The minimum trade size RMC required for a 45 ft run with four #12 THHN/THWN, three #10 THHN/THWN, and two #6 THHN/THWN conductors is which of the following?

The minimum trade size RMC required for a 45 ft run with four #12 THHN/THWN, three #10 THHN/THWN, and two #6 THHN/THWN conductors is which of the following?

• A. 1 in.
• B. 1-1/4 in.
• C. 3/4 in.
• D. 1/2 in.

Answer: (C)

The main concept is conduit fill: when you run conductors inside a rigid metal conduit, the total space with insulation around each conductor must not exceed the conduit’s allowable fill. Each conductor's diameter (including its insulation) takes up volume inside the raceway, and the NEC sets a limit on how much of the interior can be occupied. So, you determine whether a given conduit size can physically hold all the conductors. For the given set, you have four #12 conductors, three #10 conductors, and two #6 conductors, all THHN/THWN. The insulation on these sizes adds significant diameter, so you must sum the cross-sectional areas of all conductors (and count any grounding conductor per the fill rules). A 1/2 inch RMC simply does not provide enough internal space to accommodate that many and that much insulation. When you move up to 3/4 inch, there is enough internal area to fit all nine conductors without exceeding the allowed fill percentage. Length of the run (45 ft) doesn’t affect this calculation—fill is a function of how much cross-sectional area is occupied inside the conduit, not how long the conduit is. Therefore, the smallest conduit size that meets the fill requirements is 3/4 inch, making it the minimum trade size for this installation.

The main concept is conduit fill: when you run conductors inside a rigid metal conduit, the total space with insulation around each conductor must not exceed the conduit’s allowable fill. Each conductor's diameter (including its insulation) takes up volume inside the raceway, and the NEC sets a limit on how much of the interior can be occupied. So, you determine whether a given conduit size can physically hold all the conductors.

For the given set, you have four #12 conductors, three #10 conductors, and two #6 conductors, all THHN/THWN. The insulation on these sizes adds significant diameter, so you must sum the cross-sectional areas of all conductors (and count any grounding conductor per the fill rules). A 1/2 inch RMC simply does not provide enough internal space to accommodate that many and that much insulation. When you move up to 3/4 inch, there is enough internal area to fit all nine conductors without exceeding the allowed fill percentage.

Length of the run (45 ft) doesn’t affect this calculation—fill is a function of how much cross-sectional area is occupied inside the conduit, not how long the conduit is. Therefore, the smallest conduit size that meets the fill requirements is 3/4 inch, making it the minimum trade size for this installation.