Box Fill Calculations, Part I, appearing in June’s issue, introduced box fill, with emphasis on Section 370-16(a). Notably, Table 370-16(a) is used to determine the maximum number of conductors (for sizes No. 18 through No. 6) permitted within common-size metal boxes. The number listed, under each conductor size within the table, is the maximum allowed within a box containing no fittings or devices, such as fixture studs, cable clamps, hickeys, switches, or receptacles.

Last month’s In Focus (Part II) began a discussion of Section 370-16(b), concentrating on Section 370-16(b)(1)’s conductor fill. Key points addressed the calculations performed when a box contains different size conductors, as well as those instances where the box is not listed in Table 370-16(a). In either case, each conductor’s cubic-inch volume is needed. This information is found in Table 370-16(b) for No. 18 through No. 6 conductors.

Box fill calculations require full comprehension of which items are counted and how much volume is required for them.
This month’s issue of In Focus concludes the procedures for calculating box fill. Specific explanations are included for clamp fill, support fittings fill, device or equipment fill, equipment grounding conductor fill, and conduit bodies.

370-16(b)(2) Clamp Fill

Internal cable clamp(s), whether factory or field supplied, are counted as one conductor. Whenever a box containing different size conductors also contains one or more internal cable clamps, the clamps are counted as another one of the largest conductors. Cable connector(s), configured so that the clamping mechanism falls outside the box, are not counted. Likewise, do not count external connectors for conduit and tubing (EMT, PVC, ENT, etc.)

370-16(b)(3) Support Fittings Fill

Each fixture stud or hickey, within the box, counts as one conductor. If different size conductors are present in the box, the fixture stud or hickey counts as the largest conductor. The most common reference to a hickey is that of a conduit bender, used to bend small sizes of rigid metal conduit.

The obvious question is this: How can a conduit bender (hickey) be located inside an outlet box? The answer, of course, is that it can’t. A less often used type of hickey, which can occur within a box, is a type of coupling. This mechanical device joins the fixture stem to the fixture stud. First this hickey screws onto a fixture stud, and then, a fixture stem attaches to the hickey. The fixture conductors, routed through the fixture stem, exit the hickey’s opening and enter the outlet box. Once in the box, the fixture conductors are spliced to the circuit conductors, which are then concealed by the fixture canopy.

370-16(b)(4) Device or Equipment Fill

A mounting yoke or strap can contain one or more devices (or pieces of equipment), such as a single receptacle, a duplex receptacle, a single switch, a double switch, a triple switch, or any combination. Unlike fittings, which count as one, a mounting yoke or strap must be counted as two conductors, the size of which must be equal to the largest conductor connected to the device. If conductors of different sizes are present in the box, the conductor connected to the device determines its cubic-inch volume.

Example—No. 12 conductors are connected to a duplex receptacle. The volume required, within a box, for that device is 4.5 cu. in. (2 x 2.25). If, however, No. 14 conductors are used, instead of No. 12, the volume required is only 4 cu. in. (2 x 2).

When all of the conductors within the box are the same size, the calculation is easy. Simply subtract two for each mounting yoke or strap from the maximum number allowed. Seven No. 14 conductors are permitted in a 3 x 2 x 2.75-in. metal device box containing no fittings or devices. Installing a receptacle, switch, etc. in the box reduces the maximum number from seven to only five.

Of course, the longer calculation provides the same results. The same box’s volume is 14 cu. in. The receptacle, switch, etc., has a volume of four cu. in. Fourteen cu. in. minus four leaves a volume of 10 cu. in. Ten cu. in. divided by the volume required for each No. 14 conductor (two cu. in.), results in the same number of conductors permitted: five.

370-16(b)(5) Equipment Grounding Conductor Fill

One or more equipment grounding conductors, or equipment bonding jumpers, entering a box count as only one conductor. Grounding conductor fill calculations differ from those for other conductors. Each grounded (neutral) conductor and each ungrounded (hot) conductor, originating outside the box and terminating inside the box, counts as one conductor. Even if the total number of equipment grounding conductors entering the box is four, those four count as only one.Count only the largest equipment grounding conductor where multiple grounding conductors of different sizes enter the box.

One or more additional isolated equipment grounding conductors, as permitted in Section 250-146(d), count as one conductor. Say, for instance, that an isolated ground receptacle is installed to reduce electrical noise (electromagnetic interference). The receptacle’s grounding terminal is purposely insulated from the receptacle mounting. An insulated grounding conductor, running with the circuit conductors, is then attached to the receptacle’s grounding terminal. [Section 250-146(d)] Using an isolated equipment grounding conductor does not satisfy the requirement for grounding the raceway system and outlet box. [Section 250-146(d) FPN]

As with equipment grounding conductors, wherever multiple isolated equipment grounding conductors of different sizes enter the box, only the largest is counted.

370-16(c) Conduit Bodies

Conduit bodies (also referred to as condulets) enclosing conductors no larger than No. 6, must have a cross-sectional area of not less than twice the cross-sectional area of the largest conduit or tubing to which it is attached.

While this requirement seems more like a design criterion for manufacturers, it does have practical installation applications. This rule, then, obviously prohibits using a conduit body (enclosing No. 6, or smaller, conductors) which is smaller than the largest conduit or tubing to which it is attached.

Reducer fittings (on the raceway) are not allowed to reduce the conduit body size. For example, installing one-in. conduit with reducer fittings into a .75-in. conduit body is a violation, yet installing reducing fittings in the conduit body is not. For example, installing a 1-in. conduit body with reducer fittings onto .75-in. conduit is not a violation.

Compliance with this provision is not required for short radius conduit bodies, such as capped elbows and service-entrance elbows, enclosing conductors no larger than No. 6. These short radius conduit bodies must be of sufficient size to provide free space for all enclosed conductors, and must not contain splices, taps, or devices.

[Section 370-5]

The maximum number of conductors a conduit body must not exceed that specified by Table 1 of Chapter 9 for the conduit to which it is attached. Smaller-size conduit bodies can generally hold the same number and sizes of conductors as the raceway entering the conduit body, provided the conductors are No. 6 and smaller.

370-16(c)(2) Conduit Bodies with Splices, Taps, and Devices

Splices, taps, and devices are permitted inside conduit bodies, provided that the manufacturer marks the cubic-inch capacity durably and legibly on the conduit body. Calculate the maximum number of conductors in accordance with Section 370-16(b).

A conduit body must be rigidly, and securely, supported. This support may be effectively achieved by using any of the following: rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing. [370-23(e) Exception]

MILLER, owner of Lighthouse Educational Services and author of Illustrated Guide to the National Electrical Code, can be reached by phone at (615) 333-3336, or via e-mail at