CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
A raceway will contain one 4 AWG THHN copper conductor (0.0824 in²), two 8 AWG THHN copper conductors (0.0366 in² each), and two 10 AWG THHN copper conductors (0.0211 in² each). Using the 40% fill rule for three or more conductors, what is the minimum trade size EMT required?
- A1/2 inch
- B3/4 inch
- C1 inch
- D1-1/4 inch
Show answer & explanationAnswer & explanation
Correct answer: B. 3/4 inch
Total conductor area = 0.0824 + (2 × 0.0366) + (2 × 0.0211) = 0.0824 + 0.0733 + 0.0422 = 0.1979 in². Per NEC Chapter 9, Table 4, 1/2-inch EMT provides only 0.122 in² at 40% fill (insufficient), while 3/4-inch EMT provides 0.213 in² at 40% fill, which comfortably accommodates the 0.1979 in² of conductor area.
Why the other options are wrong
- A. 1/2-inch EMT's 40% fill area (0.122 in²) is too small for the 0.1979 in² of conductors.
- C. 1-inch EMT is larger than necessary and not the minimum size required.
- D. 1-1/4-inch EMT greatly exceeds the required minimum size.
Mixed Conductor Conduit Fill Calculation
When multiple conductor sizes share a raceway, total their cross-sectional areas and compare to the 40% fill area from NEC Chapter 9 Table 4 to find the minimum trade size.
- Use Table 5 for individual conductor areas
- Use Table 4 for raceway 40% fill areas
- Round up to the next trade size that fits total area
Memory trick: Add up all the wire areas, then find a pipe big enough for 40% comfort