Code Calculations

Aug 24, 2004 5:15 PM, By Mike Holt

When installing conductors of different sizes (or types of insulation) in a raceway, you can’t size the raceway based on Tables 1 through 12 in Annex C. You must instead follow this three-step process to determine the proper raceway and nipple size required for this type of installation.

Step 1: Determine the cross-sectional area (square inches) for each conductor from Table 5 of Chapter 9 for insulated conductors and Table 8 of Chapter 9 for bare conductors.
Step 2: Determine the total cross-sectional area for all conductors.
Step 3: Size the raceway according to the percent fill as listed in Table 1 of Chapter 9; 40% for three or more conductors and 60% for raceways 24 in. or less in length (nipples).

Example 1: A 400A feeder is installed in Schedule 40 rigid nonmetallic conduit. This raceway contains three 500 kcmil THHN conductors, one 250 kcmil THHN conductor, and one 3 AWG THHN conductor. What size raceway is required for these conductors?

Example 2: What size rigid metal nipple is required for three 3/0 AWG THHN conductors, one 1 AWG THHN conductor, and one 6 AWG THHN conductor?

Example 1 answer:
Step 1: Determine the cross-sectional area of the conductors per Table 5 of Chapter 9.
500 kcmil THHN = 0.7073 sq in. x 3 wires = 2.1219 sq in.
250 kcmil THHN = 0.3970 sq in. x 1 wire = 0.3970 sq in.
3 AWG THHN = 0.0973 sq in. x 1 wire = 0.0973 sq in.
Step 2: Calculate the total cross-sectional area of all conductors.
2.1219 + 0.3970 + 0.0973 = 2.6162 sq in.
Step 3: Size the conduit at 40% fill [Chapter 9, Table 1] using Table 4.
A 3-inch Schedule 40 PVC has a cross-sectional area of 2.907 sq in. for conductors, which is just the right size.

Example 2 answer:
Step 1: Determine the cross-sectional area of the conductors per Table 5 of Chapter 9.
3/0 AWG THHN = 0.2679 sq in. x 3 wires = 0.8037 sq in.
1 AWG THHN = 0.1562 sq in. x 1 wire = 0.1562 sq in.
6 AWG THHN = 0.0507 sq in. x 1 wire = 0.0507 sq in.
Step 2: Calculate the total cross-sectional area of all conductors.
0.8037 + 0.1562 + 0.0507 = 1.0106 sq in.
Step 3: Size the conduit at 60% fill [Table 1, Note 4 of Chapter 9] using Table 4.
A 1.5 inch nipple = 1.243 sq in., which is just the right size.