Nuetral in multi wire assy.

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Jesse7623 said:
is the neutral considered a current carrying conductor in a multi wire?? I thought it is but one of my guys says its not.
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That would depend on the system and application.

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.
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tlaidman said:
In a multiwire branch the neutral is considered a current carrying conductor unless the circuit is balanced at all times
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Even though it could be carrying unbalanced current it would not necessarily be counted (considered) as a CCC.

Roger
 
tlaidman said:
What says how much unbalance it has to carry to make it a ccc
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As long as it doesn't add to the total current current of the maximum of the circuits it isn't counted.

For example;

using two 20 amp circuits Leg 1= 20 amps Leg 2= 20 amps N= 0 amps for a total of 40 amps.

Now same two circuits with part of one circuit turned off

Leg 1=20 amps Leg 2= 10 amps N= 10 amps for a total of 40 amps.

This would not hold true of two phases of a wye system.

Roger
 
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tlaidman said:
What says how much unbalance it has to carry to make it a ccc
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It doesn't matter how much unbalanced load there is. Unless you have more than 50% nonlinear load, the neutral is not a current carrying conductor. The derating is based on heat from the I^2R losses in the conductors. With a 120/240 volt multiwire branch circuit, the worst case for heat is either with one ungrounded conductor fully loaded or with both fully loaded. In both cases there are two conductors carrying the full amount of current and the other carrying none.
If we use 20 amps for the load just to make the numbers easy, you will have 400R (r=conductor resistance) watts of heat on each of the two conductors for a total of 800R watts of heat. Any other combination will produce less heat. If there is 20 amps on one, 1 amp on the other there will be 19 amps on the neutral. One ungrounded conductor will have 400R watts of heat, the other will have 1R watt of heat and the neutral will have 361R watts of heat for a total of 762R watts of heat. No need to count the neutral as a current carrying conductor.
This also works for three phase circuits.
 
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