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.
