5.4 Cable Assemblies: NM, MC, AC, SE, UF and TC

Type NM — Nonmetallic-Sheathed Cable, Article 334

334.10 Uses Permitted. Type NM cable is permitted in one- and two-family dwellings; multifamily dwellings permitted to be of Types III, IV, and V construction; and other structures permitted to be of Types III, IV, and V construction. Where installed in Types III, IV, and V construction, the cable must be installed within walls, floors, or ceilings that provide a thermal barrier of material with at least a 15-minute finish rating.

334.12 Uses Not Permitted. Type NM shall not be used:

1. In any dwelling or structure not specifically permitted in 334.10;
2. Exposed within a dropped or suspended ceiling cavity in other than one- and two-family and multifamily dwellings;
3. As service-entrance cable;
4. In commercial garages having hazardous (classified) locations;
5. In theaters and similar locations, except as provided in Article 518;
6. In motion picture studios;
7. In storage battery rooms;
8. In hoistways or on elevators or escalators;
9. Embedded in poured cement, concrete, or aggregate;
10. In hazardous (classified) locations, except as permitted by specific articles.

It is also not permitted where exposed to corrosive fumes or vapors, where embedded in masonry, or in wet or damp locations.

The Ampacity Rule — 334.80

This rule is widely misquoted and deserves careful reading:

334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.14. The allowable ampacity shall not exceed that of a 60 degrees C rated conductor. The 90 degrees C rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final derated ampacity does not exceed that of a 60 degrees C rated conductor.

Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk, or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(C)(1).

Two distinct requirements live in that section, and they should not be merged:

1. The 60 degrees C basis applies always. It is not conditional on being in insulation. A 12 AWG NM conductor is limited to the 60 degrees C ampacity of 20 A whether it runs through an open joist bay or a spray-foamed wall. That is why 14/2, 12/2 and 10/2 NM land on 15, 20 and 30 ampere circuits.
2. The thermal-insulation clause triggers adjustment factors, not a column change. More than two NM cables with two or more CCCs bundled through a sealed opening in wood framing get the Table 310.15(C)(1) percentages applied.

A related trap: the 60 degrees C ampacity of 12 AWG copper is 20 A (Table 310.16) and the maximum overcurrent device on 12 AWG copper is also 20 A (240.4(D)(5)). The numbers match at 60 degrees C, which is exactly why candidates merge two independent rules. They separate as soon as a warmer column is permitted: the same 12 AWG THHN landing on 75 degrees C lugs in a raceway has a 25 A ampacity and still only a 20 A maximum device. For NM cable the 60 degrees C cap in 334.80 means you never reach that warmer column in the first place.

Support — 334.30

NM cable shall be supported and secured by staples, cable ties listed and identified for securement and support, straps, hangers, or similar fittings at intervals not exceeding 4.5 feet (1.4 m) and within 12 inches (300 mm) of every outlet box, junction box, cabinet, or fitting. Flat cables shall not be stapled on edge. Where a single-gang nonmetallic box without cable clamps is used, the cable must be secured within 8 inches of the box.

334.15(B) Protection from Physical Damage. Where exposed and subject to physical damage, NM cable shall be protected by RMC, IMC, EMT, Schedule 80 PVC, Type RTRC-XW, or other approved means. 334.15(C) requires cable passing through a floor to be protected to at least 6 inches above the floor.

Type MC — Metal-Clad Cable, Article 330

330.10 Uses Permitted. MC cable is one of the most broadly permitted assemblies: services, feeders, and branch circuits; power, lighting, control, and signal circuits; indoors or outdoors; exposed or concealed; direct buried where identified for such use; in cable tray; in any raceway; as aerial cable on a messenger; in hazardous locations as permitted by Articles 501, 502, 503, 504, and 505; in dry locations, and in wet locations under specified conditions.

330.12 Uses Not Permitted. Where subject to physical damage; or where exposed to destructive corrosive conditions such as direct burial in earth, concrete, cinder fill, or areas subject to strong chlorides or caustic alkalis, unless the metallic sheath is suitable or protected.

330.30 Securing and Supporting. MC cable shall be secured at intervals not exceeding 6 feet (1.8 m) and within 12 inches (300 mm) of every box, cabinet, fitting, or other cable termination. Cables containing four or fewer conductors sized no larger than 10 AWG shall be secured within 12 inches. Unsupported lengths are permitted in accessible ceilings for luminaire whips up to 6 feet, and in lengths not exceeding 3 feet at terminals where flexibility is necessary.

330.80 Ampacity. Determined in accordance with 310.14 for conductors 14 AWG and larger. MC cable is generally not limited to 60 degrees C the way NM is — its conductors are typically THHN, and its ampacity follows the normal rules subject to 110.14(C).

Interlocked-armor MC is not listed as an equipment grounding conductor by itself and includes a separate green EGC. MC-AP (MC cable with a bare aluminum grounding/bonding conductor in continuous contact with the armor) is listed for use as an EGC. Know which type you have before relying on the armor.

Type AC — Armored Cable, Article 320

AC cable looks like MC but differs in a way the Code cares about: it contains an internal bonding strip in continuous contact with the armor, so the armor itself is a recognized equipment grounding conductor under 250.118(8).

320.40 Boxes and Fittings. "At all points where the armor of AC cable terminates, a fitting shall be provided to protect wires from abrasion, unless the design of the outlet boxes or fittings is such as to afford equivalent protection, and, in addition, an insulating bushing or its equivalent protection shall be provided between the conductors and the armor." This is the anti-short bushing, commonly called a red head. The connector must be of a type that permits inspection of the bushing.

There is no equivalent NEC requirement for an anti-short bushing on Type MC — although many MC connectors and manufacturers' instructions include one, in which case 110.3(B) makes it mandatory.

320.30 Securing and Supporting. Secured within 12 inches of every box and supported at intervals not exceeding 4.5 feet.

320.80 Ampacity follows 310.14 with the conditions in that section.

Type SE and USE — Article 338

Type SE is service-entrance cable with a flame-retardant, moisture-resistant covering. Type USE is identified for underground use and has a moisture-resistant covering but is not required to be flame retardant, which is why 338.10(B)(4)(a) prohibits Type USE cable inside buildings except where the underground cable emerges and is terminated in an enclosure at an outside location, and the cable is protected per 300.5(D).

338.10(B)(4)(a) Interior Installations. Where Type SE cable is used for interior wiring, it shall comply with the installation requirements of Part II of Article 334, excluding 334.80. That exclusion pointer changed between editions — verify it in the 2020 book you will bring to the exam.

Type SER (with a separate insulated neutral) and Type SEU (with a concentric neutral) differ in that SEU cannot be used for a four-wire feeder because its concentric conductor cannot serve as both neutral and EGC downstream of the service.

Type UF — Underground Feeder, Article 340

340.10 Uses Permitted. UF cable may be used underground, including direct burial, subject to 300.5; as single-conductor cables where all conductors of the circuit are installed together; for wiring in wet, dry, or corrosive locations; as nonmetallic-sheathed cable installed per Article 334 where the conditions of Article 334 are met; and for solar photovoltaic systems per 690.31.

340.12 Uses Not Permitted. As service-entrance cable; in commercial garages, theaters, motion picture studios, storage battery rooms, hoistways, hazardous locations, embedded in concrete, or where exposed to direct sunlight unless identified as sunlight resistant.

340.80 Ampacity. The ampacity of Type UF cable shall be that of 60 degrees C conductors in accordance with 310.14.

Type TC — Power and Control Tray Cable, Article 336

TC cable is used in cable trays, raceways, or where supported by a messenger wire, and in industrial establishments with qualified maintenance and supervision as open wiring where continuous support is provided. Type TC-ER is listed with an equipment grounding conductor and is permitted to be installed unsupported between a cable tray and equipment for lengths not exceeding 6 feet.

Support Interval Summary