Q. What is the minimum size NM cable required for an electric space-heating branch circuit installed 3 inches above a heated ceiling (with no thermal insulation) in a dwelling unit? The branch circuit will supply a calculated load of 12A.
A) 14 AWG NM
B) 12 AWG NM
C) 10 AWG NM
D) 12 AWG Type TC
Explanation: As per 424.3(B), 12A x 1.25 (for continuous load) = 15A. This is the minimum ampere requirement for the conductor size. In addition, 424.36 requires you to apply an adjustment based on an ambient temperature of 50°C, from the correction factors on the bottom of Table 310.16. We use the 90°C column of 310.16 as a starting point to make our adjustment from (334.80).
Don’t worry about the terminal ratings related to 110.14 (C) at this point. The adjustment factor from the 90°C column of 310.16 based on a 50°C ambient is 0.82. 15A divided by 0.82 results in minimum adjusted allowable ampacity requirement of 19.5A. From the 60°C column of 310.16, a 14 AWG conductor is rated 20A, which would appear to be adequate for this calculation.
According to 240.4(D), overcurrent protection for the 14 AWG conductor is limited to 15A. However, let’s check our work. From the 90°C column we see that a 14 AWG Type NM cable with THHN insulated conductors is rated 25A. 25A x 0.82 = 20.50A. Because the calculated value of 20.50A exceeds the 20A value from the 60°C column, this calculated value is no longer in compliance with 334.80. Therefore, this installation will require a 12 AWG Type NM cable.
From the 90°C column, we see that a 12 AWG THHN is rated 30A. 30A x 0.82 = 24.60A, which is less than the 25A rating from the 60°C column. As per 240.4(D), the overcurrent device rating is 20A. Therefore, this arrangement will work.
Oh yeah, there’s one last item to note. What about the terminals as per 110.14(C)? The actual load is rated at 12A, which is less than the 25A rating of a 12 AWG conductor from the 60°C column of 310.16. This should make this installation compliant with 110.14(C).
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at firstname.lastname@example.org.