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I want to use 90C rated 8 AWG wire for wiring to circuit breakers in a location not expected to exceed 40C with 3 current-carrying wires in a conduit. By Table 310.16 the wire has ampacity 55A with 40C temperature derating to 50A. However, NEC 110.14(C)(1) suggests that the 8AWG wire is limited to the ampacity of 60C 8AWG wire in Table 310.16, which is 40A at 30C. Do I then need to derate the 40A ampacity for 40C, which would bring it to 33A? If instead I have 4 current carrying wires in the conduit, do I need to derate the ampacity of the 8AWG 90C wire further to 26A, following Table 310.15(B)(2)(a)? If I follow this logic, if I need to carry 50A at 40C with 4 current carrying conductors in a conduit, I would need to use at least 3AWG wire, even though 3AWG 90C wire is rated to 110A. If I do not need to do any of the derating of the 60C ampacities, it seems that I could use 6AWG wire. If I only need to do the conduit derating, it seems that I can use 4 AWG wire. I hope you can clarify this for me. Thank you in advance for your assistance.
Hey Finley thanks for your question. The heading of Table 310.15(B)(16) indicates that it is based on not more than 3 current carrying conductors and an ambient temperature of 86 degrees F. If the installation is in an ambient temperature of 40 degrees C (104 degrees F) then the correction factor in Table 310.15(B)(2)(a) must be applied to the values in Table 310.15(B)(16). If there are more than three current carrying conductors the adjustment factor in Table 310.15(B)(3)(a) must be applied to the values in Table 310.15(B)(16). If both conditions of use exist then both factors must be applied.
Using 90 degree C insulated copper conductors at an ambient temperature of 104 degrees F (40 C) with 4 current carrying conductors for a load of 50 amperes would need 6 AWG conductors. The 75 ampere value [from the 90 degree C column in Table 310.15(B)(16)] X 0.91 [from the 90 degree C column in Table 310.15(B)(2)(A)] X 80% [from Table 310.15(B)(3)(A)] = 54.6 amperes.
Conductors cannot be used above the terminal temperature limitations [55 amperes for 6 AWG copper in the 60 degree C column in Table 310.15(B)(16) in this example, unless they are listed and marked otherwise] but the 90 degree C values can be the starting point as stated in the second paragraph of 310.15(B).
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