CSLB C-10 Electrical ContractorPlanning and EstimatingHard
An electrician is performing a material takeoff for a new service entrance to a small commercial building. The service will consist of four 3/0 AWG THHN copper conductors. What is the minimum trade size of rigid metal conduit (RMC) required to enclose these conductors, assuming a straight run?
- A2 1/2 inch
- B2 inch
- C1 1/4 inch
- D1 1/2 inch
Show answer & explanationAnswer & explanation
Correct answer: B. 2 inch
To accommodate four 3/0 AWG THHN copper conductors, a minimum of 2-inch RMC conduit is required. This is based on calculating the total cross-sectional area of the conductors and matching it to the permissible fill area for RMC in NEC Chapter 9, Table 4 and Annex C.
Why the other options are wrong
- A. 2 1/2 inch RMC is oversized for this application.
- C. 1 1/4 inch RMC is too small for four 3/0 AWG conductors.
- D. 1 1/2 inch RMC is also too small, exceeding the 40% fill limit.
RMC Conduit Fill
The process of selecting the appropriate Rigid Metal Conduit (RMC) size to safely contain a specified number and size of conductors, adhering to NEC fill percentages.
- RMC has larger internal diameter compared to other conduits of the same trade size.
- Conductor areas are found in NEC Chapter 9, Table 5.
- Conduit fill percentages (e.g., 40% for 3 or more conductors) are in NEC Chapter 9, Table 1.
Memory trick: RMC: 'R'eal 'M'etal 'C'onduit, 'R'eally 'M'ust 'C'alculate 'Fill'.