In the 2020 NEC ®, no more than 18 inches of cable length is allowed between the cable entry to the box and the closest cable support (see image). Below is a preview of the NEC®. ORG for the complete code section. The previous code language could technically allow an unlimited length of coiled up NM cable inside the wall as long as it was secured within 12 inches of the box. If wires are too short, they may fail inspection or create hazards during. The length of wire left inside an electrical box is a matter of strict compliance, safety, and functionality. Having the correct amount of slack ensures that future maintenance, repairs, or device replacements can be performed without difficulty. This code is based upon the type of box, wires, wire sizes, wire clamps and conduit fittings. Before you run each cable to the panel.
[PDF Version]