paralleling conductors for voltage drop

[Designer69]

please see attached pic. I need to parallel 2 conductors of a multi-conductor cable to keep voltage drop within acceptable range.

my question is... I know I need to parallel the line conductors for this to work but do I also need to parallel the neutral conductors as well?

thank you

 

[luckylerado]

View attachment 14602 please see attached pic. I need to parallel 2 conductors of a multi-conductor cable to keep voltage drop within acceptable range.

my question is... I know I need to parallel the line conductors for this to work but do I also need to parallel the neutral conductors as well?

thank you

If it is smaller than 1/0AWG the NEC will prohibit paralleling at all.?

 

[jumper]

In that application, a series circuit, the neutral would carry the same amount of current as the line conductors, so yeah the neutral would need to be paralleled/sized the same as the line conductors.

 

[Smart $]

In that application, a series circuit, the neutral would carry the same amount of current as the line conductors, so yeah the neutral would need to be paralleled/sized the same as the line conductors.

He's only paralleling for voltage drop, not ampacity as I understand it.

I don't see any problem with paralleling two ungrounded conductors (1/0 or larger) and not the neutral.

 

[jumper]

He's only paralleling for voltage drop, not ampacity as I understand it.

I don't see any problem with paralleling two ungrounded conductors (1/0 or larger) and not the neutral.

Must resist urge to ask to explain.

Must resist urge to ask to explain.

Must resist urge to ask to explain.


Urge not resisted.

Explain please.

 

[iwire]

You can decrease voltage drop by just increasing the size or quantity of conductors used for one side of a two wire circuit.

As long as the smaller conductors are rated for the application the NEC does not prohibit it is.


Plus there is the fun of providing a What the heck? moment for the next guy that goes to service it.

 

[Smart $]

Must resist urge to ask to explain.

Must resist urge to ask to explain.

Must resist urge to ask to explain.


Urge not resisted.

Explain please.

What's to explain? Assume one ungrounded and one neutral complies with ampacity requirements. Parallel one side of the circuit increases ampacity on that side, but does not reduce ampacity on the other.

 

[jumper]

You can decrease voltage drop by just increasing the size or quantity of conductors used for one side of a two wire circuit.

As long as the smaller conductors are rated for the application the NEC does not prohibit it is.


Plus there is the fun of providing a What the heck? moment for the next guy that goes to service it.

What's to explain? Assume one ungrounded and one neutral complies with ampacity requirements. Parallel one side of the circuit increases ampacity on that side, but does not reduce ampacity on the other.

Yeah, but.......makes my head hurt. I understand it, just have never saw or considered it.

It would definitely be a what the heck moment for me.

 

[Designer69]

He's only paralleling for voltage drop, not ampacity as I understand it.

I don't see any problem with paralleling two ungrounded conductors (1/0 or larger) and not the neutral.

That is correct. Ampacity is no issue here at all. Just voltage drop. Conductors are #10 though not 1/0 or larger.

I got thrown off because I know in voltage drop calculations, total circuit length resistance is calculated.

I guess if I don't parallel the neutral, I cannot take credit for the decreased resistance for that portion of the circuit?

 

[Carultch]

View attachment 14602 please see attached pic. I need to parallel 2 conductors of a multi-conductor cable to keep voltage drop within acceptable range.

my question is... I know I need to parallel the line conductors for this to work but do I also need to parallel the neutral conductors as well?

thank you

In an application such as this, the neutral carries the same current that the line conductors carry. This is in contrast to other common situations where the line/phase conductors take full round trip path for the lion's share of the current on their own, and the neutral is mainly there for carrying the return imbalance. Or a situation where the neutral is just there for voltage measurement purposes and carries negligible current in the first place.


In this situation, it is mandatory that both the line set of conductors and the neutral set of conductors have sufficient ampacity for the load (of course). So as long as both "polarities" have sufficient ampacity, it is technically permissible for the line "polarity" to have significantly more conductor material than the neutral.


However, anyone looking at this will be completely confused about you felt it necessary to curtail voltage drop on only half of the circuit, and not on the other half. It is an inconsistency that the inspector may question of you, and an inconsistency that will leave any future site personnel stuck guessing at why it would be built that way. It is much easier to defend a construction that uses a consistent design for all conductors that carry the full current load of the circuit. If 2 sets were necessary to curtail voltage drop on the line conductor, then when the same current flows the same distance through the neutral, I would expect to also see the same 2 sets of wire for the return path.

 

[jumper]

Conductors are #10 though not 1/0 or larger.

Uh, Houston we may have a problem.

Unless you meet the exceptions in 310.10(H) parallel conductors need to be 1/0 or larger.

 

[charlie b]

Conductors are #10 though not 1/0 or larger.

In that case, you are not allowed to parallel at all. Reference 310.10(H)(2).

 

[winnie]

Code does not require parallel neutrals when you parallel line conductors. However there are potentially lots of detail devils in this sort of installation.

1) For the same total amount of copper, you get better reduction in voltage drop if that copper is evenly distributed. For a single phase circuit, half of the voltage drop is in each side, half on the 'line' and half on the 'neutral'.

If you double the 'line' conductors, then the _total_ voltage drop goes to 75% of the original value, because you still have the original voltage drop of the neutral.

2) As others have mentioned, code has a minimum size requirement for paralleling conductors.

3) When you parallel conductors, they must have the same 'characteristics'. In a multi-conductor cable you may need to consider symmetry and conductor lay to determine which conductors may be placed in parallel.

-Jon

 

[GoldDigger]

Ignoring for now that you may not be allowed to do it, the change in the sketch will result in 3/4 of the original voltage drop. Paralleling the neutral too will give you 1/2 the original VD.

 

[luckylerado]

That is correct. Ampacity is no issue here at all. Just voltage drop. Conductors are #10 though not 1/0 or larger.

I got thrown off because I know in voltage drop calculations, total circuit length resistance is calculated.

I guess if I don't parallel the neutral, I cannot take credit for the decreased resistance for that portion of the circuit?

I assume that increasing the voltage was already considered and ruled out?

 

[Designer69]

I assume that increasing the voltage was already considered and ruled out?

yes because you're talking about adding step-up transformers.

I fully agree, not paralleling the neutral gives only 75% reduction. There are available conductors however to parallel the neutrals as well. The cables are 1-4C #10. Only 2 of the 4 conductors are being used. So the available 2 spares could be used.

These are control circuits so exception 1 to 310.10(H) could apply. This is for a power generating station anyway so the NEC doesn't apply as much as commercial/industrial installs.

I will check with the owner to get buy-in as well.

I appreciate your help

 

[Carultch]

Code does not require parallel neutrals when you parallel line conductors. However there are potentially lots of detail devils in this sort of installation.

1) For the same total amount of copper, you get better reduction in voltage drop if that copper is evenly distributed. For a single phase circuit, half of the voltage drop is in each side, half on the 'line' and half on the 'neutral'.

If you double the 'line' conductors, then the _total_ voltage drop goes to 75% of the original value, because you still have the original voltage drop of the neutral.

2) As others have mentioned, code has a minimum size requirement for paralleling conductors.

3) When you parallel conductors, they must have the same 'characteristics'. In a multi-conductor cable you may need to consider symmetry and conductor lay to determine which conductors may be placed in parallel.

-Jon

Does the code require that you have symmetry in your conductor sizing for voltage drop curtailment in general?


Take a 40A line-to-neutral single phase load with a default size of #8 wire. Suppose you can either curtail voltage drop with a #6 wire for both line and neutral (what most people would do), or you can curtail voltage drop with a #2 line and keeping the #8 neutral as the default size. Why you would do the latter, I do not know. But that is the kind of solution that the OP is proposing, except involving parallel sets instead of a general upsizing.


This does seem like it opens a loophole in the requirement to proportionally upsize the EGC. Because it only requires this when you increase the size of the ungrounded conductors. If you only increase the size of the neutral to curtail voltage drop, and keep the line conductor equal to the minimum local size, then technically you haven't upsized the ungrounded conductors. It feels wrong, but like I said, it is a loophole.

 

[GoldDigger]

Does the code require that you have symmetry in your conductor sizing for voltage drop curtailment in general?


Take a 40A line-to-neutral single phase load with a default size of #8 wire. Suppose you can either curtail voltage drop with a #6 wire for both line and neutral (what most people would do), or you can curtail voltage drop with a #2 line and keeping the #8 neutral as the default size. Why you would do the latter, I do not know. But that is the kind of solution that the OP is proposing, except involving parallel sets instead of a general upsizing.


This does seem like it opens a loophole in the requirement to proportionally upsize the EGC. Because it only requires this when you increase the size of the ungrounded conductors. If you only increase the size of the neutral to curtail voltage drop, and keep the line conductor equal to the minimum local size, then technically you haven't upsized the ungrounded conductors. It feels wrong, but like I said, it is a loophole.

If you look at what happens when there is a ground fault, you will see that the size of the grounded conductor is not important.
Not a loophole IMHO.

 

[winnie]

Does the code require that you have symmetry in your conductor sizing for voltage drop curtailment in general?

The code does not require that you have symmetry in conductor sizing (between line and neutral) for voltage drop.

_Physics_ says that for a given amount of copper you will have the lowest voltage drop when you have symmetric sizing.

Taking your example, if you change from two #8 wires (line and neutral) to 2 #6 wires, your 'total copper cross section' goes from 33 kcmil to 52.6 kcmil, and your voltage drop goes to 63% of the original value.

If, instead you go to a #8 for the 'line' and a #2 for the 'neutral' your total copper cross section is 82.9 kcmil, and your voltage drop is now 62% of the original value.

Lets go to a #8 for line and a 4/0 for the 'neutral'. Total copper cross section is 228 kcmil, and voltage drop is now 54% of the original value. In the limit of having a superconductor for the neutral, the voltage drop remains 50% of the original value.

-Jon

 

[don_resqcapt19]

That is correct. Ampacity is no issue here at all. Just voltage drop. Conductors are #10 though not 1/0 or larger.

I got thrown off because I know in voltage drop calculations, total circuit length resistance is calculated.

I guess if I don't parallel the neutral, I cannot take credit for the decreased resistance for that portion of the circuit?

It doesn't matter why you are paralleling the conductors, in general the code does not permit parallel conductors smaller than 1/0.

I submitted a PI to permit conductors of any size to be paralleled as long and the OCPD was not rated higher than the ampacity of a single conductor of the paralleled set, but it was rejected. The very purpose was to permit smaller conductor to be paralleled for voltage drop applications and to clearly permit the use of ring circuits for the same reason. There is no hazard with paralleled conductors of any size where the OCPD is sized to protect a single conductor.

Yes, in a two wire circuit you have equal voltage drop on both conductors. Reducing the voltage drop on only one of the two conductors only acts to decrease the voltage drop on one half of the circuit.