OUTLET BOX SIZING
By Mike Holt for EC&M Magazine
Outlet boxes shall be of sufficient size to provide free space for all conductors. An outlet box is generally used for the attachment of devices and luminaires and has a specific amount of space (volume) for conductors, devices and fittings. The volume taken up by conductors, devices and fittings in a box must not exceed the box fill capacity. The volume of a box is the total volume of its assembled parts, including plaster rings, industrial raised covers and extension rings. The total volume includes only those fittings that are marked with its volume in cubic inches [314.16(A)].
SIZING BOX - CONDUCTORS ALL THE SAME SIZE [Table 314.16(A)]
When all of the conductors in an outlet box are the same size (insulation doesnt matter), Table 314.16(A) of the NEC can be used to:
(1) Determine the number of conductors permitted in the outlet box, or
(2) Determine the size outlet box required for the given number of conductors.
Outlet Box Size
What size outlet box is required for six 12 AWG THHN conductors and three 12 AWG THW conductors? Figure 5-14.
(a) 4 x 11/4 in. square (b) 4 x 11/2 in. square
(c) 4 x 11/4 in. round (d) 4 x 11/2 in. round
Answer: (b) 4 x 11/2 in. square
Table 314.16(A) permits nine 12 AWG conductors; insulation is not a factor.
Number of Conductors in Outlet Box
Using Table 314.16(A), how many 14 AWG THHN conductors are permitted in a 4 x 11/2 in. round box?
(a) 7 conductors (b) 9 conductors
(c) 10 conductors (d) 11 conductors
Answer: (a) 7 conductors
CONDUCTOR EQUIVALENTS [314.16(B)]
Table 314.16(A) does not take into consideration the fill requirements of clamps, support fittings, devices or equipment grounding conductors within the outlet box. In no case can the volume of the box and its assembled sections be less than the fill calculation as listed below:
(1)(a) Conductor Terminating in the Box. Each conductor that originates outside the box and terminates or is spliced within the box is considered as one conductor. Figure 5-15.
(1)(b) Conductor Running Through the Box. Each conductor that runs through
the box is considered as one conductor. Figure 5-15. Conductors, no part of which leave the box, shall
not be counted. This includes equipment bonding jumpers and pigtails. Figure 5-16.
Exception. Fixture wires smaller than 14 AWG from a domed luminaire or similar canopy are not counted. Figure 5-17.
(2) Cable Fill. One or more internal cable clamps in the box are considered
as one conductor volume in accordance with the volume listed in Table 314.16(B) based on the largest
conductor that enters the outlet box. Figure 5-18.
Note: Small fittings such as locknuts and bushings are not counted [314.16(B)].
(3) Support Fittings Fill. One or more luminaire studs or hickeys within the box are considered as one conductor volume, based on the largest conductor that enters the outlet box. Figure 5-19.
(4) Device or Equipment Fill. Each yoke or strap containing one or more devices or equipment is considered as two conductors, based on the largest conductor that terminates on the yoke. Figure 5-20.
(5) Grounding Conductors. One or more grounding conductors are considered as one conductor volume in accordance with the volume based on the largest grounding conductor that enters the outlet box. Figure 5-21.
Whats not Counted in outlet boxes.
Wirenuts, cable connectors, raceway fittings and conductors that originate and terminate within the outlet box (such as equipment bonding jumpers and pigtails) are not counted for box fill calculations [314.16(A)].
Number of Conductors
What is the total number of conductors required for box fill calculations for the following? Figure 5-22?
One 14/3 NM cable for a three-way switch
One 14/2 NM cable for a duplex receptacle
(a) 5 conductors (b) 7 conductors
(c) 9 conductors (d) 11 conductors
Answer: (d) 11 conductors
SIZING BOX - DIFFERENT SIZE CONDUCTORS [314.16(B)]
To determine the size of the outlet box when the conductors are of different sizes (insulation is not a factor), the following steps can be used:
Outlet Box Sizing
What size outlet box is required for 14/3 Type NM cable (with ground) that terminates on a switch, with 14/2 NM that terminates on a receptacle, if the box has internal cable clamps factory installed? Figure 5-22.
(a) 4 x 11/4 square (b) 4 x 11/2 square
(c) 4 x 21/8 square (d) any of these
Answer: (c) 4 x 21/8 square
Domed Fixture Canopy [314.16(B)(1), Ex.].
A round 4 x 1/2 in. box has a total volume of 7 cubic inches and has factory internal cable clamps. Can this pancake box be used with a lighting luminaire that has a domed canopy? The branch-circuit wiring is 14/2 nonmetallic-sheath cable and the luminaire has three fixture wires and one ground wire all smaller than 14 AWG. Figure 5-23.
(a) Yes (b) No
Answer: (b) No, The box is limited to 7 cu in., and the conductors total 8 cu in. [314.16(B)(1) Ex.].
Conductors Added to Existing Box
How many 14 AWG THHN conductors can be pulled through a 4 x 21/8 square box that has a plaster ring of 3.6 cu in.? The box already contains two receptacles, five 12 AWG THHN conductors and one 12 AWG bare grounding conductor. Figure 5-24.
(a) 4 conductors (b) 5 conductors
(c) 6 conductors (d) 7 conductors
Answer: (b) 5 conductors
Step 1: Determine the number and size of the existing conductors.
Two Receptacles 4 - 12 AWG conductors (2 yokes x 2 conductors)
Five 12 AWGs 5 - 12 AWG conductors
One ground 1 - 12 AWG conductor
Total 10 - 12 AWG conductors
Step 2: Determine the volume of the existing conductors [Table 314.16(B)].
12 AWG conductor = 2.25 cu in., 10 wires x 2.25 cu in. = 22.5 cu in.
Step 3: Determine the space remaining for the additional 14 AWG conductors.
Remaining space = Total space less existing conductors
Total space = 30.3 cu in. (box) [Table 314.16(A)] + 3.6 cu in. (ring) = 33.9 cu in.
Remaining space = 33.9 cu in. - 22.5 cu in. (ten 12 AWG conductors)
Remaining space = 11.4 cu in.
Step 4: Determine the number of 14 AWG conductors permitted in the spare
Conductors added = Remaining space/added conductors volume
Conductors added = 11.4 cu in./2 cu in. [Table 314.16(B)]
Conductors added = 5
Copyright © 2002 Mike Holt Enterprises,Inc.