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.
- It is not permitted to install conductors operating at 300V or more in a surface metal raceway.
Assuming a 20% fill, the number of conductors allowed is calculated as follows:
- 4 in. x 2 in. = 8 sq. in. (total sq. in. area)
- 20% (fill allowed) x 8 = 1.6 sq. in. (maximum usable space allowed)
- 1.6 sq. in. ÷ 0.0133 (sq. in. area for a single 12 AWG THHN conductor from Chapter 9, Table 5) = 120 (maximum number of conductors allowed)
Reference NEC Secs. 386.10, 386.12, 386.22, and Table 5 in Chapter 9.
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