Article 314: Boxes, Conduit Bodies, Fittings and Manholes

314.16(A) Box Volume Calculations

Article 314 of the National Electrical Code covers the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes depending on their use. This article also covers manholes and other electric enclosures intended for personnel entry. The last sentence of Article 314’s scope (314.1) states that the article also includes installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies. Last month’s In Focus concluded by covering 314.16(A). This month, the discussion continues with Standard Boxes [314.16(A)(1)].

The volumes of standard boxes that are not marked with their volume shall be as given in Table 314.16(A). While most boxes must be marked with their volume, it is not mandatory for all boxes. A metal box matching the trade size of one in Table 314.16(A) may or may not be marked by the manufacturer with the volume because it is not required. When the volume is not marked on or in a box, use the cubic inch (or cubic centimeter) volume listed in Table 314.16(A).

The box’s volume must be durably and legibly marked on the box by the manufacturer if the size of the box is 100 cubic inches or less, or if the box is nonmetallic [314.16(A)(2)]. For example, a 4-inch octagon concrete box having a depth of 2 inches will be installed on a job. In order to determine the maximum number of conductors permitted in this box, the volume of the box must be known. Table 314.16(A) lists 4-inch octagon boxes, but not ones having a depth of 2 inches. Since this box is less than 100 cubic inches and it is not listed in the table, the volume must be marked on the box. The volume marked on the box in this example is 23.0 cubic inches (See Figure 1).

All nonmetallic boxes must also be durably and legibly marked with their volume. Although not required, some nonmetallic boxes are marked with the maximum number of conductors permitted within the box. Usually, several size conductors are listed, such as 14 AWG, 12 AWG and 10 AWG. When other size conductors are installed, or when different sizes of conductors are installed, calculate the box fill in accordance with 314.16(B) and Table 314.16(B) (See Figure 2).

The conductors marked on a nonmetallic box are the maximum numbers that can be installed where no volume allowances are required by 314.16(B)(2) through 314.16(B)(5). This means the box must not contain any fittings or devices. For example, a nonmetallic box will be installed as a junction box for three 12-2 (with ground) nonmetallic-sheathed cables. The maximum numbers of conductors are marked inside the box as follows: nine 14 AWG or eight 12 AWG or seven 10 AWG conductors. Because this will be a junction box, no fittings or devices will be in the box. After the conductors are joined together, a blank cover will be installed on the box.

The maximum number of conductors can be installed because there are no volume allowances for items such as fittings or devices in this box. Wire connectors (wirenuts) will be in the box, but they are not counted in box-fill calculations. The installation of this box complies with 314.16 (See Figure 3).

Sometimes there is some confusion regarding the numbers of conductors listed inside a nonmetallic box. Some people think a receptacle or switch can be installed in addition to the numbers of conductors marked on the box, but this is not correct. A reduction of conductors must occur in boxes containing fittings or devices such as fixture studs, cable clamps, hickeys, switches or receptacles. For example, the markings inside a nonmetallic box showing the maximum numbers of conductors are as follows: nine 14 AWG or eight 12 AWG or seven 10 AWG conductors. As previously mentioned, if the box is used as a junction box, the maximum number of conductors can be installed. The conductors marked on the box cannot be installed if the box contains a receptacle, switch or device (See Figure 4).

Boxes described in Table 314.16(A) that have a volume larger than is designated in the table can have their volume marked as required by 314.16 [314.16(A)(2)]. It is permissible to use the larger volume even though it is more than the volume listed in the table for the same size box. For example, a 4-inch octagon metal box that is 21/8 inches deep has a marked volume of 22.5 cubic inches. The same size box listed in Table 314.16(A) has a volume of only 21.5 cubic inches. Although the volume in the table shows 21.5 cubic inches for a 4-inch octagon box that has a depth of 21/8 inches, the actual volume is 22.5 cubic inches because it is marked on the box (See Figure 5). Since the box in this example does not match the volume of a box in Table 314.16(A), the maximum number of conductors will have to be calculated. This type of box-fill calculation will be discussed at a later time.

Table 314.16(A) could be helpful even if the box is a nonmetallic box. If the volume marked on a box matches the volume for one of the boxes in the table, the table could provide the maximum number of conductors permitted in the box. The box could be a different size than the one listed or it could be a nonmetallic box. For example, what are the maximum numbers of 18 through 6 AWG conductors permitted in a single-gang nonmetallic junction box that has a volume allowance of 18 cubic inches? Since 18 cubic inches is one of the volumes listed in Table 314.16(A), no calculation is necessary. Find 18 cubic inches in the “minimum volume” column and follow the row across to find the conductors permitted. The maximum conductors permitted are: 12 18 AWG, 10 16 AWG, nine 14 AWG, eight 12 AWG, seven 10 AWG, six 8 AWG and three 6 AWG conductors. Remember, these numbers are based on the box not containing any fittings or devices.

Next month’s In Focus continues the discussion of 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, or