NEC MC Cable Installation Rules
NEC Article 330 governs Metal-Clad (MC) cable. Permitted uses, prohibited locations, bundling per 330.80, support per 330.30
NEC Article 330 of NFPA 70 (2023) governs Metal-Clad (MC) cable - the spiral-wrapped armored cable assembly with an interlocked aluminum or steel armor over insulated conductors. MC has become the dominant feeder cable in commercial construction because it ships ready to install, eliminates separate conduit pulls, and is recognized for nearly all the same locations as NM-B plus several where NM cannot go. Article 330 is concise but loaded with installation rules that AHJ inspectors actively check.
What Article 330 Covers
NEC 330.1 defines MC cable as a "factory assembly of one or more insulated circuit conductors with or without optical fiber members enclosed in an armor of interlocking metal tape or a smooth or corrugated metallic sheath." Article 330 covers permitted uses (330.10), prohibited uses (330.12), construction (330.104-118), installation (330.10 through 330.130), and ampacity (330.80).
Permitted Uses - NEC 330.10(A)
MC cable is permitted:
- For services, feeders, and branch circuits
- For power, lighting, control, and signal circuits
- In any approved raceway
- As open runs of cable
- As aerial cable on a messenger
- In hazardous locations as permitted in 501.10(B), 502.10(B), 503.10(A), 504.20, 505.15
- In dry locations and, where identified for the use, in wet locations
- Where embedded in plaster finish or run in air voids of masonry block walls (with restrictions on wet locations)
- In direct burial applications where identified for the use
Prohibited Uses - NEC 330.12
MC cable shall not be used:
- Where subject to physical damage (unless suitably protected)
- Where exposed to corrosive conditions (unless metallic sheath is resistant)
- Direct buried unless identified for that purpose
- In wet locations unless the metallic sheath is impervious to moisture and the conductor jacket is suitable
Bundling and Ampacity - NEC 330.80
MC ampacity is determined per 310.16 or 310.17 for the conductor type inside the cable assembly. When multiple MC cables are installed in close proximity:
- NEC 330.80(A)(1) - For cables containing more than 3 CCCs or cables installed without spacing maintained, the adjustment factors of 310.15(C)(1) apply
- Note exception: Cables installed in cable trays with maintained spacing per 392.80(A) have less restrictive rules
Practical impact: when you staple 5 home-run MC cables side by side along a stud bay for more than 24 inches, you have more than 9 CCCs bundled together (assuming 12/2 MC has 2 CCCs each), triggering 70% adjustment per Table 310.15(C)(1).
Support and Securing - NEC 330.30
| Requirement | Section | Value |
|---|---|---|
| Maximum support spacing | 330.30(B) | 6 ft (1.8 m) |
| Securing distance from box | 330.30(C) | 12 in (300 mm) for cables with 4 or fewer conductors no larger than 10 AWG |
| Unsupported permitted | 330.30(D) | Up to 6 ft in finished walls/ceilings where fishing is required; up to 6 ft from luminaire/equipment for tap connections |
| Cable tray | 330.30(E) | Per Article 392 |
Bending Radius - NEC 330.24
MC cable shall not be bent tighter than:
- Smooth sheath: 10x external diameter for cables 3/4 in or less; 12x for larger
- Interlocked or corrugated sheath: 7x external diameter
- Shielded conductors: 12x cable diameter
Tighter bends crack the armor and can damage insulation.
Worked Example
A commercial tenant fit-out runs 8 home runs of 12/2 MC cable from a panel to lighting whips, stapled tightly together along a top plate for 12 feet before fanning out:
- Each 12/2 MC = 2 CCCs (2 ungrounded; the EGC and any neutral on a 2-wire 120V circuit also counts as CCC because the neutral carries the same current as the hot)
- 8 cables x 2 CCCs = 16 CCCs bundled
- Bundling > 24 inches, so per 330.80 / 310.15(C)(1) the 16-CCC factor is 50%
- 12 AWG THHN at 90C = 30A; 30 x 0.50 = 15A adjusted
- 240.4(D) caps 12 AWG at 20A; the lower of (15A, 20A) = 15A
- A 20A breaker is too large - either upsize to 10 AWG MC or break the bundle into smaller groups
This single example is why bundled MC pulls in commercial construction are a frequent inspection failure.
MC Versus NM-B Versus Conduit
| Attribute | MC | NM-B | Conduit with THHN |
|---|---|---|---|
| Code Article | 330 | 334 | 358 (EMT), etc. |
| Dwelling allowed | Yes | Yes | Yes |
| Commercial >3 stories | Yes | Limited | Yes |
| Wet locations | If identified | No | Yes |
| Exposed in metal frame | Yes | With protection | Yes |
| Physical protection | Armor included | Plates required | Conduit is protection |
| Field labor | Moderate | Low | High |
See our Romex installation guide for the NM-B side of the comparison.
Cross-References
- NEC 250.118 - MC armor as EGC (only if cable contains an additional grounding conductor or armor is listed)
- NEC 310.15(C)(2) - ampacity of installed conductors and cables
- NEC 392 - cable trays
- NEC 110.26 - working space (applies to terminations)
How WireFillChart Supports MC
Our conduit fill calculator is dedicated to raceway fill, but MC cable users still need to size the breaker against bundled-cable derating. See ampacity reference and the 310.15 derating guide for the math that applies to bundled MC.