Have you ever wondered if the outlet box you're planning to install is large enough? What if a box doesn't even have its capacity marked on it? The answers to those questions lie in the requirements of Art. 314. In fact, Table 314.16(A) tells you the size of the most common standard boxes and how much they can hold. The minimum box volume must allow for conductors, devices, and fittings. The volume of a box includes its assembled parts, including plaster rings, industrial raised covers, and extension rings.

Sizing boxes when all conductors are the same size.

When the conductors running to the box are all the same size (insulation doesn't matter), you can use Table 314.16(A) to determine the number of conductors permitted in the outlet box or 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, although this is unlikely.

Let's work through a few examples to clarify the calculation.

Example No. 1: What size outlet box is required for six 12 AWG THHN conductors and three 12 AWG THW conductors?

(a) 4×1.25-inch square box

(b) 4×1.5-inch square box

(c) 4×1.25-inch round box

(d) 4×1.5-inch round box

The answer is (b), 4×1.5-inch square box. This answer comes directly from Table 314.16(A), which permits nine 12 AWG conductors in this size box. Note that insulation isn't a factor.

Example No. 2: Using Table 314.16(A), how many 14 AWG THHN conductors are permitted in a 4×1.5-inch round box?

(a) 7 conductors

(b) 9 conductors

(c) 10 conductors

(d) 11 conductors

The answer is (a), 7 conductors.

Table 314.16(A) doesn't 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 following fill calculations.

Conductor terminating in the box.

Each conductor that originates outside the box and terminates or is spliced within the box must be counted as one conductor.

Conductor running through the box Each conductor that runs through the box is counted as one conductor. If no part of a conductor leaves the box, it shall not be counted. This includes equipment bonding jumpers and pigtails (Fig. 1). Fixture wires smaller than 14 AWG from a domed luminaire or similar canopy aren't counted.

Cable fill.

One or more internal cable clamps in the box are counted 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 (Fig. 2). Small fittings, such as locknuts and bushings, aren't counted [314.16(B)].

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.

Device or equipment fill

Each yoke or strap that contains one or more devices or pieces of equipment is counted as two conductors, based on the largest conductor that terminates on the yoke.

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. Fixture ground wires smaller than 14 AWG from a domed luminaire or similar canopy aren't counted [314.16(B)(1) Ex.].

Wirenuts, cable connectors, raceway fittings and conductors that originate and terminate within the outlet box, such as equipment bonding jumpers and pigtails, aren't counted for box fill calculations [314.16(A)].

Let's work through another example calculation to help solidify your understanding of the requirements.

Example No. 3: What is the total number of conductors required for box fill calculations for one 14/3 NM cable for a three-way switch and one 14/2 NM cable for a duplex receptacle (Fig. 3)?

(a) 5 conductors

(b) 7 conductors

(c) 9 conductors

(d) 11 conductors

The answer is (d), 11 conductors. The tally breaks down as follows:

  • Five 14 AWG conductors for the switch — two conductors for the device and three conductors terminating

  • Four 14 AWG conductors for the receptacle — two conductors for the device and two conductors terminating

  • One conductor for the ground wire

  • One conductor for the cable clamps

Sizing boxes when working with different size conductors.

To determine the size of the outlet box when the conductors are different sizes (insulation isn't a factor), use the following steps:

Step 1: Determine the number and size of conductors' equivalents in the box.

Step 2: Use Table 314.16(B) to determine the volume of the conductors' equivalents.

Step 3: Size the box by using Table 314.16(A).

We'll reinforce the steps used in this procedure by working through another couple of examples.

Example No. 4: What size outlet box is required for 14/3 Type NM cable (with ground) that terminates on a switch, with 12/2 NM that terminates on a receptacle, if the box has internal, factory installed cable clamps (Fig. 4)?

(a) 4×1.25-inch square box

(b) 4×1.5-inch square box

(c) 4×2.125-inch square box

(d) any of these boxes

Step 1: Determine the number and size of conductors.

  • Three 14 AWG conductors for the 14/3 NM

  • Two 14 AWG conductors for the switch

For a total of five 14 AWG conductors

  • Two 12 AWG conductors for the 12/2 NM

  • One 12 AWG conductor for the cable clamps

  • Two 12 AWG conductors for the receptacle

  • One 12 AWG conductor for the ground wires

For a total of six 12 AWG conductors

When there are different size conductors for internal cable clamps, the volume is based on the largest conductor entering the box [314.16(B)(2)]. Where more than one equipment grounding conductor enters a box, the volume is based on the largest grounding conductor that enters the box [314.16(B)(5)].

Step 2: Determine the volume of the conductors per Table 314.16(B).

14 AWG=2 cu in.

14 AWG conductors volume=5 wires×2 cu in.=10 cu in.

12 AWG=2.25 cu in.

12 AWG conductors volume=6 wires×2.25 cu in. = 13.5 cu in.

Total conductor volume=10 cu in.+13.5 cu in.=23.5 cu in.

Step 3: Select the outlet box from Table 314.16(A).

4×1.25-in. square box=21 cu in. (too small)

4×2.125-in. square box=30.3 cu in. (just right)

Example No. 5: A round 4×0.5-inch pancake box has a total volume of 7 cubic inches. It's equipped with factory installed internal cable clamps. Can this pancake box be installed 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 (Fig. 5).

Step 1: Determine the number and size of conductors within the box.

  • Two 14 AWG conductors for the 14/2 NM

  • One 14 AWG conductor for the cable clamps

  • One 14 AWG conductor for the ground wire

For a total of four 14 AWG conductors

Step 2: Determine the volume of the conductors per Table 314.16(B).

14 AWG=2 cu in.

Four 14 AWG conductors=4 wires×2 cu in.=8 cu in.

Since the box is limited to 7 cubic inches of fill, this installation would violate the requirements of the Code. Therefore, the answer is no.

Example No. 6: How many 14 AWG THHN conductors can be pulled through a 4 × 2.125-inch square box that has a plaster ring of 3.6 cubic inches? The box already contains two receptacles, five 12 AWG THHN conductors, and one 12 AWG bare grounding conductor.

(a) 4 conductors

(b) 5 conductors

(c) 6 conductors

(d) 7 conductors

Step 1: Determine the number and size of the existing conductors.

  • Four 12 AWG conductors (two yokes×two conductors) for the two receptacles

  • Five 12 AWG conductors

  • One 12 AWG conductor for the ground wires

For a total of 10 12 AWG conductors

Step 2: Determine the volume of the existing conductors per Table 314.16(B).

12 AWG conductor=2.25 cu in.

10 wires×2.25 cu in.=22.5 cu in.

Step 3: Determine the space remaining for the additional 14 AWG conductors. Remaining space is the total space less the volume of the 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.

Remaining space = 11.4 cu in.

Step 4: Determine the number of 14 AWG conductors that would be permitted in the spare space by dividing the remaining space by the added conductors' volume.

Additional conductors permitted=11.4 cu in.÷2 cu in. [Table 314.16(B)]

Five more conductors are permitted.

Therefore, you could add five 14 AWG conductors to this box and still meet the requirements of the Code.