Per 376.22 derating factors should be applied where the number of current carrying conductors exceeds 30. Does this mean that once you exceed 30 conductors all of them need to be derated to 40% or just those in excess of 30? IE conductors 1-30 can be rated at 100% but conductors 31-40 need to be rated at 40%. Separately and I'm sure this has been answered in the forum elsewhere (but I could not find it) if I am using branch circuits with dedicated neutral conductors wouldn't the neutral then be considered a non CCC per 310.154a?
Derating of conductors in a wireway
- Thread starter mak134
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Once you cross 30, every wire in the tray gets derated. Think about. If there are 31 wires, which one was number 30 and which one was number 31, so which one gets derated and which one doesn't?
Also, in a single phase circuit consisting of one ungrounded wire and one neutral wire, all the current that flows away from the source on the one wire will return on the other wire. So the neutral is not just carrying the unbalanced current from other phases, it is carrying all of the current. So it does count as a CCC.
Also, in a single phase circuit consisting of one ungrounded wire and one neutral wire, all the current that flows away from the source on the one wire will return on the other wire. So the neutral is not just carrying the unbalanced current from other phases, it is carrying all of the current. So it does count as a CCC.
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The derating applies to all conductors, since they will all be subject to the increased heat.
(And as a practical matter, how would you tell how to number the conductors.)
If the neutral is dedicated to one single phase (not a 120/240 MWBC), that pretty much guarantees that the neutral will carry the same current as the hot conductor.
In a three phase wye circuit with linear loads, the neutral will only carry current when there is an imbalance and the current in one or two of the phase wires will be reduced by the same amount.
Tapatalk!
(And as a practical matter, how would you tell how to number the conductors.)
If the neutral is dedicated to one single phase (not a 120/240 MWBC), that pretty much guarantees that the neutral will carry the same current as the hot conductor.
In a three phase wye circuit with linear loads, the neutral will only carry current when there is an imbalance and the current in one or two of the phase wires will be reduced by the same amount.
Tapatalk!
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The derating only applies where you have more than 30 current carrying conductors at any cross section. Also, often you can apply the provisions of the exception to 310.15(A)(2) and not have to derate the conductors in the wireway.
texie
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I agree and key point. While the 2011 does not make it clear that it is only at any given cross section that was always the intent. The 2014 has been revised to make clear this is the case.
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