A common accessory with auxiliary gutters or wireways is a divider that extends the length of the gutter/wireway and creates two wiring compartments instead of one. This is more commonly used to separate service conductors from branch/feeder conductors or low voltage conductors from line voltage conductors, but can be used between two sets of branch/feeder conductors as well.

Wireway divider

When a divider is used between two sets of branch/feeder conductors though, does the 30 conductor limit on the 310.15(B)(3)(a) derate exception (376.22(B), 366.22(A)) apply to each subdivided compartment within the wireway/gutter, or to the wireway/gutter as a whole?

  • Would this mean the two groups of wires are not close to/touching each other? Aug 5 '17 at 17:19
  • @Harper -- you could lay it out so no wire was adjacent to the divider, if that helps. (The 20% rule comes out in the wash, BTW, thank you associative law!) Aug 5 '17 at 17:21
  • Gutters have different derate rules than pipe, having a divider creates 2 separate spaces for example low voltage cable can be run on 1 side and standard power on the other where the insulation on the low voltage prevents it being in the same space with the power wiring.
    – Ed Beal
    Aug 5 '17 at 20:34
  • @EdBeal -- yeah, 366.22(A)/376.22(B) -- (it's only true for metal ones though, nonmetallic ones must not provide enough of a heat-sink to allow it) Aug 5 '17 at 20:47
  • My bad I did not read the entire question. I think metallic breaks the cross coupling and reduces the heating.
    – Ed Beal
    Aug 5 '17 at 20:56

CMP-8 says it's all one wireway, or at least they rejected changing the Code to say otherwise

Well, making it so the 30 conductor limit applies to each compartment of a partitioned wireway was proposed as a Code change for the 2014 cycle and roundly rejected by CMP-8, so I think that's a definitive "one wireway" answer to my question, or at least the closest I'll get so far (quoting modulo markdown limitations from this NFPA doc):

8-134 Log #798 NEC-P08

Final Action: Reject


Submitter: Paul E. Guidry, Fluor Enterprises, Inc. / Rep. Associated Builders and Contractors

Recommendation: This is an updated proposal to one previously sent. Part of the text was left out. Please reject the previous proposal.

Add new text to read:

376.22 Number of Conductors and Ampacity. The number of conductors and their ampacity shall comply with 376.22(A) and (B).

(A) Cross-Sectional Areas of Wireway. The sum of the cross-sectional areas of all contained conductors at any cross section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway.

(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.

For the purposes of this Section, wireways with metallic dividers installed constitutes separate wireways.

Substantiation: Wireways are often being used with dividers that are a listed part of the equipment. Often these divided sections contain up to 30 current-carrying conductors in each divided section. Where dividers are used, this proposal would allow each divided section of the wireway to be counted as individual wireways and allow up to 30 current-carrying conductors and 20% fill without derating of ampacities.

Panel Meeting Action: Reject

Panel Statement: CMP-8 acknowledges that the use of internal barriers in wireway is not prohibited. The submitter has not provided adequate technical substantiation to permit increasing the number of conductors in a divided metal wireway above 30 without applying the provisions of Section 310.15(B)(5).


Alright so we go back to what is the "intent" of the NEC. Wiring in anything is a matter of fill and heat displacement. So the way I was trained. You would consider it two wireways and fill requirements pertain to each side. But you could also get a better definition if you posted this over on the IAEI (International Assoc of Electrical Inspectors) website or appeal it to your local chapter. They would give you the definitive AHJ answer. They usually post questions and answers every month.

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