CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A commercial building requires a new branch circuit for a motor. The circuit will use 10 AWG THHN copper conductors. The raceway will be a 15-foot run of flexible metal conduit (FMC) connecting to the motor. Per NEC 348.60, what is the maximum number of 10 AWG THHN conductors permitted in a 3/4 inch FMC, assuming no more than 40% fill?
- A5 conductors
- B4 conductors
- C7 conductors
- D6 conductors
Show answer & explanationAnswer & explanation
Correct answer: A. 5 conductors
According to NEC Annex C, Table C.3, a 3/4 inch flexible metal conduit (FMC) can accommodate a maximum of five 10 AWG THHN conductors while adhering to the 40% fill limitation. This table is specific to FMC.
Why the other options are wrong
- B. Four 10 AWG THHN conductors is less than the maximum allowed in 3/4 inch FMC.
- C. Seven 10 AWG THHN conductors greatly exceeds the maximum allowed in 3/4 inch FMC.
- D. Six 10 AWG THHN conductors exceeds the maximum allowed in 3/4 inch FMC.
FMC Conduit Fill
Flexible metal conduit (FMC) fill limits are specified in NEC Annex C tables, similar to rigid conduits, to ensure proper conductor installation and prevent overheating.
- FMC fill limits are generally found in tables like C.3 for specific conductor types and sizes.
- The 40% fill rule applies to FMC with three or more conductors.
- Overfilling FMC can damage conductors during installation and lead to overheating.
Memory trick: Flexible conduit, like a noodle, has fill limits, don't be a doodle.