CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
A raceway will contain three 4/0 AWG THHN copper service conductors plus one 4 AWG THHN equipment grounding conductor. Using Chapter 9, Table 5 conductor areas (4/0 = 0.3237 in², 4 AWG = 0.0824 in²) and the 40% fill rule, what is the minimum trade size EMT required?
- A1-1/2 inch
- B3 inch
- C2-1/2 inch
- D2 inch
Show answer & explanationAnswer & explanation
Correct answer: D. 2 inch
Total conductor area = (3 × 0.3237) + 0.0824 = 0.9711 + 0.0824 = 1.0535 in². A 1-1/2-inch EMT allows only 0.814 in² at 40% fill (insufficient). A 2-inch EMT allows 1.342 in² at 40% fill, which comfortably exceeds the 1.0535 in² required, making it the minimum acceptable size.
Why the other options are wrong
- A. 1-1/2-inch EMT's 40% fill area (0.814 in²) is smaller than the 1.0535 in² needed.
- B. 3-inch EMT is significantly oversized for this conductor group.
- C. 2-1/2-inch EMT is larger than necessary since 2-inch already satisfies the requirement.
Mixed Conductor Conduit Fill
When conductors of different sizes share a raceway, sum their individual cross-sectional areas from Table 5 and compare to the raceway's 40% fill area from Table 4.
- Sum all conductor areas including the EGC
- Use 40% fill allowance for 3 or more conductors
- Round up to the next standard trade size if the total area doesn't fit the smaller size
Memory trick: 'Add them all, then find the box that fits 40%.'