7.4 Cable Types & Applications (NM, AC, MC, UF, SE)

Cable Article Map

NEC Chapter 3 (Wiring Methods and Materials) devotes one article per cable type. The Virginia journeyman exam tests the article number, permitted/prohibited locations, and the equipment grounding means for each:

Type NM and NMC — Article 334

Nonmetallic-sheathed cable (NM-B is the modern dry-location version) is the dominant residential wiring method. Per 334.10, NM is permitted in one- and two-family dwellings and attached/detached garages, in multifamily dwellings of Types III, IV, and V construction, and in other structures of Types III, IV, V construction when concealed behind a thermal barrier with a 15-minute finish rating. 334.12 prohibits NM in wet or damp locations, commercial garages with hazardous locations, theaters, hoistways, embedded in concrete, and as service-entrance cable.

334.80 limits NM cable ampacity to the 60°C column of Table 310.16 even though the underlying conductors are rated 90°C, but you may use the 90°C ampacity as the derating starting point for ambient and bundling. So a 12-2 NM-B cable on a 20 A breaker uses the 60°C column value of 20 A for final sizing, even though the THHN inside is 90°C-rated.

334.24 requires bends to have a radius of not less than 5× the cable diameter. 334.30 requires the cable to be secured within 12 in. of each box, cabinet, or fitting and at intervals not exceeding 4.5 ft. NM run exposed where subject to physical damage must be protected per 334.15; in exposed work it must closely follow the building surface.

Type AC — Article 320

Armored cable (Type AC, e.g., BX) is a fabricated assembly of insulated conductors in a flexible interlocked metallic armor with an internal bonding strip in intimate contact with the armor for its full length. Per 250.118(A)(8), the armor plus bonding strip is an equipment grounding conductor — no separate wire EGC is required (although modern AC often includes one anyway). An anti-short bushing (redhead) is required at each termination per 320.40.

AC is dry locations only (320.10/320.12). It is prohibited in wet/damp locations, where subject to physical damage, in plaster or masonry in wet locations, and in corrosive conditions. 320.24 sets the bending radius at not less than 5× the cable diameter. 320.30 secures AC within 12 in. of boxes and at intervals not exceeding 4.5 ft.

Type MC — Article 330

Metal-clad cable (Type MC) is the most versatile metallic cable: it may be used for branch circuits, feeders, and services; indoors and outdoors; in cable trays and raceways; in wet locations when the sheath or jacket is corrosion-resistant; and direct-buried when identified for that use. The interlocked-tape MC sheath by itself is not an EGC under 250.118(A)(10); a wire-type EGC is required unless the MC cable is the listed type with a bare bonding conductor in intimate contact with the sheath (the "MCAP" style).

330.24 bending radii depend on sheath type: smooth sheath up to 0.75 in. diameter = 10× diameter; larger smooth sheath and interlocked tape = 7× diameter. 330.30 secures MC within 12 in. of boxes and at intervals not exceeding 6 ft for cables with four or fewer conductors sized 10 AWG or smaller.

Type UF — Article 340

Underground feeder and branch-circuit cable (Type UF) is a moisture-resistant cable assembly suitable for direct burial. UF has solid or stranded conductors in a nonmetallic jacket rated for wet locations; it can also be used indoors where allowed for NM (334.10) but is harder to strip. Minimum burial depths are in 300.5(A) — typically 24 in. for direct-buried UF (12 in. with a listed GFCI-protected circuit). UF ampacity is taken from Table 310.16 using the 75°C column for #14 Cu and larger when used in wet locations; in dry locations the 60°C column applies.

Type SE and USE — Article 338

Service-entrance cable comes in two flavors:

• Type SE (including SER, round with insulated neutral and bare EGC) — flame-retardant and moisture-resistant; used for service-entrance conductors per Article 230, and for interior feeders and branch circuits. For interior installations, SE follows the Article 334 rules (except 334.80), so ampacity is the 60°C column. Uninsulated conductors in SE may be used only as equipment grounding conductors, not as current-carrying conductors.
• Type USE (and USE-2) — moisture-resistant but not flame-retardant, so it is for underground service only and may not be run indoors or aboveground unless protected per 300.5(D).

338.12 prohibits SE where subject to physical damage unless protected per 230.50(B), and underground with or without raceway. USE is prohibited for interior wiring and aboveground use unless protected.

Comparison Table at a Glance

Common Traps

• Treating MC sheath as an EGC. Standard interlocked MC needs a wire EGC unless it is the listed bonding type.
• Using NM outdoors or in a wet conduit. 334.12 prohibits it; use UF, MC with wet-location jacket, or a raceway method.
• Forgetting 334.80's 60°C cap on NM. Even with 90°C conductors inside, NM final ampacity is the 60°C column.
• Confusing SE with USE. USE is underground-only; SE/SER is the round service cable with insulated neutral and bare EGC.