National Electrical Installation Standards

Standards as High as Your Own

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Wednesday, February 7, 2018

Question:

A Residential Air Conditioning Unit has a max fuse rating of 60a. The minimum circuit amps is 33.3a at 240v, single phase. Assuming the unit is 50 ft from the panel and no temperature factors or other corrections are necessary. Is #10 wiring prudent for this install? Is it better to install 2 - #8 ungrounded conductors with a #10 grounded conductor or does installing a #10 ground with #8 phase conductors in this case violate Code or affect available fault currents in any way? Which wire size is better to install? Does this A.C. load push the limits of a #10 conductor? Is Article 250.122(B) violated, in this case if I increase the size of phase conductors from a #10 to a #8, but install a #10 ground, even though Code only requires a #10? Thank you for your time. Jimmy

A

Answer:

Hey Jimmy thanks for your question. Conductors must be sized to at least the minimum circuit ampacity as stated in 440.35. The Informational Note to 310.15(A)(2) refers us to 110.14(C) for conductor limitations based on termination provisions. Unless all the terminations are marked with temperature ratings higher than 60 degrees C we are limited to using the 60 Degree column values in Table 310.15(B)(16) as stated in 110.14(C)(1). A 10 AWG copper conductor has an allowable ampacity of 30 amperes in the 60 degree C column of Table 310.15(B)(16) so is not large enough for your example. If all terminations are marked 75 degrees C or higher then the 75 degree column of Table 310.15(B)(16) can be used an a 10 AWG copper conductor would be large enough if it had 75 degree C insulation or higher.

A 10 AWG copper equipment grounding conductor is acceptable for the 60-ampere overcurrent device as stated in Table 250.122. If you increased the size of the ungrounded conductors, from the minimum size that has sufficient ampacity (either the 10 AWG or 8 AWG depending on the termination ratings) then the equipment grounding conductor size would need to be larger as stated in 250.122(B).

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