12.1 NM, MC, and AC Cable Assemblies

12.1 NM, MC, and AC Cable Assemblies

Quick Answer: NM (nonmetallic-sheathed cable) is Article 334—dry locations, typically dwellings and light commercial, secured within 12 in of boxes and supported every 4½ ft. MC (metal-clad cable) is Article 330—broader commercial use with interlocking or smooth metallic sheath. AC (armored cable) is Article 320—older armored assembly that depends on the armor and bonding strip with listed fittings for grounding continuity. Know uses not permitted before you worry about strap spacing.

On the Arkansas Journeyman Electrician exam (Prov / ADLL, NEC 2023, 60 questions, 3 hours, 70% pass), Conductors is 7 items. Cable assemblies sit next to ampacity and identification in that bucket. Prov loves stems that mix “Romex,” “MC,” and “BX,” then ask which method is legal in a wet crawlspace or which support rule applies. This section trains you to open the correct article first.

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Map the three articles before you dig

Exam vocabulary traps:

• “Thin-wall” and “setscrew coupling” → raceway (358), not a cable article.
• “Flexible metal conduit whip” → 348, not AC cable.
• “Liquidtight flex” → 350, not MC.

If the stem says cable, stay in Chapter 3 cable articles. If it says conduit or tubing, leave this chapter.

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NM cable — Article 334

What NM is

Type NM and NMC are factory assemblies of insulated conductors with an outer nonmetallic jacket. Modern dwelling cable is usually NM-B (90°C conductor rating for ampacity adjustment, but terminations still follow 110.14(C) temperature limits—often 60°C for NM in practice). Conductors are typically copper; aluminum NM exists but is less common on residential stems.

Uses permitted — 334.10

334.10 permits NM in:

• One- and two-family dwellings and their attached/detached garages and storage buildings.
• Multifamily dwellings of construction types where the Code and building construction allow NM (read the current 334.10 subsections—construction-type limits are classic open-book lookups).
• Other structures of permitted construction types for exposed and concealed work where not forbidden by 334.12.

NM may be run exposed or concealed in dry locations when protected from physical damage (furring strips, running boards, or other protection where required).

Uses not permitted — 334.12 (high-yield)

334.12 is where Prov scores easy points. NM shall not be used:

• In any dwelling or structure not specifically permitted in 334.10.
• As service-entrance cable (use Article 230 / Type SE methods instead).
• In wet or damp locations (with limited exceptions for NMC where the article allows).
• Embedded in poured concrete or aggregate (unless a permitted Type addresses it—do not invent an exception).
• Exposed to corrosive vapors or in hoistways, most theaters, and other restricted spaces listed in the section.
• Where subject to physical damage unless protected.

Practical exam filter: Wet basement walls, underground, outdoor exposed on a roof, or a commercial wet process room → NM is usually wrong. Reach for MC, conduit, or another wet-rated method.

Securing and supporting — 334.30

Two numbers to memorize like FMC support rules:

1. Secure within 12 in of every cabinet, box, or fitting.
2. Support at intervals not exceeding 4½ ft.

Horizontal runs through framing openings not more than 4½ ft apart may count as supported. Staples and cable ties must not damage the jacket. Where finished walls make intermediate support impracticable, the article’s finished-construction exceptions apply—read them; do not invent a general “no staples needed” rule.

Boxes, bushings, and physical protection

NM entering a box needs a cable clamp or listed connector (334.40 family). Do not leave the jacket short of the clamp so the insulated conductors take the strain. Where NM is exposed within 7 ft of the floor in unfinished basements or similar, or where subject to damage, protect it with raceway, guard strips, or other Code-accepted means (334.15).

Worked NM examples

Example A: 12/2 NM-B from a panel to a receptacle in a wood-frame dwelling. Staple within 12 in of the panel and box; intermediate staples ≤ 4½ ft; keep cable dry and protected where exposed on basement joists.

Example B: Stem asks for a cable method in an outdoor wet location feeding a yard light. NM fails 334.12; look to UF, MC listed for wet, or raceway.

Example C: NM fished in a finished wall without opening every stud bay. Finished-construction support exceptions may apply—still clamp at the boxes.

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MC cable — Article 330

What MC is

Metal-clad cable has one or more insulated conductors in a metallic sheath (interlocking metal tape, corrugated tube, or smooth tube). Modern MC for branch circuits usually includes an insulated equipment grounding conductor. MC is the commercial go-to when you want a cable assembly with better mechanical protection and broader location permission than NM.

Uses permitted and not permitted — 330.10 / 330.12

330.10 permits MC in many dry, damp, and wet locations when the cable and fittings are listed for the environment; exposed and concealed work; cable trays; and many commercial applications. 330.12 restricts use where subject to physical damage (unless protected), where corrosive conditions attack the sheath, and in other listed prohibitions. Always match the listing (wet-location MC, hazardous-location MC, etc.) to the stem.

Support — 330.30

General pattern (verify exact 2023 text in your book):

• Secure within 12 in of every box, cabinet, or fitting (some constructions allow different first-support distances—read the subsection).
• Support at intervals not exceeding 6 ft for most interlocking-type MC (smooth/corrugated tubular constructions may differ).

Prov may ask “MC support interval” specifically to catch candidates who answer 4½ ft from NM muscle memory. Open 330.30.

Fittings and grounding

Use connectors listed for MC. Cutting interlocking armor cleanly and installing anti-short / insulating bushings where required protects conductor insulation. For equipment grounding, follow the cable construction: if an insulated EGC is present, terminate it on the grounding terminal. Armor may also be recognized under 250.118 when continuity and listing conditions are met—do not assume every MC armor is an EGC substitute without reading the product and Code text.

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AC cable — Article 320

What AC is

Armored cable is an assembly of insulated conductors in a flexible interlocking metal armor, typically with a bonding strip (bonding wire) under the armor. It is the historical “BX” family. AC is still Code-recognized but less dominant than MC on new commercial work; exams still test it because older buildings and certain stems use AC language.

Uses and limits — 320.10 / 320.12

AC is generally limited to dry locations. 320.12 does not permit AC in wet/damp locations, where subject to physical damage (unless protected), in corrosive atmospheres, or for direct burial/embedded concrete unless specifically allowed. If the stem paints moisture, AC is usually the wrong armored cable—MC listed for wet is the contrast answer.

Support — 320.30

Secure within 12 in of terminations and support at intervals not exceeding 4½ ft (same rhythm as NM for many stems). Use listed AC connectors; the bonding strip must be properly terminated with the listed fitting so the armor/bonding path remains continuous.

AC vs MC on exam day

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Common Prov traps

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Timed open-book tips

Tab 334.12, 334.30, 330.30, 320.12, and 320.30. On every cable stem: (1) Which article—334, 330, or 320? (2) Is the location dry/wet/damaged? (3) What is the first-support and interval rule? (4) How is the EGC accomplished? That four-step filter clears most Arkansas Prov cable-assembly items under the Conductors subject area.