The cross-sectional area of three or more THW conductors in a conduit must not exceed what percent of the cross-sectional area of the conduit?

• A. 40%
• B. 30%
• C. 60%
• D. 50%

Answer: (A)

Conduit fill is about how much space inside a conduit you can use for conductors without crowding. When there are three or more THW conductors in a conduit, the rule limits the total cross-sectional area of those conductors to 40% of the conduit’s inner cross-sectional area. This keeps enough room for heat dissipation and for pulling wires through the conduit. So you add up the cross-sectional areas of all the conductors, and that sum must not exceed 40% of the conduit’s interior area. The other percentages would either unnecessarily restrict space or risk overheating and difficulty in installation, so 40% is the standard limit for three or more conductors.