# Box-Fill Calculations, Part X

314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies

The National Electrical Code has specific requirements detailing how to calculate the maximum number of conductors in boxes (outlet, device, junction, etc.) and conduit bodies. Provisions for boxes and conduit bodies enclosing conductors 6 AWG, or smaller, are in 314.16. Boxes and conduit bodies enclosing conductors larger than 6 AWG must also comply with the provisions of 314.28.

Box-volume calculations are in 314.16(A) and box-fill calculations are in 314.16(B). Requirements for the maximum numbers and sizes of conductors in conduit bodies are in 314.16(C). Under certain conditions, conduit bodies are permitted to contain splices, taps or devices ... these provisions are in 314.16(C)(2). Last month's Code in Focus started a discussion of conduit-body requirements in 314.16(C)(1). This month, the discussion continues with conduit bodies.

314.16(C) Conduit Bodies

The maximum number of conductors a conduit body can contain must not exceed that specified by Table 1 of Chapter 9 for the conduit to which it is attached. [314.16(C)(1)]. Unless they contain splices, taps or devices, conduit bodies are permitted to contain the same number of 6 AWG and smaller conductors as the raceway that enters the conduit body.

For example, a Type C conduit body with ½-in. raceway entries has two ½-in. rigid-metal conduits attached. In accordance with Table 1 of Chapter 9 (and Table C.8 of Annex C), six 10 AWG THHN conductors can be installed in ½-in. rigid-metal conduit. Therefore, six 10 AWG THHN conductors can be installed in this conduit body with ½-in. raceway entries (see Figure 1).

Although the numbers and sizes of conductors contained in a conduit body must not exceed that specified by Table 1 of Chapter 9 for the conduit to which it is attached, sometimes a conduit body is not permitted to contain as many conductors as allowed by Chapter 9. Larger conduit bodies are marked by the manufacturer with the maximum number and maximum size of conductors permitted (see Figure 2).

Most, if not all, manufacturers use the term “MCM” instead of “Kcmil” for conductor sizes larger than 4/0 AWG. The term “MCM” was changed to “Kcmil” in the 1990 edition of the National Electrical Code but is still used today. Both terms have the same meaning; they both stand for 1,000 circular mils. In Roman numerals, “M” represents one thousand. The “CM” stands for circular mils.

For the purpose of the Code, metric units of measurement are in accordance with the modernized metric system known as the International System of Units (SI). Since the metric symbol “M” represents one million, the term “MCM” had to be changed. The letter “K” represents one thousand, and “cmil” stands for circular mils. Conductor sizes up to 4/0 AWG are sized in accordance with the American Wire Gage (AWG). Conductors larger than 4/0 AWG are sized in circular mils. For example, a 4/0 AWG conductor has an area of 211,600 circular mils, and a 250 Kcmil conductor has an area of 250,000 circular mils. For conductors larger than 4/0 AWG, the circular mil area of the conductor is the name of the conductor (see Figure 3).

Pull or junction boxes having raceways containing conductors of 4 AWG or larger must be installed in accordance with the requirements of 314.28. The dimensions for conduit bodies can be less than required by 314.28(A)(1) and (A)(2) as long as the specifications in 314.28(A)(3) are met. First, the installations of combinations of conductors must be less than the maximum conduit or tubing fill (of conduits or tubing being used) permitted by Table 1 of Chapter 9. Also, the conduit body must be listed for, and be permanently marked with, the maximum number and maximum size of conductors permitted.

Not all conduit bodies (having the same-size raceway entries) are permitted to contain the same size and number of conductors. While a 4-in. Type LB conduit body from one manufacturer is permitted to contain three 500 MCM conductors, the same type and size conduit body from a different manufacturer states that the maximum is only three 300 MCM conductors (see Figure 4).

The maximum number marked in the conduit body overrides the maximum number permitted by Table 1 of Chapter 9. As stipulated in 110.3(B), listed and labeled equipment must be installed and used in accordance with any instructions included in the listing or labeling. For example, a run of 3½-in. rigid-metal conduit attaches to a conduit body with 3½-in. raceway entries. The markings on the inside of the conduit body are as follows: “3 500 MCM wire max.” In accordance with Table 1 of Chapter 9 (and Table C.8 of Annex C), five 500 Kcmil THHN conductors are permitted in 3½-in. rigid. But, because of the marking on the inside of the conduit body, no more than three 500 Kcmil conductors are permitted in the conduit body. If this installation requires four 500 Kcmil conductors, this conduit body cannot be installed (see Figure 5).

Since conduit bodies cannot contain the same number and size conductors (4 AWG and larger) as the conduit or tubing being used, it is important to determine the number and size conductors that will be installed before selecting the conduit body. Different types or forms of conduit bodies are manufactured; some are permitted to contain a larger size and/or a greater number of conductors. Some companies manufacture a mogul style or type of conduit body, and as the name applies, it is larger than other conduit bodies. For example, four 500 Kcmil THHN conductors will be installed in a run of 3½-in. rigid-metal conduit. A 3½-in. Type LB conduit body is needed in the run of conduit. The markings on the inside of the 3½-in. conduit body are as follows: “3 500 MCM wire max.” Because the conduit will contain four 500 Kcmil conductors, installing this conduit body would be a violation. Installing a mogul-type conduit body is one option to make this installation Code compliant. The markings on the inside of one 3½-in. mogul conduit body are as follows: “5 500 MCM wire max.” Since only four conductors will be installed, installing this mogul type conduit body is permissible (see Figure 6). Given that not all conduit bodies (having the same-size raceway entries) are permitted to contain the same size and number of conductors, it is imperative to install a conduit body large enough to contain all the conductors that will be installed in the conduit or tubing.

Next month's Code in Focus continues the discussion of box-fill calculations in conduit bodies. EC

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615.333-3336, charles@charlesRmiller.com or www.charlesRmiller.com.