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Friday, October 13, 2017
Question:
Good morning Charlie,
I need a clarification of a calculation for AC branch circuit conductors:
A respected author, in one of his training manuals, gave an example regarding the OCP and Minimum circuit ampacity for a central air conditioning condenser. Name plate read Minimum Circuit ampacity of 23 amps and Maximum HACR breaker sized at 40 amps. He initially indicates that a #12 THWN copper conductor which is rated in the 60, 75 and 90 degree columns at 25 amps as the correct conductor size, referencing 240.4(G), 440.4(A), 440.22 (A). In December 2016 a correction was released revising the conductor size to a #10 THWN, which in my opinion is correct.
The July/August IAEI publication again gives a similar example with a nameplate Minimum Circuit Ampacity of 24A (already included the 125% Calculation) indicating that a #12 AWG copper conductor@ 60 degrees is acceptable since it’s rated at 25A, which one is correct?
Thank You, Carl Gurney
A
Answer:
Hey Carl thanks for your question. Conductor ampacities are often determined using Table 310.15(B)((16). Using 12 AWG copper as an example it has an allowable ampacitiy of 20 for 60 degree C rated insulation, 25 for 75 degree C rated insulation , and 30 for 90 degree C rated insulation. I don't see how you are getting 25 amperes for 60 degree C rated 12 AWG.
We are not allowed to exceed the terminal temperature ratings as stated in 110.14(C). Using 110.14(C)(1)(a) we are limited to the 60 degree column ampacity - unless the equipment is listed and marked otherwise so the 20 ampere value applies. If the equipment nameplate minimum circuit ampacity is 23 or 24 a 10 AWG copper conductor is the minimum size allowed.