30 CFR § 57.12008
Insulation and fittings for power wires and cables.
Power wires and cables shall be insulated adequately where they pass into or
out of electrical compartments. Cables shall enter metal frames of motors,
splice boxes, and electrical compartments only through proper fittings. When
insulated wires, other than cables, pass through metal frames, the holes
shall be substantially bushed with insulated bushings.