Wiring Methods and Materials
Wiring-method questions ask: is this method permitted in this location, and is it installed and supported correctly?
Know the families
Non-metallic sheathed cable for dry, protected locations in dwellings; armoured and metal-clad cable where physical protection or flexible connections are needed; rigid metal conduit, IMC, and EMT for exposed and hazardous work; PVC and rigid non-metallic for corrosive and underground use; liquid-tight flexible conduit for wet or vibrating equipment connections; flexible cord only for portable equipment. Each has its own support spacing, bending radius, fitting requirements, and permitted locations.
Rules that appear constantly
- Conduit fill: no more than 40 percent of the interior cross-section for three or more conductors; fewer conductors allow higher percentages.
- Bends must not kink or reduce the internal diameter; total bends between pull points are limited.
- Supports are required within a set distance of boxes and at intervals along the run.
- Raceways must be complete, connected, and closed before conductors are pulled, and bushings protect conductor insulation.
- Wet, damp, and dry location designations decide which fittings and boxes are acceptable.
- Burial depths, expansion fittings, and corrosion protection are frequent calculation items.
Traps
Using a dry-location cable in a wet location, omitting a bonding bushing on a concentric knockout, ignoring expansion in long PVC runs, supporting a raceway from a ceiling grid, or exceeding fill because "it pulls fine."
Rule of thumb
Derive the allowable number of conductors from the 40 percent fill rule and the published conduit and conductor areas, then subtract for fittings and bends. If the pull is long or the bends are many, the practical limit is lower than the code limit.