Use this box fill calculator to total NEC-style wire space and see if your marked electrical box volume is enough. Do not include ground wires here. It takes the incoming power and safely distributes it to different circuits throughout your building. However, the key to. (a) Conductors entering cutout boxes, cabinets, or fittings shall be protected from abrasion, and openings through which conductors enter shall be effectively closed. The National Electrical Code (NEC) requirements might seem like bureaucratic red tape, but they"re more like the safety rails that. A distribution box is a low-voltage electrical enclosure that receives incoming power and distributes it safely to multiple outgoing circuits through protective and switching devices such as MCBs, RCDs, RCBOs, fuses, isolators, busbars, neutral bars, earth bars, and surge protective devices. Key. Summary: The National Electrical Code explains the Maximum Number of Wires that can be installed into a box, otherwise known as Box Fill.