# NEC 250.122(B) - Upsizing for voltage drop



## Carultch (May 14, 2013)

NEC 250.122(B) requires that when I increase my current-carrying wire size to accommodate long distance voltage drops, that I must also upsize the ground wire proportionally in metal cross sectional area.

All wires are copper with 90C insulation and 75C terminals.

Consider 400A of wire for a 400A breaker. There are three solutions:
1. Single set 600 kcmil, #3 ground
2. Parallel sets of #4/0 in common conduit, #3 ground
3. Parallel sets of #3/0 in separate conduits, #3 ground in each

Suppose I upsize this to parallel 400 kcmil for distance.
*What should my starting point be for proportionally upsizing the EGC?*

Start with 600 kcmil? Conclusion: ground size = #1
Start with 2*#4/0 = 424 kcmil? Conclusion: ground size = #1/0
Start with 2*#3/0 = 336 kcmil? Conclusion: ground size = #2/0

I've previously asked this question, and I didn't get the conclusion I was expecting. Now I'm in another situation where it matters.
http://www.electriciantalk.com/f5/q...ound-wires-voltage-drop-parallel-feeds-66876/


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## Black Dog (Oct 16, 2011)

Carultch said:


> NEC 250.122(B)


How many feet of wire?


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## Carultch (May 14, 2013)

Black Dog said:


> How many feet of wire?


Let's say it is 470ft.
http://www.southwire.com/support/voltage-drop-calculator.htm

Enter 160A (160 = half of 320), 240V, single phase, 3% voltage drop.

"A maximum distance of 477.302 feet will limit the voltage drop to 3% or less with a #400 Copper conductor delivering 160.0 amps on a 240 volt system."


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## Black Dog (Oct 16, 2011)

Carultch said:


> *What should my starting point be for proportionally upsizing the EGC?*
> /


In my opinion that would be at the beginning of the run.

If it's 470ft then you need to use a lager size feeder so the ground would increase From Point A to Point B.


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## Carultch (May 14, 2013)

Black Dog said:


> In my opinion that would be at the beginning of the run.
> 
> If it's 470ft then you need to use a lager size feeder so the ground would increase From Point A to Point B.


Is that a joke or a misunderstanding?


What I mean by "starting point", is the size of conductors before any increasing for voltage drop, that I use in order to multiply the size of the EGC.

In otherwords, complete this equation, by filling in X.
EGC KCMIL = 52.3 * (400 + 400) / X

Does X = 600?
Does X = 424?
Does X = 336?

52.3 kcmil = #3
424 kcmil = 2 * #4/0
336 kcmil = 2 * #3/0


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## Black Dog (Oct 16, 2011)

Carultch said:


> Is that a joke or a misunderstanding?


Not a joke, misunderstanding maybe.

Do you have more than 2 points in this pull?


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## Carultch (May 14, 2013)

Black Dog said:


> Not a joke, misunderstanding maybe.
> 
> Do you have more than 2 points in this pull?


Well, it got me to laugh.

Let's say it's one continuous pull.


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## Black Dog (Oct 16, 2011)

Carultch said:


> Well, it got me to laugh.
> 
> Let's say it's one continuous pull.


The ground wire must be sized according to the ungrounded conductors 250.122(B)

So that article should take you to 250.66 table.

Unless there are calculation examples elsewhere in the code, and article 250.122(B) should tell you where in the code to find it such as chapter 9 or the table in 250.66.

I could be wrong but that is where I would go..


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## Carultch (May 14, 2013)

Black Dog said:


> The ground wire must be sized according to the ungrounded conductors 250.122(B)
> 
> So that article should take you to 250.66 table.
> 
> ...


I couldn't find any example on this in Chapter 9.

Did you understand what I meant, when I said there are 3 possible values of X? And why there are three ways of interpreting what X should be?

EGC KCMIL = 52.3 * (400 + 400) / X

Does X = 600?
Does X = 424?
Does X = 336?

This is one of those borderline situations, where you come to a jump discontinuity between code rules by changing the number of parallel sets to solve the problem.


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## don_resqcapt19 (Jul 18, 2010)

For the increase in size of the EGC, it would be based on the smallest permitted size for the ampacity of the circuit. If you are using a single conductor, it would be based on the 600 kcmil, assuming that the code would not permit the use of 500 kcmil for this application. 

For the parallel runs, the 3/0s are the smallest permitted by the code and any increase in size from the 3/0s requires an increase in the size of the EGC.


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## Carultch (May 14, 2013)

don_resqcapt19 said:


> For the increase in size of the EGC, it would be based on the smallest permitted size for the ampacity of the circuit. If you are using a single conductor, it would be based on the 600 kcmil, assuming that the code would not permit the use of 500 kcmil for this application.
> 
> For the parallel runs, the 3/0s are the smallest permitted by the code and any increase in size from the 3/0s requires an increase in the size of the EGC.


So for this example, should it be interpreted as an increase in size from 600, or an increase in size from parallel 3/0s?


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## Pete m. (Nov 19, 2011)

Carultch said:


> So for this example, should it be interpreted as an increase in size from 600, or an increase in size from parallel 3/0s?


The calculation is based on the conductors that are actually used.

Pete


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## Carultch (May 14, 2013)

Pete m. said:


> The calculation is based on the conductors that are actually used.
> 
> Pete


Well, which one should it be interpreted as?

An increase from 2 x 3/0 to 2 x 400?
Or an increase from 1 x 600 to 2 x 400?


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## Pete m. (Nov 19, 2011)

Carultch said:


> Well, which one should it be interpreted as?
> 
> An increase from 2 x 3/0 to 2 x 400?
> Or an increase from 1 x 600 to 2 x 400?


I would say that either one would be correct as far as the NEC is concerned. If it were my money I would use which ever one netted the smallest permitted EGC.

Pete


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## don_resqcapt19 (Jul 18, 2010)

Carultch said:


> So for this example, should it be interpreted as an increase in size from 600, or an increase in size from parallel 3/0s?


That would be my opinion.


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## don_resqcapt19 (Jul 18, 2010)

Carultch said:


> Well, which one should it be interpreted as?
> 
> An increase from 2 x 3/0 to 2 x 400?
> Or an increase from 1 x 600 to 2 x 400?


In my opinion, the increase is from the smallest permitted conductor for the number of conductors that you are using. 

If you are using a single run, it would be based on the increase from 600kcmil.
If you have two runs in parallel it would be based on the increase from two sets of 3/0s.


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