Question 1

When do I need to derate conductors?

Accepted Answer

Conductors must be derated when ambient temperature exceeds 30°C (NEC 310.15(B)(1)) and when more than 3 current-carrying conductors are bundled in a raceway or cable (NEC 310.15(C)(1)).

Question 2

How do I derate for ambient temperature?

Accepted Answer

Multiply the conductor's ampacity from Table 310.16 by the correction factor from Table 310.15(B)(1) based on the actual ambient temperature.

Question 3

Does the neutral conductor count for derating?

Accepted Answer

Per NEC 310.15(E)(3), the neutral counts as a current-carrying conductor if it carries the unbalanced portion of a nonlinear load. In most balanced 3-phase 4-wire systems with linear loads, the neutral does not count.

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