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Would you help us resolve a question of derating and maximum number of conductors in a raceway?

Please refer to the 2002 NEC Handbook for the following question:

We are discussing how to properly derate conductors in raceway when the quantity of conductors exceeds 85.

The wiring application in question is for theaters and involves stage, concert and house lighting circuits controlled by solid state dimming racks. Many dimming racks in large theaters have 90 or more dimmer modules resulting in at least 180 current carrying conductors plus ground conductors. Metal wireway is often used to route conductors from the dimmer racks to the fixtures. The wireway is sized for the maximum 20 percent fill required by Article 376. We are applying Article 520.6 that grants exception to the 30 conductor maximum rule in Article 376.

The question of properly derating remains. Note: Load diversity is often 50% or greater.

Table B.310.11 appears to apply. The problem is that this table does not seem to address more than 85 conductors. The FPN following the table refers to "special calculations required" but does not specify further reading or guidelines to perform the calculations.

Jeffrey M. Leavenworth, P.E.

2017 / 2014 NEC & Related Books and Study Guides
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My NEC [1999] is in the Van and it's getting late, but I would like to address this message, so I'll toss some numbers and stuff in this thread later.

Scott SET


Scott " 35 " Thompson
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Bob Offline
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Jeff,
I do not have the 2002 code yet but the
99 code section 318-11 seems to apply to your installation.

Joined: Jan 2001
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Tom Offline
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This query has been lying here festering for a couple of days. Seems like no one really wants to get out in the lead (and become a target).

Here goes. If you are applying 520.6, and are otherwise in compliance with that article, no derating of the conductors is required.

The last sentence waives the 30 conductor limitation of 366.6 & 376.22 Both of these require derating when exceeding 30 current carrying conductors. Since the rule is waived, derating is not required.

Do I need to put on my asbestos coveralls?

[This message has been edited by Tom (edited 04-15-2002).]


Few things are harder to put up with than the annoyance of a good example.

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