The maximum number of 4/0 THWN/THHN conductors that can be installed in an 8 by 8 by 5 ft gutter is which value?

• A. 4
• B. 25
• C. 30
• D. 39

Answer: (D)

The concept being tested is raceway (gutter) fill. In a gutter, the interior cross-sectional area can only be filled to a certain percentage with conductors. For THHN/THWN conductors in gutters, the NEC commonly uses a 40% fill limit. So, first find the gutter’s usable cross-sectional area: 8 inches by 8 inches gives 64 square inches. Forty percent of that is 25.6 square inches available for conductor cross-sectional area.

Next, know the cross-sectional area of a 4/0 THHN/THWN conductor including its insulation (from NEC tables). That area is about 0.66 square inches per conductor. Dividing the total available area by the area per conductor gives 25.6 / 0.66 ≈ 38.8, which rounds down to 39 conductors. Therefore, the maximum number is 39.

This calculation assumes all conductors are the same THHN/THWN type and uses the 40% fill rule for gutter interiors; derating for ampacity is a separate consideration and doesn’t change the maximum count.