National Electrical Installation Standards

Standards as High as Your Own

Thursday, July 18, 2019


Re: CQD answer published Wednesday, July 10, 2019 -NM Cable Adjustment Factor

Hi Charlie 2.0, Regarding the answer provided to the question for Tuesday 7/9/2019, just wanted to point out the 80% adjustment factor mentioned in the answer is applied to the ampacity allowed for the maximum conductor temperature. For NM-B Cable, the conductor insulation is rated 90C therefore the ampacity adjustment is applied to the 90C ampacity. In this situation (12/4 or 12/2/2 NM-B), that would give an allowable ampacity of 0.8 x 30 Amps = 24 Amps. Since this calculated result is higher than the 60C ampacity, we must use the lowest of the two values which is the 60C ampacity.  The net result is that a 4 conductor 12 AWG NM-B Cable (whether it is one circuit or two circuit) has an ampacity of 20 Amps – just like the 12/2 and 12/3 NM-B Cables. If multiple cables are bundled together, further ampacity adjustment may be required.

Dave Watson, Southwire


Hey Dave thanks for your comment, it is a point we didn't cover well enough. The adjustment factor for more than 3 current carrying conductors applies based on 310.15(B)(3)(a) but the value for the 90-degree insulation in Table 310.15(B)(16) can be used for applying that factor as stated in 334.80.

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