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

The National Electrical Code contains provisions that limit the numbers and sizes of conductors that can be installed in boxes and conduit bodies. Article 314 covers the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes, depending on their use. Specifications for calculating the number of conductors in outlet, device, junction boxes and conduit bodies are in 314.16. While box-volume calculations are in 314.16(A), box-fill calculations are in 314.16(B). Provisions pertaining to conduit bodies (also referred to as condulets) are in 314.16(C). Boxes and conduit bodies enclosing conductors larger than 6 AWG (American Wire Gauge) must comply with the provisions of 314.28. Code in Focus columns in March and April covered some of the general requirements for conduit bodies in 314.16(C)(1). This month, the discussion continues with conduit bodies.

314.16(C) Conduit Bodies

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 are allowed by Chapter 9.

The dimensions for conduit bodies containing conductors of 4 AWG, or larger, can be less than required by 314.28(A)(1) and (A)(2), providing the installations of combinations of conductors is less than the maximum conduit or tubing fill (of conduits or tubing being used) permitted by Table 1 of Chapter 9, and the conduit body is listed for, and permanently marked with, the maximum number and maximum size of conductors permitted. [314.28(A)(3)] Since conduit bodies cannot contain the same number and size conductors (4 AWG and larger) as the conduit or tubing being used, determine the number and size conductors that will be installed before selecting the conduit body.

One option is to install a mogul-type conduit body. If a mogul conduit body is not readily available or if the mogul is still not large enough, another option is to install a larger conduit body with reducer fittings. For example, four 300 Kcmil THHN conductors will be installed in a run of 2_-inch rigid metal conduit.

A Type LB conduit body is needed in the run of conduit. The markings on the inside of one brand of conduit body having 2_-inch raceway entries are as follows: 3 300 MCM wire max. Because the conduit will contain four 300 Kcmil conductors, installing this 2_-inch conduit body would be a violation. The manufacturer of this conduit body provides a chart showing maximum number and size 4 AWG, and larger, conductors allowed in their conduit bodies. In accordance with the chart, the next size up conduit body of the same style is listed for a maximum of five 300 MCM conductors. Although the conduit is 2_-inch rigid, a 3-inch conduit body will be installed with reducing (RE) fittings (see Figure 1).

It is not always possible to add one more conductor by increasing the conduit body by one size. For example, the markings on the inside of one conduit body having 3_-inch raceway entries are as follows: 3 500 MCM wire max. A 4-inch conduit body of the same style from the same manufacturer is marked with the following: 3 500 MCM wire max. Although the trade size for one conduit body is 3_ inches and the other is 4 inches, they both hold the same size and number of conductors (see Figure 2).

If the manufacturer does not have a chart showing the maximum number and size conductors allowed in their conduit bodies, a calculation may be necessary. If the conductors installed in the conduit are smaller in size (and greater in number) than what is shown on the inside of the conduit body, the approximate area for the conductors must be calculated.

For example, the markings on the inside of a conduit body having 2_-inch raceway entries are as follows: 3 300 MCM XHHW wire max or equal. The size conductors installed will be 4/0 AWG THHN, not 300 Kcmil XHHW. The number of conductors will be four, not three. It will be necessary to calculate the approximate area of four 4/0 AWG THHN conductors and compare it to the approximate area of three 300 Kcmil conductors. If the area of the 4/0 AWG conductors is less than the area of the 300 Kcmil conductors, the installation will be Code-compliant. First, calculate the area (in square inches or square millimeters) for the conductors that are marked inside the conduit body. This particular conduit body permits installing three 300 Kcmil XHHW conductors. In Table 5 of Chapter 9, the area in square inches for one 300 Kcmil XHHW conductor is 0.4536. The area for three 300 Kcmil XHHW conductors is 1.3608 (3 x 0.4536) square inches (see Figure 3).

Next, calculate the area for the conductors that will be installed in the conduit. Since four 4/0 AWG THHN conductors will be installed, find the area for one of those conductors. The area in square inches for one 4/0 AWG THHN conductor is 0.3237.

The area for all four 4/0 AWG THHN conductors is 1.2948 (4 x 0.3237) square inches. Because the area of the 4/0 AWG conductors (1.2948 square inches) is less than the area of the conductors that are marked in the conduit body (1.3608 square inches), this installation is permitted (see Figure 4).

The conductor size marked in a conduit body is the largest conductor permitted in that conduit body. Even if the approximate area for the installed conductor(s) is less than the area for the marked conductors, installing even one conductor larger than the marked conductor is a violation.

For example, the markings on the inside of a conduit body having 2_-inch raceway entries are as follows: 3 300 MCM XHHW wire max. As calculated in Figure 3, the area for three 300 Kcmil XHHW conductors is 1.3608 (3 x 0.4536) square inches.

Even though the approximate area for one 350 Kcmil THHN conductor is only 0.5242 square inches, installing a conductor larger than the marked conductor is a violation (see Figure 5).

Although there are a variety of configurations for conduit bodies, obtaining the right size for the installed conductors may not always be possible. One option, instead of installing a conduit body, is to install a pull box.

For example, four 500 Kcmil THHN conductors will be installed in a 3-inch raceway. A Type LB conduit body is needed in the run of conduit. The largest readily available conduit body that could be located had the following markings: 3 500 MCM wire max. Since a conduit body capable of enclosing four 500 Kcmil conductors could not be found in time, the installation called for another option.

A pull box, with a conduit entering one end and another entering the back of the opposite end, will be installed. The conductors are larger than 6 AWG and therefore the pull box must be sized in accordance with the specifications in 314.28. The distance between the raceway entries enclosing the same conductors is 18 inches (see Figure 6). Pull and junction-box sizing requirements will be discussed in an upcoming Code in Focus.

Installing splices or taps inside a conduit body may sound like a Code violation. But, under certain conditions, splices, taps or devices can be installed in conduit bodies. Next month's Code in Focus covers these conditions and concludes box-fill calculations. 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.