Question

What’s the allowable ampacity of a No. 12 copper wire with type THHN insulation installed in a raceway with 35 wires? Question 3 options: A) 10 A B) 15 A C) 8 A D) 12 A

Answer 1

D) 12 A

Step-by-step explanation:

You want to know the allowable ampacity of AWG 12 type THHN copper wire in a raceway with 35 wires.

Nominal ampacity

Table 310.15(B)(16) of the 2017 National Electrical Code gives the nominal rating of AWG 12 type THHN copper wire as 30 A when installed in an environment with a temperature of 30 °C or less.

Derating

Table 310.15(B)(3)(a) gives the adjustment factor when there are more than three conductors in a raceway or cable. For 31–40 conductors, the ampacity must be derated to 40% of its nominal value.

In a raceway with 35 wires, the allowable ampacity of THHN AWG 12 wire is ...

(30 A)(0.40) = 12 A

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Additional comment

For environmental temperatures above 30 °C, Table 310.15(B)(2)(a) gives additional derating factors. The adjustment factor for temperatures up to 50 °C (122 °F) is 0.82, which would bring the allowable ampacity down to about 9.8 A.

Answer 2

The allowable ampacity of a No. 12 copper wire with type THHN insulation installed in a raceway with 35 wires is approximately 12 amperes. This is best represented by option D.

According to the National Electrical Code Table (NEC), a derating factor must be applied for any raceway with more than three current-carrying conductors (in our case, copper wires). In a raceway with 31-40 current-carrying conductors, the derating factor is 50%. Because the ampacity of a No. 12 copper wire usually is 25 amperes (A), we can multiply this by our derating factor to find that:

25 • 0.50 = 12.5

The exact allowable ampacity of this copper wire is 12.5 amperes; however, we can round down to find that the closest answer is 12 A.

Hence, option D represents the best answer.