366.23 Ampacity of Conductors.
Code Change Summary: Revised code language on conductor ampacity adjustment in a sheet metal auxiliary gutter.
In the 2017 NEC®, Section 366.22 had redundant language also appearing in 366.23.
Section 366.22 is all about how many conductors are permitted in an auxiliary gutter whereas Section 366.23 is all about the ampacity of those conductors. There was no need to have language in 366.22 on conductor ampacity adjustment when it was already contained in 366.23.
In the 2020 NEC®, changes were made to ensure that only Section 366.23 contained conductor ampacity adjustment requirements for those installed in a sheet metal auxiliary gutter.
There are two important rules to remember when installing sheet metal auxiliary gutters. One is about how many wires can fill the gutter while the other is about when to apply derating or adjustment factors to those wires. According to the 2020 NEC:
- Conductors cannot fill more than 20% of the interior cross-sectional area of an auxiliary gutter per 366.22(A & B).
- Adjustment factors from Table 310.15(C)(1) must be applied when the number of current carrying conductors (CCC) exceeds 30 “at any cross section of a sheet metal auxiliary gutter”.
For example, if there are 31 CCC at a particular cross section in a sheet metal gutter but at a different cross section, there are 10 CCC, adjustment factors from Table 310.15(C)(1) must only be applied to the 31 CCC’s. See image.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.
2017 Code Language:
366.23 Ampacity of Conductors.
(A) Sheet Metal Auxiliary Gutters. Where the number of current-carrying conductors contained in the sheet metal auxiliary gutter is 30 or less, the adjustment factors specified in 310.15(B)(3)(a) shall not apply. The current carried continuously in bare copper bars in sheet metal auxiliary gutters shall not exceed 1.55 amperes/mm2 (1000 amperes/in.2) of cross section of the conductor. For aluminum bars, the current carried continuously shall not exceed 1.09 amperes/mm2 (700 amperes/in.2) of cross section of the conductor.
(B) Nonmetallic Auxiliary Gutters. The adjustment factors specified in 310.15(B)(3)(a) shall be applicable to the current carrying conductors in the nonmetallic auxiliary gutter.
2020 Code Language:
366.23 Ampacity of Conductors.
(A) Sheet Metal Auxiliary Gutters. The adjustment factors in 310.15(C)(1) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under 310.15(E), exceeds 30 at any cross section of the sheet metal auxiliary gutter. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors. The current carried continuously in bare copper bars in sheet metal auxiliary gutters shall not exceed 1.55 amperes/mm2 (1000 amperes/in.2) of cross section of the conductor. For aluminum bars, the current carried continuously shall not exceed 1.09 amperes/mm2 (700 amperes/in.2) of cross section of the conductor.
(B) Nonmetallic Auxiliary Gutters. The adjustment factors specified in 310.15(C)(1) shall be applicable to the current-carrying conductors up to and including the 20 percent fill specified in 366.22(B).