Neutral as a current carrying conductor question

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dm9289

Industrial Maintenance Electrician
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Pennsylvania
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Industrial process repair/ maintenance Electrician
I think I may have found an error in a test question help appreciated.

The neutral conductor that carries only the unbalanced current from a 3 wire delta circuit or 4 wire three phase wye circuit is considered a current carrying conductor for the purpose of table 310.15(b)(3)(a) .

I would say true, answer key says false stating 310.15(B)(5)(a) which I think refers to single phase. My point is
310.15(B)(5)(b) basically there will be neutral current so it should be counted?​
 
The answer is false, If the conductor is only carrying "unbalanced" current it is not additive to the total. In the delta example it would be the same as a single phase supply, if the wye example were a three wire circuit it would carry the full current of the others and your answer of true would be correct.

Roger
 
Although is does carry current it is not considered a CCC for derating purposes.

310.15(B)(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).
(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye-connected
system, a common conductor carries approximately the same
current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions
of 310.15(B)(3)(a).
(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor.
Click to expand...
 
Thanks for the input I can sort of see with the delta because I guess you don't use the high leg it acts like single phase. Any sort of wye that uses a neutral you will have current flow thus heat if I'm thinking correctly like(b) that's where my thinking disconnects. In practice with my situation anything I doubt I count like my feelings expressed here, but for a test which I'm taking soon its confusing to me.
 
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dm9289 said:
Any sort of wye that uses a neutral you will have current flow thus heat if I'm thinking correctly like(b) that's where my thinking disconnects.
Click to expand...
What you're not grasping is that any current the neutral is carrying is current not being carried by one or more line conductors.

If you had a balanced three-phase circuit, let's say with 10a of various loads on each line, you'd have a total heating current of 30 amps. No matter how much you imbalance those loads, creating neutral current, the total will still not exceed 30 amps.

The same total power used requires the same total current, regardless of how it's shared among the individual conductors.
 
dm9289 said:
I think I may have found an error in a test question help appreciated.

The neutral conductor that carries only the unbalanced current from a 3 wire delta circuit or 4 wire three phase wye circuit is considered a current carrying conductor for the purpose of table 310.15(b)(3)(a) .
Click to expand...
Neutral in a delta circuit?
 
There is something wrong with the question wording. If it is a high leg delta then it is a 4 wire delta, not a 3 wire delta.
 
texie said:
There is something wrong with the question wording. If it is a high leg delta then it is a 4 wire delta, not a 3 wire delta.
Click to expand...

The same applies to 4-wire deltas; total current still supplies the load's total power.

A 4-wire delta is a 3ph delta superimposed on a standard 120/240v 1ph supply.

The delta's power is carried by the three lines, and the neutral current is still the imbalance between the two flanking lines.
 
LarryFine said:
The same applies to 4-wire deltas; total current still supplies the load's total power.

A 4-wire delta is a 3ph delta superimposed on a standard 120/240v 1ph supply.

The delta's power is carried by the three lines, and the neutral current is still the imbalance between the two flanking lines.
Click to expand...

You're missing my point. The wording of the question is incorrect. A 3 wire delta does not have a neutral.
 
Ah, I see what you mean now. They specify a 3-wire delta circuit, not supply. I believe they're referring to a 3-wire 1ph circuit supplied from a delta.

A 3-wire 3ph circuit has no neutral, as you point out, to be considered in a CCC count, so the rule need not refer to that, and I believe it does not.
 
texie said:
You're missing my point. The wording of the question is incorrect. A 3 wire delta does not have a neutral.
Click to expand...

The question could been worded something like, in a three wire circuit with a neutral fed from a Delta supply ……., but I think just by including a neutral in the question gets the point across regardless.

Roger
 
Irrespective of all that, the neutral does not count as a CCC when all the wires are there. Any current it carries is subtracted from the current through the others.
 
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