CSLB C-10 Electrical ContractorWiring Methods and MaterialsEasy
A contractor is installing a new service mast on the exterior of a commercial building. The service mast is made of 2-inch Rigid Metal Conduit (RMC). The service-entrance conductors are 4/0 AWG THWN copper. What is the maximum number of 4/0 AWG THWN copper conductors permitted in this 2-inch RMC service mast, assuming no more than 40% fill?
- A3 conductors
- B4 conductors
- C5 conductors
- D2 conductors
Show answer & explanationAnswer & explanation
Correct answer: A. 3 conductors
According to NEC Annex C, Table C.8, a 2-inch Rigid Metal Conduit (RMC) can accommodate a maximum of three 4/0 AWG THWN copper conductors while adhering to the 40% fill limitation. This table is specifically for RMC.
Why the other options are wrong
- B. Four conductors exceeds the maximum allowed.
- C. Five conductors significantly exceeds the maximum allowed.
- D. Two conductors is less than the maximum allowed.
RMC Conduit Fill
Rigid Metal Conduit (RMC) fill limits are specified in NEC Annex C tables for various conductor types and sizes, ensuring proper installation and preventing conductor damage or overheating.
- Annex C tables simplify conduit fill calculations when all conductors are of the same type and size.
- RMC is a robust conduit type, often used for service masts and other demanding applications.
- The 40% fill limit for three or more conductors is implicitly applied in these tables.
Memory trick: RMC, strong and tough, check Annex C, that's enough!