Counting the neutral

[Volta]

Many of us think of it as "not current-carrying if it only carries the unbalanced current", give or take a few words.

But 310.15(B)(4) says, ". . . shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).

What are the "provisions"?

So we don't count it as current-carrying, but 310.15(B)(2)(a) also says in part, " . . . the allowable ampacity of each conductor shall be reduced as shown in Table . . ."

Is that something else not required to be counted?

Has the NEC allowed neutrals of balanced multi-wire circuits to remain at a full ampacity rating while derating the current-carrying conductors?

Suppose a black, red, blue and green each on #10 for derating and 250.122(B) purposes, with a #12 white? Some good reasons not to do it, sure, but does it meet code?

 

[infinity]

Are you asking when to count the neutral as a CCC?

310.15(B)(4) 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)(2)(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)(2)(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.

 

[Twoskinsoneman]

Suppose a black, red, blue and green each on #10 for derating and 250.122(B) purposes, with a #12 white? Some good reasons not to do it, sure, but does it meet code?

There are some places in the code you can derate gthe neutral for specific loads.
You can't derate on gen pur MWBC because the load on the the neutral can easily be as much as that on the ungrounded conductor. Think about a 3 wire MWBC with one line loads off and the other maxed out... the neutral would be maxed out.... derating would be bad.

 

[Volta]

Are you asking when to count the neutral as a CCC?

No. I am considering an existing installation where the neutrals are #12, and the phase conductors are all #10. I will certainly use a #10 for the neutral of my new MWBC, but can it be considered compliant to leave the existing #12 wires since they "shall not be required to be counted when applying the provisions"?

(We have actually pulled in replacement #10 white conductors while adding our new circuit, but it will take special scheduling to disconnect the 12's and reconnect the 10's. We will probably do it, but do we have to do it?)

There are some places in the code you can derate gthe neutral for specific loads.
You can't derate on gen pur MWBC because the load on the the neutral can easily be as much as that on the ungrounded conductor. Think about a 3 wire MWBC with one line loads off and the other maxed out... the neutral would be maxed out.... derating would be bad.

I agree it would be bad. :smile: But does it comply with 310.15(B)(4) if it only carries the unbalanced current?

As I say above, this is existing, but we are adding to the raceway. I wouldn't install it that way, but it is that way now.

 

[SEO]

310.15(B)(4) (a) is used when applying the provisions of 310.15(B)(2)(a) for adjustment factors of conductors in raceways. If you have a ballanced 3-wire multiwire circuit the neutral in theory carrys 0 current. In that case the #12 will carry 0 current but what happens if one circuit is carrying 0 current and the other is carrying 24 amps the #12 will carry 24 amps as well.( Not even concidering 4-wire circuits with nonlinear loads and harmonics.)

 

[weressl]

310.15(B)(4) (a) is used when applying the provisions of 310.15(B)(2)(a) for adjustment factors of conductors in raceways. If you have a ballanced 3-wire multiwire circuit the neutral in theory carrys 0 current. In that case the #12 will carry 0 current but what happens if one circuit is carrying 0 current and the other is carrying 24 amps the #12 will carry 24 amps as well.( Not even concidering 4-wire circuits with nonlinear loads and harmonics.)

I wonder when will the Code catch up with the already recognized fact that high proportionate non-linear loads can actually cause the neutral to carry MORE current that each individual phase. I think engineering practices will catch the resultant voltage drop issue before.

Lets see who will be the winner.

 

[chris kennedy]

No. I am considering an existing installation where the neutrals are #12, and the phase conductors are all #10.

These phase conductors are #10 on 20A breakers?