety issue.  OTOH, using URD overhead could lead to a broken wire leaving a live wire whipping in the wind or laying on the ground.

Also, consider the value of your time (even if it is donated.)  It might make the cost of doing it one time correctly seem more reasonable.  Of course, if the load is just incandescent lighting or electric heat-or a number of other loads, voltage drop is not as critical.

Just some things to consider.   
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Steve Swaffer

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Re: Voltage drop question
« Reply #9 on: August 09, 2015, 11:50:59 pm »


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