You’ve seen it time and again: an outlet box busting at the seams because someone tried to stuff a few too many devices and wires in too small a box. So how do you know how much is too much? The NEC provides guidelines on how to properly size outlet boxes.

It’s really pretty simple. You must size outlet boxes to be of sufficient size to provide adequate space for conductors, fittings, and devices.

Space used by conductors, fittings, and devices must not exceed the values as specified in Tables 370-16(a) and (b) of the NEC. The volume of a box is the sum of the values of its assembled parts, including plaster rings, covers, and extension rings. Manufacturers don’t have to mark the volume capacity of standard electrical boxes and extension rings with their cubic inch capacity, if their size is listed in Table 370-16(a). Splicing devices and raceway connectors, lock nuts, or bushing don’t affect sizing.

Box volume calculations. When all of the conductors to be enclosed in an outlet box are the same size, use Table 370-16(a)—only if the box contains no fittings or devices—to determine the size outlet box required for the condition or the number of conductors permitted in the outlet box. You don’t have to factor insulation into the calculation.

Example No. 1:

Q. What size outlet box is necessary to accommodate six No. 12 THHN conductors and three No. 12 THW conductors?

(a) 4 x 1 1/4 square
(b) 4 x 1 1/2 square
(c) 4 x 1 1/4 round
(d) 4 x 1 1/2 round

A. The answer is (b), 4 x 1 1/2 square. Refer to Table 370-16(a) and select the type of box that will accommodate nine No. 12 conductors.

Example No. 2: Q. How many No. 14 THHN conductors can be installed in a 421½ round box?

(a) 7
(b) 9
(c) 10
(d) 11

A. The answer is (a), 7 conductors. Again, you refer to Table 370-16(a) to obtain the answer.

Box fill calculations. Only use Table 370-16(a) if the outlet box does not contain any cable clamps, fixture supporting fittings, wiring devices, or equipment-grounding conductors (EGCs). To properly size an outlet box that contains more than just conductors, you must refer to Table 370-16(b). How do you take these devices into account in your calculations? Let’s discuss them individually.

Conductors. Calculate the volume for each conductor that originates outside an outlet box and then terminates or is spliced within that box as the volume of one conductor, as listed in Table 370-16(b). Calculate the volume for each conductor running through the box (without splice) as the volume of one conductor, as listed in Table 370-16(b). You don’t count conductors that originate and terminate within a box for box fill.

When working with a domed fixture or canopy, you can omit an EGC(s) and not more than four fixture wires (smaller than No. 14) from the calculations if they enter the outlet box from a domed fixture or similar canopy.

Cable clamps. Where one or more internal cable clamps (factory or field supplied) are in the box, a single conductor’s volume allowance shall be made in accordance with the volume listed in Table 370-16(b)—based on the largest conductor that enters the outlet box. One or more cable or raceway connectors do not count in box fill calculations.

Fixture support fittings. When installing one or more fixture studs or hickeys, the volume allowance shall be one conductor volume per type, based on the largest conductor entering the box.

Device or equipment fill. For each yoke (strap) with one or more devices, a double volume allowance in accordance with Table 370-16(b) shall be made for each yoke (strap) based on the largest conductor connected to the device(s) or equipment supported by that yoke (strap).

Grounding conductors. When one or more grounding conductors enter a box, consider these as one conductor in your fill calculation. Base the volume calculation on the largest grounding conductor that enters the box.

If you have one No. 14 and two No. 12 grounding conductors entering a box, consider these conductors as one No. 12 conductor. Add a conductor volume for an isolated ground conductor (to reduce electrical noise) [per Sec. 250-146(d)] installed with an EGC. The NEC isn’t clear on how to calculate volumes if you’re installing more than one isolated ground conductor within the same raceway.

Example No. 3:

Q. What is the total number of conductor volumes for a switch wired with 14/2 NM cable and a receptacle wired with 14/2 NM cable? Assume the outlet box contains two cable clamps.

(a) 5
(b) 7
(c) 9
(d) 11

A. The answer is (d), 11 conductors. The sum of individual devices is calculated as follows:

• Switch—Five No. 14 conductors (two conductors for device and three conductors terminating);

• Receptacle—Four No. 14 conductors (two conductors for the device and two terminating);

• Ground wire—One No. 14 conductor; and

• Cable clamps—One No. 14 conductor.

Then add each for a total of 11 No. 14 conductors.

Next month, we’ll continue our discussion on sizing outlet boxes with sample calculations for working with different size conductors, domed fixtures or paddle fan canopies.