One reason behind this rule is that if a failure occurs, it is likely to occur at a joint, splice, or termination. In completed installations, each box is required to have a cover, faceplate or luminaire canopy. Other boxes that enclose devices or utilization equipment are to be sized based on the supply conductors that supply the devices or utilization equipment (see Table 1). Large equipment is defined as devices or utilization equipment that projects more than 48 mm (1 ⅞ in.) rearward from the mounting plane of the box. Boxes that enclose large equipment are required to have a depth that is not less than the depth of the equipment plus 6 mm (¼ in.). Outlet boxes that do not contain devices or utilization equipment are permitted to have an internal depth of less than 12.7 mm (½ in.). All boxes (enclosures) must be large enough to provide for sufficient free space for all conductors and devices that will be enclosed within them to prevent overcrowding and possible physical damage when the devices or splices are installed or completed. The Code includes general requirements for boxes, such as sizing and support provisions. Let’s look closer at these requirements for box fill calculations. Most of these requirements can be found in Article 314 (Outlet, Device, Pull, and Junction Boxes Conduit Bodies Fittings and Handhole Enclosures). The National Electrical Code ( NEC) contains sufficient rules and requirements that apply to box fill calculation requirements. The same principle applies to conductors enclosed in a confined space, such as a switch box. A crowded elevator heats up fairly quickly because you and the other heat radiators (bodies) don’t have ample room to dissipate the produced body heat. These conductors must have ample free space to allow proper dissipation of heat from the conductor to not cause damage to the surrounding insulation of these said conductors. One might say, “OK, what’s the big deal what’s the harm?” When put to work carrying the load they are intended to carry (such as a lighting load), the enclosed conductors produce heat from current flow associated with these loads. Unfortunately, these over-crowded boxes are all too common at both residential and commercial applications. More conductors, devices, fittings, etc., are often installed in a box than what that particular box was designed and listed to contain. One of the most frequently encountered Code violations, in my experience, would have to be those concerning box fill requirements.
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