Example Problem: What size round or octagon box is required to support a luminaire with 2 - 12-2 w/ ground nonmetallic-sheathed cables (romex) with a fixture stud, two cable clamps, two pigtails, and 2 - 18 AWG fixture wires? (No luminaire canopy)
Step 1: Combination conductors
314.16(B)(1) through (B)(5) and Table 314.16(B)
2 - 12 AWG ungrounded (phase) conductors
2.25 cu. in. x 2 = 4.5 cu. in.
2 - 12 AWG grounded (neutral) conductors
2.25 cu. in. x 2 = 4.5 cu. in.
2 - 12 AWG equipment grounding conductors
2.25 cu. in. x 1 = 2.25 cu. in.
1 fixture stud
2.25 cu. in. x 1 = 2.25 cu. in.
2 cable clamps
2.25 cu. in. x 1 = 2.25 cu. in.
2 pigtails = 0 cu. in.
2 - 18 AWG fixture wires
1.5 cu. in. x 2 = 3 cu. in.
Total = 18.75 cu. in.
Step 2: Selecting box
Table 314.16(A)
18.75 cu. in. requires 4" x 2-1/8" (100 mm x 54 mm) octagon box
Solution: A 4 in. x 2-1/8 in. (100 mm x 54 mm) octagon box is required.
