CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A technician must install 8 current-carrying 10 AWG THHN conductors in a single EMT raceway. Using a 40% fill allowance, what is the minimum trade size of EMT required?
- A3/4 inch
- B1/2 inch
- C1 inch
- D1-1/4 inch
Show answer & explanationAnswer & explanation
Correct answer: A. 3/4 inch
Each 10 AWG THHN conductor has a cross-sectional area of 0.0211 in² (NEC Chapter 9, Table 5). Total area = 8 × 0.0211 = 0.1688 in². A 1/2-inch EMT allows only 0.122 in² at 40% fill, which is insufficient. A 3/4-inch EMT allows 0.213 in² at 40% fill, which comfortably accommodates the 0.1688 in² total.
Why the other options are wrong
- B. 1/2-inch EMT's 40% fill area (0.122 in²) is too small for 0.1688 in² of conductors.
- C. 1-inch EMT is oversized; 3/4-inch already satisfies the fill requirement.
- D. 1-1/4-inch is unnecessarily large for this conductor count.
Conduit Fill (40% Rule)
When more than two conductors occupy a raceway, NEC Chapter 9 Table 1 limits fill to 40% of the raceway's internal cross-sectional area.
- Use Table 5 for conductor area, Table 4 for raceway area
- 40% applies to 3+ conductors; 31% for 2, 53% for 1
- Always round up to next larger trade size if exact fit is close
Memory trick: '40 for more than two' — the fill rule for crowded raceways.