Q: How many 10 AWG THHN conductors fit in a 1-1/4" EMT conduit at 40% fill?

28 conductors (NEC NEC Chapter 9, Tables 1 and 4). 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.

Question 1

Per NEC Chapter 9 Table 1, when installing 3 or more conductors in a single conduit, the maximum conduit fill percentage allowed is:

Accepted Answer

40% (NEC NEC Chapter 9, Table 1). NEC Chapter 9, Table 1 specifies 40% fill for 3 or more conductors. One conductor = 53%, two = 31%, three+ = 40%.

Question 2

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)

Accepted Answer

1/2 inch (NEC NEC Chapter 9, Tables 1, 4, and 5). 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.

Question 3

How many 12 AWG THHN conductors can be installed in a 3/4" EMT conduit?

Accepted Answer

16 conductors (NEC NEC 300.17 and Chapter 9, Table 1). 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.

Question 4

What is the minimum EMT conduit size required for 10 conductors of 14 AWG THHN?

Accepted Answer

3/4" EMT (NEC NEC Chapter 9, Tables 1 and 4). 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.

Question 5

How many 10 AWG THHN conductors fit in a 1-1/4" EMT conduit at 40% fill?

Accepted Answer

28 conductors (NEC NEC Chapter 9, Tables 1 and 4). 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.

Practice Questions: Conduit Fill