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)].

Author’s Comment: The volume capacity of standard electrical boxes and extension rings are not required to be marked with their cubic inch capacity if their size is listed in Table 370.16(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 doesn’t 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.

Note: Table 314.16(A) only applies if the outlet box contains no switches, receptacles, luminaire studs, luminaire hickeys, manufactured cable clamps, or grounding conductors (not likely).

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.

Note: Fixture ground wires smaller than 14 AWG from a domed luminaire or similar canopy are not counted [314.16(B)(1) Ex.].

What’s 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:

Step 1: Determine the number and size of conductors’ equivalents in the box.
Step 2: Determine the volume of the conductors’ equivalents from Table 314.16(B).
Step 3: Size the box by using Table 314.16(A).

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

Step 1: Determine the number and size of conductors.
          14/3 NM                     3 - 14 AWG
          14/2 NM                     2 - 14 AWG
          Cable Clamps           1 - 14 AWG
          Switch                     2 - 14 AWG
          Receptacles           2 - 14 AWG
          Ground wires           1 - 14 AWG
          Total                     11 - 14 AWG
 
Step 2: Determine the volume of the conductors, [Table 314.16(B)].
          14 AWG = 2 cu in.
          14 AWG conductors volume = 11 wires x 2 cu in. = 22 cu in.
 
Step 3: Select the outlet box from Table 314.16(A).
          4 x 11/2 square, 21 cu in. - too small
          4 x 21/8 square, 30.3 cu in. - just right

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.].

Step 1: Determine the number and size of conductors within the box.
          14/2 NM                     2 - 14 AWG
          Cable clamps           1 - 14 AWG
          Ground wire           1 - 14 AWG
          Total                     4 - 14 AWG conductors
 
Step 2: Determine the volume of the conductors [Table 314.16(B)].
          14 AWG = 2 cu in.
          Four 14 AWG conductors = 4 wires x 2 cu in. = 8 cu in.

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 AWG’s          
    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 space.
          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.
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