CSLB C-10 Electrical ContractorPlanning and EstimatingEasy
When performing a material takeoff for a series of recessed lighting fixtures, the electrician needs to identify the type of cable specified. The blueprint legend indicates 'NM-B' cable. Which of the following best describes the appropriate use and characteristics of NM-B cable?
- ASuitable for wet locations, direct burial, and exposed outdoor use.
- BPermitted in concealed dry locations within walls and ceilings of dwellings.
- CHigh-temperature rated for use in raceways above 90°C.
- DPrimarily used for underground service entrance conductors in conduit.
Show answer & explanationAnswer & explanation
Correct answer: B. Permitted in concealed dry locations within walls and ceilings of dwellings.
NM-B (Nonmetallic-Sheathed Cable, Type NM-B) is commonly known as Romex. It is permitted in normally dry locations in dwelling units and some other structures, typically concealed within walls, floors, and ceilings. It is not permitted in wet or damp locations, direct burial, or exposed outdoors (CEC 334.10, 334.12).
Why the other options are wrong
- A. Incorrect. NM-B is not permitted in wet or damp locations, or for direct burial (CEC 334.12).
- C. Incorrect. While individual conductors within NM-B might have a 90°C rating, the ampacity is limited by the 60°C column of Table 310.16 (unless specific conditions allow otherwise), and its general application is not for high-temperature raceway use.
- D. Incorrect. NM-B is not for service entrance conductors, especially underground. RMC or PVC in conduit is more typical.
NM-B Cable (Nonmetallic-Sheathed Cable)
A common type of electrical cable consisting of two or more insulated conductors and an insulated or bare equipment grounding conductor enclosed within a nonmetallic jacket, primarily used for interior wiring.
- Often referred to as 'Romex' (a brand name).
- Permitted in normally dry locations (CEC 334.10).
- Not permitted in damp/wet locations, direct burial, or exposed outdoors (CEC 334.12).
Memory trick: NM-B is for 'N-ice and M-ainly D-ry' inside walls.