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When I apply NEC 110.14(C) for wiring between equipment using 90C copper wire, do I include temperature and conduit derating for the 60C copper column?

Question asked by finley.shapiro on Oct 3, 2017

I need to carry 55A between breakers and other equipment in an environment that may reach 38C at a location where the 2008 NEC is in force. The breakers will be rated at least 55*1.25=68.75, or 70A breakers. At 30C Table 310.16 would allow 90C 6AWG copper wire. When including the derated ampacities, the table requires the use of 4AWG copper 90C wire. However, 110.14C indicates that the equipment terminals can be expected to be rated 60C, and therefore the ampacities in the 60C column of Table 310.16 must be used even with 90C wire. While the 60C copper column shows a 70A ampacity at 30C, is temperature derating also required if the 60C copper column is used? In this case, 3AWG copper wire would be called for. Also, if the ampacities need to be derated for more than 3 current carrying wires, then even larger wire will be necessary.

 

Therefore the question: For 90C copper wiring installed on-sight between equipment, does 110.14C call for the use of 60C ampacities from Table 310.16 with temperature and conducting wire deratings, or can the 60C ampacities be used without temperature and conducting wire deratings.

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