The ampacity adjustment factors of 31 0.1 5(B)(3)(a) shall not apply to conductors installed in surface metal raceways where:
(a) the cross-sectional area exceeds 4 sq in.
(b) the current-carrying conductors do not exceed 30 in number
(c) the total cross-sectional area of all conductors does not exceed 20 percent of the interior cross-sectional area of the raceway
(d) all of these
The correct answer and explanation is :
The correct answer is: (d) all of these
Explanation:
The ampacity adjustment factors referred to in 310.15(B)(3)(a) of the National Electrical Code (NEC) are designed to prevent overheating of conductors that are bundled together, as heat cannot dissipate effectively when many current-carrying conductors are installed in close proximity. This rule typically requires derating (reducing) the ampacity of conductors when there are more than three current-carrying conductors grouped together.
However, the NEC allows exceptions where derating does not apply, particularly when certain installation conditions reduce the risk of excessive heat build-up. One such exception involves conductors installed in surface metal raceways, such as Wiremold or other metallic surface-mounted raceway systems.
According to NEC 310.15(A)(2) Exception No. 4, derating is not required if all the following conditions are met:
- The total number of current-carrying conductors does not exceed 30.
This limits the potential heat buildup within the raceway. - The total cross-sectional area of all conductors does not exceed 20% of the interior cross-sectional area of the raceway.
This ensures adequate air space for heat dissipation within the raceway. - The raceway has a cross-sectional area exceeding 4 square inches.
A larger raceway allows better heat dissipation and reduces the need for ampacity adjustment.
Because the exemption is conditional on all three of these criteria being met, the correct answer is (d) all of these. When these conditions are satisfied, the conductors are considered to be in an environment that permits sufficient cooling, so the ampacity adjustment factors in 310.15(B)(3)(a) do not apply, maintaining both safety and installation efficiency.