Consider the following situation: Three black 12 AWG THHN conductors pass directly through a box, uncut and unspliced. Three red 12 AWG THHN conductors are looped in the box. The loop measures 12 inches for each conductor. Six white (grounded) conductors are also looped with 12-inch loops. As would be expected, a green 12 AWG equipment grounding conductor originates at the panelboard and is installed with the branch-circuit conductors, cut into two pieces (each measuring 6 inches) within the box. An equipment bonding jumper is also pigtailed within the box (to the equipment grounding conductors), bonding the equipment grounding conductors to the box, but it doesn’t leave the box. What’s the minimum cubic inch capacity for the box?
A) 29.25 cu. in.
B) 31.50 cu. in.
C) 33.75 cu. in.
D) 49.50 cu. in.
Per a 2005 NEC change, 314.16(B)(1) was revised to address looping of conductors within boxes. If the looped conductor measures 12 inches or more on its exposed length within the box, with reference to section 300.14, each looped conductor will now be considered to be two conductors. As has been the case for some time, a conductor that passes through the box, uncut, unspliced, and not looped 12 inches or more counts as only one conductor. Any conductor that is cut is counted as two conductors. One or more equipment grounding conductors count only as one conductor.
To find the mimimum size in cubic inches of a box for this application, you have to perform the following calculations:
Three black 12 AWG THHN conductors at 2.25 cubic inches each per 314.16(B).
2.25 X 3 = 6.75 cu. in. (uncut, unspliced)
Three red 12 AWG THHN conductors at 2.25 cubic inches each per 314.16(B).
2.25 X 3 X 2 = 13.5 cu. in. (because of the loop)
Six white 12 AWG THHN conductors at 2.25 cubic inches each per 314.16(B).
6 X 2.25 X 2 = 27 cu. in. (because of the loop)
Two green 12 AWG THHN conductors at 2.25 cubic inches each per 314.16(B).
2.25 X 1 = 2.25 cu. in. (because equipment grounding conductors count only as one)
One green 12 AWG THHN pigtail equipment bonding conductor, which originates in the box and doesn’t leave the box does not count.
Minimum box size:
6.75 + 13.5 + 27 + 2.25 = 49.5 cu. in.
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 firstname.lastname@example.org.