Wiring Methods and Materials
Wiring Methods and Materials
This domain covers NEC Chapter 3: choosing cables and raceways, support and securing, boxes, wet and damp locations, and permitted uses. Exam items often ask whether a method is allowed in a location.
Core rules
- NM-B is for dry, protected interior locations; it is not permitted in wet locations, exposed to weather, or in most hazardous areas.
- MC cable and AC cable have different grounding and bonding construction; use listed connectors and support per Article 330 or 320.
- EMT must be supported within 3 ft of boxes and every 10 ft; PVC schedule 40 support intervals differ and expansion fittings are required.
- Boxes must be accessible and sized per 314.16; conductors need 6 in free length, 3 in beyond the box face.
- Wet-location raceways need listed fittings and conductors; use WP boxes and gaskets.
Worked example
A 25 ft NM-B run in a dry attic needs support. NEC 334.30 requires support every 4.5 ft and within 12 in of each box. Supports at 0, 4.5, 9, 13.5, 18, and 22.5 ft satisfy the interval rule; add supports within 12 in of both boxes. If the same run enters a damp crawlspace, NM-B is not permitted-switch to a wet-location method such as PVC conduit with THWN conductors.
Traps
- Using NM cable outdoors or in a wet location because it is "inside" a wall.
- Confusing support and securing intervals.
- Omitting bushings or connectors where cable enters a box.
- Assuming all gray PVC is sunlight-resistant; use listed UV-resistant raceway where exposed.