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310.15(B)(3)(a) Derating and not counting the Equipment Grounding Conductor or When not to Count the Neutral

Posted on 11 April, 2016 at 14:41
NABCEP PV Installation Professional Exam Prep Course Discussion Board from HeatSpring Course

Q:

on question 52, looking at the derating part for the number of conductors in the conduit on table 310.15b3a we have two source circuits which is 4 conductors plus an equipment grounding conductor. The table shows 4 to 6 which at the end of the day the derating would be the same for 4 or 5 but you only consider 4. Could you explain why you don't count the 5th equipment ground conductor?? The footnote states the count shall not include conductors that that are connected to electrical components but cannot be simultaneously energized.?? Thanks

A:

For table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current Carrying Conductors, we only carry current carrying conductors, which means that we do not count equipment grounding conductors (EGC).

This is because current carrying conductors, when carrying current generate heat that needs to be dissipated.

In one way, you can look at a non-current carrying conductor (EGC) as a heat sink. To a small degree, an equipment grounding conductor could transmit heat out of the system as a heat sink (this is in theory).

The 310.15(B)(3)(a) that is not a table and states:

"More Than Three Current-Carrying Conductors.
Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24" and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in table 310.15(B)(3)(a)."

310.15(B)(3)(a) says:

"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)"

This means that when we have a neutral that is not part of a 2 wire circuit, that we do not need to count the neutral. This would apply mostly to ac circuits for PV systems or perhaps a bipolar circuit where all of the conductors of both monopole subarrays are in the same raceway.

The footnote under table 310.15(B)(3)(a) that refers to circuits that are not simultaneously energized, would apply to circuits where either one circuit is energized or the other. If we have a backup generator and a double throw switch with 2 sets of conductors in the same raceway, then as with a double throw switch, either one set or another set of conductors will be energized, but not both at once. We should not have to count both sets of conductors as generating heat, of only one at a time would be energized.

Thanks,
Sean White

Categories: NABCEP PV Installation Professional Exam Prep Course

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