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©. 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).