paralleling conductors for voltage drop

[Carultch]

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

I tried to select my example, so it would be nearly the same curtailment of voltage drop in each case. I initially thought I could make one work with a #8 to #4 symmetric upsizing example, and #8-#8 / #8-#2 might be an equivalent asymmetric upsizing. But the algebra equation showed me a negative value for resistance that I'd need in the asymmetrically upsized solution, and it surprised my initial guess.


Now that I've calculated it out, it does make sense that it would seldom (if ever) be an advantage to asymetrically curtail voltage drop. Not to mention, it is only going to confuse anyone else who has to look at it.

 

[Carultch]

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.

I suppose they do not necessarily have to give a reason why they reject your public input, but I am curious to know what the reason is, if there is any at all.

I never thought about your proposed exception before, but it does make complete sense that there wouldn't be a hazard. I can't conceive of an application where this would be an advantage over the single conductor per phase solution to curtail voltage drop, but in theory I agree with the proposed exception.

 

[don_resqcapt19]

I suppose they do not necessarily have to give a reason why they reject your public input, but I am curious to know what the reason is, if there is any at all.

I never thought about your proposed exception before, but it does make complete sense that there wouldn't be a hazard. I can't conceive of an application where this would be an advantage over the single conductor per phase solution to curtail voltage drop, but in theory I agree with the proposed exception.

Actually in general they do have to give a reason.

Public Input No. 4435-NFPA 70-2014 [ New Section after 310.10(H)(1) ]
Exception Number 3
Exception Number 3. Conductors of any size or combination of sizes shall be permitted to be connected in parallel where the circuit overcurrent protective device has a rating equal to of less than the ampacity of the smallest paralleled conductor.

Statement of Problem and Substantiation for Public Input
Many of the states have adopted energy codes that include mandatory voltage drop requirements. This change would permit non-conventional methods, such as ring circuits, to be used to satisfy these voltage drop requirements There is no safety hazard where the OCPD has a rating that is equal to or less than the ampacity of the smallest paralleled conductor.

Submitter Information Verification
Submitter Full Name: DON GANIERE
Organization: [ Not Specified ]
Street Address:
City:
State:
Zip:
Submittal Date: Thu Nov 06 22:00:02 EST 2014
Committee Statement
Resolution: PI4435 No study has been conducted to prove that this practice does not pose a safety hazard. The submitted language would permit this practice for installations other than addressing voltage drop.

Existing exception #1 permits this practice for high frequency circuits with small electrical loads and it provides specific requirements for these installations. Based on the conditions prescribed in exception #1, and no testing done- the panel will continue to support the existing requirements.

PI510 The submitter failed to provide adequate testing to prove that the paralleling of the grounding electrode conductor, system bonding jumper and main bonding jumper will not affect performance. The largest size grounding electrode conductor required by the NEC is a 3/O copper which is only 2 sizes larger than minimum required for parallel installations. The system and main bonding jumpers are typically
shorter in length and required to be sized 12 1/2% of the circular mil area of the ungrounded conductors.

PI3073 does not add clarity or usability to the Code.

Note this shows one of the problems with the new code change process....they post panel statements for multiple PIs in one common response.

 

[Designer69]

I can't conceive of an application where this would be an advantage over the single conductor per phase solution to curtail voltage drop, but in theory I agree with the proposed exception.

I agree there aren't many applications but trust me there are some. It would allow for improved efficiency and flexibility. I personally cannot conceive of a reason why this would not be allowed by the NEC.

 

[Carultch]

Actually in general they do have to give a reason.

Note this shows one of the problems with the new code change process....they post panel statements for multiple PIs in one common response.

It sounds like they didn't even think, when they provided that reason. If one individual wire already has sufficient ampacity, we already have the test data to show that there isn't a hazard. It's called 310.15(B)(16).

 

[Designer69]

could this have to do with short circuit rating of the OCPD? the added conductor would add additional short circuit current. although I don't know how this would differ from just installing a very large conductor there instead

 

[Carultch]

could this have to do with short circuit rating of the OCPD? the added conductor would add additional short circuit current. although I don't know how this would differ from just installing a very large conductor there instead

That is unrelated to whether or not you would be able to parallel the conductors in the first place.

Plus, if the available fault current at the ultimate source is already less than the KAIC rating, that kind of concern is irrelevant.

 

[Smart $]

It sounds like they didn't even think, when they provided that reason. If one individual wire already has sufficient ampacity, we already have the test data to show that there isn't a hazard. It's called 310.15(B)(16).

You'll see this type of response quite often. Never expect the CMP to use self-imposed rationale. You must specifically present the rationale in the substantiation.

 

[jim dungar]

could this have to do with short circuit rating of the OCPD? the added conductor would add additional short circuit current.

No.
According to 110.9, the AIC is based on the fault current available on the line side of the protective device.
Through current does not come into consideration.

 

[Sahib]

No.
According to 110.9, the AIC is based on the fault current available on the line side of the protective device.
Through current does not come into consideration.

More specifically, the protective device is rated for a short across its load terminals.