376.22(B) Metal Wireways. Adjustment factors.

Correction factors for the number of current carrying conductors must be applied only to the conductors in cross section B.
Click to Enlarge
Correction factors for the number of current carrying conductors must be applied only to the conductors in cross section B.

Code Change Summary: A clarification was made on how to determine when adjustment factors must be applied to conductors in a metal wireway.

There are two important rules to remember when installing metal wireways. One is about how many wires can fill the wireway while the other is about when derating or adjustment factors must be applied to those wires. According to the 2014 NEC:

  1. Conductors cannot fill more than 20% of the interior cross-sectional area of a wireway per 376.22(A).
  2. Adjustment factors from Table 310.15(B)(3)(a) must be applied when the number of current carrying conductors (CCC) exceeds 30 “at any cross section of the wireway”.

The 2014 code change was to rule 2 above. The 2011 NEC® required derating when the number of CCC in the wireway exceeded 30. The 2014 NEC® requires derating when the number of CCC in the wireway exceeds 30 “at any cross section of the wireway”.

For example, if there are 35 CCC at a particular cross section in the wireway but at a different cross section, there are 3 CCC, adjustment factors from Table 310.15(B)(3)(a) must only be applied to the 35 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 the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2011 Code language:

376.22(B) Adjustment Factors. The adjustment factors in 310.15(B)(3)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(5), exceeds 30. 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.

2014 Code Language:

376.22(B) Adjustment Factors. The adjustment factors in 310.15(B)(3)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(5), exceeds 30 at any cross section of the wireway. 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.

Did You Like This? Let Us Know With A Like! Thanks!

376.22(B) Metal Wireways. Adjustment factors.

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

When must adjustment factors be applied to a group of conductors in a metal wireway?

A: When the number of current carrying conductors exceeds 20.
B: When the number of current carrying conductors is less than 30 at any cross section.
C: When the number of current carrying conductors exceeds 30 at any cross section.
D: When the number of current carrying conductors exceeds 20%.
Please register or sign in for electrical continuing education courses.

If you were already signed in, your session probably expired, please sign back in.