What is the maximum number of 4 AWG THHN conductors permitted in 11/2" RMC?

What is the maximum number of 4 AWG THHN conductors permitted in 11/2" RMC?

• A. 7
• B. 10
• C. 8
• D. 6

Answer: (B)

Conduit fill is the main idea here: a raceway can only hold a certain amount of conductors based on how big the inside of the raceway is and how much of that space the conductors are allowed to occupy. For THHN conductors in a rigid metal conduit of a given size, the NEC sets a maximum fill percentage so you don’t crowd the raceway and impede heat dissipation or pulling. In an 11/2" RMC, the allowed fill for conductors is such that you can fit about ten 4 AWG THHN conductors. That comes from comparing the cross-sectional area of each 4 AWG THHN conductor (including its insulation) to the interior cross-sectional area of the 11/2" conduit and applying the standard conduit-fill limit (roughly 40% of the interior area for multiple conductors). When you do that math for 4 AWG THHN in that size of conduit, the total equals about ten conductors. So the reason this is the best answer is that it reflects the conduit-fill limit for 4 AWG THHN in a 1-1/2 inch RMC, not just how many wires you might be able to physically squeeze in. The other counts would either exceed that fill limit or fall short of what the NEC allows for this combination.

Conduit fill is the main idea here: a raceway can only hold a certain amount of conductors based on how big the inside of the raceway is and how much of that space the conductors are allowed to occupy. For THHN conductors in a rigid metal conduit of a given size, the NEC sets a maximum fill percentage so you don’t crowd the raceway and impede heat dissipation or pulling.

In an 11/2" RMC, the allowed fill for conductors is such that you can fit about ten 4 AWG THHN conductors. That comes from comparing the cross-sectional area of each 4 AWG THHN conductor (including its insulation) to the interior cross-sectional area of the 11/2" conduit and applying the standard conduit-fill limit (roughly 40% of the interior area for multiple conductors). When you do that math for 4 AWG THHN in that size of conduit, the total equals about ten conductors.

So the reason this is the best answer is that it reflects the conduit-fill limit for 4 AWG THHN in a 1-1/2 inch RMC, not just how many wires you might be able to physically squeeze in. The other counts would either exceed that fill limit or fall short of what the NEC allows for this combination.