CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A metal wireway has an interior cross-sectional area of 12 square inches. Per NEC 376.22(A), conductors may occupy no more than 20% of this area. What is the maximum allowable conductor fill area?
- A1.2 in²
- B4.8 in²
- C2.4 in²
- D3.6 in²
Show answer & explanationAnswer & explanation
Correct answer: C. 2.4 in²
The 20% fill rule for wireways is calculated as 12 in² × 0.20 = 2.4 in². This limit ensures adequate heat dissipation and ease of conductor installation within the wireway.
Why the other options are wrong
- A. This reflects a 10% fill, not the 20% rule.
- B. This reflects a 40% fill, far exceeding the wireway fill limit.
- D. This reflects a 30% fill, exceeding the allowed limit.
Wireway Fill (376.22)
Conductors in a metal wireway may not exceed 20% of the wireway's interior cross-sectional area.
- 20% max fill for general conductor fill
- Multiply total area by 0.20 to find max fill
- Additional derating applies if more than 30 current-carrying conductors are present
Memory trick: One-fifth full keeps the wireway cool.