In a surface metal raceway that measures 4 in. by 2 in., what is the maximum number of 12 AWG THHN, 600V insulated, single conductors, operating at 480V nominal permitted? The surface metal raceway is designed to contain a maximum of 255 conductors, and the thickness of the metal is 0.040 in. nominal.

  1. 60
  2. 120
  3. 255
  4. It is not permitted to install conductors operating at 300V or more in a surface metal raceway.

Answer: C

Assuming a 20% fill, the number of conductors allowed is calculated as follows:

  1. 4 in. x 2 in. = 8 sq. in. (total sq. in. area)
  2. 20% (fill allowed) x 8 = 1.6 sq. in. (maximum usable space allowed)
  3. 1.6 sq. in. รท 0.0133 (sq. in. area for a single 12 AWG THHN conductor from Chapter 9, Table 5) = 120 (maximum number of conductors allowed)
However, the surface metal raceway is designed for a maximum of 255 - 12 THHN or THWN conductors. Therefore, the maximum number of conductors allowed is actually 255 (per the manufacturer's design).

Reference NEC Secs. 386.10, 386.12, 386.22, and Table 5 in Chapter 9.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.