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Paralled Conductors-Derate Q

Question asked by matt.rudolph on Dec 3, 2019
Latest reply on Jan 7, 2020 by opes

I have parallel feeders to a 800 Amp MCC in separate raceways from the distribution breaker.  Does the wire have to be derated according to Table 310.15(B)(3)(a) from NFPA 70 code.  We are having a debate on the meaning of the highlighted sentence.  One interpretation is that it needs to be derated regarless because it's  a parallel feed.  The other interpretation is this only apply's to the raceway that the conductors are within.


This could be a huge difference in wiring size of parrell 750 MCM's vs parrell 500 MCM's.


Appreciate your feedback..




(3) Adjustment Factors. (a) 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 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.