CSLB C-10 Electrical ContractorEquipment and InstallationMedium
A technician must run 9 current-carrying 10 AWG THHN conductors (0.0211 sq in each) in a single EMT raceway. Using the 40% fill allowance from NEC Chapter 9, Table 4 (1/2 inch EMT = 0.122 sq in at 40%; 3/4 inch EMT = 0.213 sq in at 40%), what is the minimum trade size EMT conduit required?
- A1 inch
- B3/4 inch
- C1-1/4 inch
- D1/2 inch
Show answer & explanationAnswer & explanation
Correct answer: B. 3/4 inch
Total conductor area = 9 x 0.0211 = 0.1899 sq in. This exceeds the 1/2 inch EMT 40% fill area of 0.122 sq in but fits within the 3/4 inch EMT 40% fill area of 0.213 sq in, so 3/4 inch EMT is the minimum size.
Why the other options are wrong
- A. 1 inch conduit is oversized; 3/4 inch already satisfies the fill requirement.
- C. 1-1/4 inch is unnecessarily large for this conductor count.
- D. 0.122 sq in is too small to accommodate 0.1899 sq in of conductor fill.
Conduit Fill Calculation
When more than two current-carrying conductors occupy a raceway, total conductor cross-sectional area cannot exceed 40% of the raceway's internal area per NEC Chapter 9, Table 1 and Table 4.
- Multiply individual conductor area by quantity for total fill
- Compare against 40% fill area for the raceway type/size in Table 4
- Round up to the next size if the calculated fill exceeds the allowance
Memory trick: Never pack more than 40% — wires need room to breathe.