Wiring Methods, Materials, and Installation Practices
Choosing a method
Chapter 3 is a menu with conditions. NM cable (Article 334) suits dry, protected interior locations; UF (340) suits direct burial; EMT (358) suits exposed and concrete work; PVC (352) suits corrosive and underground locations; MC (330) gives flexibility with metallic armor. Article 300 supplies rules common to all methods: 300.15 requires a box at every splice and outlet, 300.14 requires at least 6 in. of free conductor with 3 in. outside the box, and 300.22 restricts installations in ducts and plenums.
Support and box fill
- NM: secured within 12 in. of boxes and every 4.5 ft (334.30).
- EMT: within 3 ft of boxes and every 10 ft (358.30).
- PVC: within 3 ft of boxes, then per Table 352.30, generally 3 ft for 1/2 through 1 in.
Box fill under 314.16: each current-carrying conductor counts one volume; all equipment grounds together count one; one or more internal clamps count one; a device yoke receives a double volume allowance, equal to two conductors.
Worked example
A 4 × 4 × 1 1/4 in. box has 18.0 cu in. available. Three 14/2 NM cables enter: six current-carrying conductors, one EGC allowance, and one clamp allowance give 8 × 2.0 = 16.0 cu in. Adding a receptacle adds a double allowance of 2 × 2.0 = 4.0 cu in., for 20.0 cu in., which exceeds 18.0. Use a deeper box or a 4 11/16 in. box.
Traps
- Counting cable diameter instead of conductor volume.
- Forgetting the clamp or device allowance.
- Supporting PVC on the EMT schedule.
- Installing NM cable in a plenum or wet location.