Total volume allowance is the volume of the box plus the volume of any attached plaster rings, extension rings, or domed cover. If the extension ring is made of metal and corresponds to a size listed in NEC Table 314.16(A), then the appropriate volume for that size box is used as the volume of the extension ring. Where used, the space provided by plaster rings, domed covers, and extension rings - all of which are marked with their volume in cubic inches - is included. The volume of a box is the total volume in cubic millimeters or cubic inches of the assembled sections. Boxes that enclose devices or utilization equipment are to be sized based on the conductors that supply the devices or utilization equipment. This will prevent overcrowding and possible physical damage when the devices or splices are installed. ![]() All boxes (enclosures) must be large enough to provide sufficient free space for all conductors and devices that will be enclosed within them. The NEC includes general rules for boxes, such as sizing and support provisions. 314 (outlet, device, pull, and junction boxes conduit bodies fittings and handhole enclosures). The National Electrical Code (NEC) contains sufficient rules and requirements applying to box fill calculation requirements. These conductors must have ample free space to allow proper dissipation of heat and not cause damage to the surrounding insulation of said conductors. When put to work carrying the loads they were designed to carry, the enclosed conductors produce heat from current flow. ![]() Jamming too many conductors, devices, fittings, etc., into a box - more than what it was designed to hold and listed to contain - is a recipe for disaster. Overcrowded boxes are all too common in residential and commercial buildings. 1 on the list, it certainly comes close to the top. As a representative of the electrical inspection community, I am often asked, “What are some of the most common Code violations you encounter?” While violation of box fill requirements might not be No.
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