Wiring Methods, Materials, and Installation Practices
This domain tests whether you can select and install the correct Chapter 3 method: raceway versus cable, support spacing, box fill, bend limits, raceway fill, and physical protection, plus the separation rules that keep low-voltage cable clear of power wiring.
Core rules
- Securing and supporting: EMT (358.30) within 3 ft of a box and at least every 10 ft; Type NM cable (334.30) every 4.5 ft and within 12 in of boxes; power-limited fire alarm cable every 5 ft and within 12 in of boxes.
- Bends: no run may contain more than 360 degrees of bends between pull points.
- Fill: Chapter 9 Table 1 caps raceway fill at 40% for three or more conductors; use Table 4 for raceway area and Table 5 for conductor area.
- 300.4: protect conductors less than 1.25 in from the framing edge with a listed steel plate.
- 300.15: every splice, angle, and termination needs an accessible box or fitting. No buried splices.
- Plenum: 300.22(C) limits an "other space used for environmental air" to metal raceways and armored cable or listed plenum cable such as CMP, CL2P/CL3P, and FPLP.
Worked example
A 4 x 4 x 1.5 in. square box holds 21.0 in³ per Table 314.16(A) and receives two 12/2 NM cables, one device, and internal cable clamps. At 2.25 in³ per 12 AWG: 4 circuit conductors + 1 equipment ground + 1 for the clamps + 2 for the device = 8 allowances x 2.25 = 18.0 in³. That fits within 21.0 in³, but a third cable would add another 2.25 in³ and exceed the box, so move up a size.
Traps that cost marks
Mixing up the two NM cable rules: the 4.5 ft support interval and the 12 in box distance both apply. Plenum and riser ratings are not interchangeable in reverse: CMP works in a riser, CMR does not work in a plenum. The equipment grounding conductor is never free in box fill.