Practice Questions: Raceway Sizing
Real questions from our bank, all referenced to NEC 2026. Tap “Show explanation” after you answer.
Per NEC Chapter 9 Table 1, when installing 3 or more conductors in a single conduit, the maximum conduit fill percentage allowed is:
- A31%
- B40%
- C53%
- D60%
Show explanation
NEC Chapter 9, Table 1 specifies 40% fill for 3 or more conductors. One conductor = 53%, two = 31%, three+ = 40%.
You need to install four 12 AWG THHN conductors in EMT. Per NEC Chapter 9 Table 5, each 12 AWG THHN conductor has an area of 0.0133 sq in. What is the minimum trade size EMT required? (1/2" EMT at 40% = 0.122 sq in; 3/4" EMT at 40% = 0.213 sq in)
- A1/2 inch
- B3/4 inch
- C1 inch
- D1-1/4 inch
Show explanation
Total conductor area = 4 × 0.0133 sq in = 0.0532 sq in. Per NEC Chapter 9 Table 4, 1/2" EMT has an internal area of 0.304 sq in; at 40% fill (Table 1, three or more conductors) the allowable fill is 0.122 sq in, which exceeds the required 0.0532 sq in. Therefore 1/2" EMT is the minimum trade size required. 3/4" EMT is not needed.
How many 12 AWG THHN conductors can be installed in a 3/4" EMT conduit?
- A12 conductors
- B16 conductors
- C18 conductors
- D20 conductors
Show explanation
Per NEC Chapter 9, Table 1, the fill for 3 or more conductors is limited to 40%. From NEC Chapter 9, Table 4, the 40%-fill area of 3/4" EMT = 0.213 sq in. From NEC Chapter 9, Table 5, each 12 AWG THHN conductor = 0.0133 sq in. Maximum conductors = floor(0.213 / 0.0133) = floor(16.0) = 16. This is confirmed by NEC Annex C, Table C.1 (16 conductors of 12 AWG THHN in 3/4" EMT). Correct answer: 16 conductors.
What is the minimum EMT conduit size required for 10 conductors of 14 AWG THHN?
- A1/2" EMT
- B3/4" EMT
- C1" EMT
- D1-1/4" EMT
Show explanation
Per NEC Chapter 9, Table 1, the fill for 3 or more conductors is limited to 40%. Total conductor area = 10 × 0.0097 sq in (14 AWG THHN, per Table 5) = 0.097 sq in. From NEC Chapter 9, Table 4, the 40%-fill area of 3/4" EMT = 0.213 sq in. 0.097 sq in < 0.213 sq in — 3/4" EMT is sufficient. This is confirmed by NEC Annex C, Table C.1: 3/4" EMT accommodates up to 22 conductors of 14 AWG THHN, which exceeds the 10 required. Correct minimum conduit size: 3/4" EMT.
How many 10 AWG THHN conductors fit in a 1-1/4" EMT conduit at 40% fill?
- A16 conductors
- B20 conductors
- C24 conductors
- D28 conductors
Show explanation
Per NEC Chapter 9, Table 1, the fill for 3 or more conductors is limited to 40%. From NEC Chapter 9, Table 4, the 40%-fill area of 1-1/4" EMT = 0.598 sq in. From NEC Chapter 9, Table 5, each 10 AWG THHN = 0.0211 sq in. Maximum conductors = floor(0.598 / 0.0211) = floor(28.34) = 28. This is confirmed by NEC Annex C, Table C.1 (28 conductors of 10 AWG THHN in 1-1/4" EMT). Correct answer: 28 conductors.
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