Box Fill Calculations: Article 314: Boxes; Conduit Bodies; Fittings and Manholes

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. As indicated by the title, this article covers more than just boxes, it also includes manholes and other electric enclosures intended for personnel entry. Article 314 also includes installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies. Fittings used to join raceways include, but are not limited to, couplings. Likewise, fittings used to connect raceways and cables to boxes and conduit bodies include, but are not limited to, connectors (See Figure 1).

The middle sentence in 314.1 clarifies that cast, sheet metal, nonmetallic and other boxes such as FS, FD and larger boxes are not classified as conduit bodies. A conduit body, as defined in Article 100, is a separate portion of a conduit or tubing system that provides access through a removable cover(s) to the interior of the system at a junction of two or more sections of the system or at a terminal point of the system.

Article 314 is divided into five parts: I. Scope and General; II. Installation; III. Construction Specifications; IV. Manholes and Other Electric Enclosures Intended for Personnel Entry; and V. Pull and Junction Boxes for Use on Systems Over 600 Volts, Nominal. Specifications for box fill calculations are in the section titled Installation (Part II).

314.15 Damp, Wet or Hazardous Locations

Boxes, conduit bodies and fittings in damp or wet locations, must be placed or equipped so as to prevent moisture from entering or accumulating within the box [314.15(A)]. Damp locations, as defined in Article 100, are locations protected from weather and are not subject to saturation with water or other liquids. Locations subject to moderate degrees of moisture are classified as damp locations. Partially protected external locations under canopies, marquees, roofed open porches and similar sites are considered damp locations. Interior locations such as certain basements, barns and cold-storage warehouses are also classified as damp locations. Dry locations are not normally subject to dampness or wetness, except on a temporary basis, such as a building under construction.

Boxes, conduit bodies, and fittings installed in wet locations must be listed for that specific use [314.15(A)]. Installations in any of the following categories are wet locations: underground, within concrete slabs, in masonry (directly contacting the earth), areas subject to saturation with water or other liquids (such as vehicle washing areas), and locations unprotected from weather [Article 100]. Weatherproof, also defined in Article 100, is constructed or protected so that exposure to the weather will not interfere with successful operation. The fine print note (FPN) under the definition for weatherproof states that rainproof, raintight or watertight equipment can fulfill the requirements for weatherproof where varying weather conditions other than wetness, such as snow, ice, dust or temperature extremes, are not a factor.

Two fine print notes are under 314.15(A). The first FPN refers the reader to 314.27(C) for boxes in floors, and the second references 300.6 for protection against corrosion.

Hazardous (classified) location installations must comply with Articles 500 through 517 [314.15(B)].

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

Boxes and conduit bodies must be of sufficient size to provide free space for all enclosed conductors (See Figure 2). The term “free space” should not be confused with “free conductor” located in 300.14. Contrary to the opinion of some electricians, there are requirements that limit the number of conductors allowed within outlet, device, pull and junction boxes. Many times, when covers are removed from junction boxes, it is discovered that the box is packed full of conductors, and no consideration was given to the sizes and numbers of conductors. Disregarding box fill requirements can create safety as well as fire hazards (See Figure 3).

Article 314 contains detailed requirements for box fill and junction box sizing. Two sections refer to box fill calculations: 314.16(A) Box Volume Calculations, and 314.16(B) Box Fill Calculations. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as determined in 314.16(B). Requirements for calculating the minimum volume for conduit bodies are located in 314.16(C). Conduit bodies, also referred to as condulets, are manufactured with a variety of configurations. The Type (i.e., LB, LR, LL, C, T, etc.) indicates the number and location of conduit entries. Unlike junction boxes and conduit bodies, terminal housings supplied with motors are not required to meet box fill requirements.

Use 314.16 to determine the size box required for conductors 18 AWG through 6 AWG. These boxes are calculated by using the number of conductors and their respective sizes. Boxes and conduit bodies enclosing conductors 4 AWG or larger must also comply with the provisions of 314.28.

314.16(A) Box Volume Calculations

Various sizes of metal boxes are listed in Table 314.16(A). The table’s first section contains the box trade size in millimeters, and inches, as well as a description of the box. The middle section gives the minimum volume in cubic millimeters and cubic inches. The last section provides the maximum number of 18 through 6 AWG conductors permitted in each of the boxes. The maximum number of conductors is based on the same size conductors in the box. For example, what is the maximum number of 12 AWG conductors permitted in a 4-inch-square box with a depth of 11/2 inches? There will be no fittings or devices in the box. First, look at the left side of Table 314.16(A) and find the row with a 4-inch-square box having a depth of 11/2 inches. Next, follow the row across to the column under 12. The maximum number of 12 AWG conductors permitted in a 4-inch-square box with a depth of 11/2 inches is nine (See Figure 4). This means a total of nine conductors, not nine conductors from each conduit entering the box. This also means the box cannot contain any fittings or devices (See Figure 5). Slang words and phrases are used in every trade, and the electrical trade is no exception. A 4-inch-square metal box is also known as a 1900 box, a 4-S box, or a 4-square box.

The maximum number of conductors in the right section of Table 314.16(A) is based on the box not containing any fittings or devices, such as fixture studs, cable clamps, hickeys, switches, or receptacles. A few items within boxes are not counted such as cable connectors (if the clamping mechanism is outside the box), locknuts and bushings. Where the box contains fittings or devices, the number of conductors in the box must be reduced.

Next month’s Code 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


About the Author

Charles R. Miller

Code Contributor

Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.”...

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