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3 Phase, 4 wire Feeder De-rating

Question asked by isaacmand on Dec 4, 2018
Latest reply on Dec 17, 2018 by fourseng
  1. If I have a hypothetical 480/277V, 3 Phase, 4 wire feed to a distribution panel. (Spare panel for future use, we do not know the load) Do you de-rate because it is 4 wire?
  2. Following 310.15(B)(5)(c) If the majority is non-linear (meaning greater than 50%?) we would de-rate because of 4 wire. (Correct?)
  3. Or does this apply only to a branch circuit, like an appliance, because it says a 3 Phase, 4 wire circuit?
  4. If it is a branch panel that is being designed for only motor loads, it would normally be a 3 wire panel and the question wouldn’t come into play. If it is a branch panel that also has 277 loads and is 4 wire, but the majority is motor loads, then the feed going to it should be a de-rated 4 wire, feed if I am reading the code right. Correct?
  5. Back to question number (1). You are feeding a distribution panel from the switchgear. Do you need to calculate all the loads from all the branch panels that will be connected to it to figure out if you need to de-rate the distribution panel feeders?
  6. I have seen many drawings that have 3 and 4 wire feeder schedules printed on the drawing. There is never a difference in the wire size between 3 and 4 wire feeders of the same amperage, just the obvious quantity of wire difference (3 or 4) and possibly conduit size. Are these Feeder schedules incorrect, or does the 310.15(B)(5)(c) rule not apply to feeders?


Sorry if I have asked the same question more than once in different ways, I am just trying to understand correctly. It is not clear to me from the codebook. If you agree, maybe we can make an enhancement in the future.