NEC · Article 330

MC Cable (NEC Article 330)

Metal-clad cable shows up on journeyman wiring-method questions because it is permitted almost everywhere — and because its grounding works differently from the AC cable it sits next to on the shelf.

NEC 2017 2020 2023 Last updated 2026-06-26

What MC cable is

Metal-clad (MC) cable is a factory assembly of one or more insulated circuit conductors enclosed in a metallic sheath, most often an interlocked aluminum or steel armor. Inside that armor you find the circuit conductors, a separate equipment grounding conductor, and usually a protective wrap. Because the armor is a continuous mechanical jacket, MC gives solid physical protection while staying flexible enough to fish and route like a cable rather than a rigid raceway.

The detail that matters most for the journeyman exam is the grounding means. Standard MC cable includes a dedicated, full-size equipment grounding conductor (EGC) — typically a green-insulated or bare wire — sized to Table 250.122. The interlocked armor by itself is generally not recognized as the equipment grounding conductor. That single fact is the cleanest way to separate MC from AC cable, and it appears again and again on wiring-method questions.

MC cable vs AC cable

MC cable (Article 330) and AC cable (Article 320, the old "BX") look almost identical on the rack, but they handle a ground fault differently:

Feature MC cable (Art. 330) AC cable (Art. 320)
Ground-fault path Separate full-size EGC inside Armor plus internal bonding strip
Internal bonding strip No Yes
Support interval Not over 6 ft Not over 4½ ft

AC cable relies on a thin aluminum bonding strip that runs in continuous contact with the armor; the strip and armor together form the fault-return path, so AC cable carries no separate green EGC. MC flips that: the armor is not the ground path, so a dedicated EGC is run inside. Expect at least one question that hinges on which cable uses a bonding strip and which uses a dedicated EGC.

Where MC cable is permitted and how it is secured

Article 330 permits MC cable very broadly: services, feeders, and branch circuits; for power, lighting, control, and signal; indoors and outdoors; exposed or concealed. With a cable listed and identified for the condition, it may be used in wet locations, direct buried, or embedded in concrete. That wide latitude is exactly why MC is common in commercial work, and why the exam pairs it against NM cable (Article 334), which is limited largely to dry, protected residential locations.

The main restrictions are the common-sense ones: MC cannot be installed where it would be exposed to physical damage, and it must be suitable for any corrosive conditions it will face. Where it passes through framing it follows the same bored-hole and protection rules as other cable wiring methods.

MC cable must be secured at intervals not exceeding 6 feet and fastened within 12 inches of every box, cabinet, fitting, or termination for the common smaller cables. Specific allowances exist for fishing in concealed spaces and for cables with larger conductor counts. The bending radius is also limited so the armor is not crushed or kinked, which would damage the conductors inside.

Key rule: 6-foot support spacing with a 12-inch securing distance at boxes is the MC pattern worth memorizing. AC cable, by contrast, uses a 4½-foot support interval — a favorite "which number goes with which cable" distractor on the journeyman exam.

How MC cable shows up on the exam

The conductors inside MC cable are still ordinary building wire, so they are sized for ampacity under Article 310 like any other conductor — the armor does not change that math. What the armor changes is mechanical protection and the grounding scheme, and that is where the questions cluster. Most MC items fall into a few recognizable buckets:

  • identifying that the armor is not the equipment grounding conductor, and that a separate EGC is run inside;
  • contrasting MC with AC cable on the bonding strip and the support interval;
  • knowing the broad list of permitted uses against NM cable's narrow one; and
  • spotting the prohibited conditions — physical damage and unsuitable corrosive environments.

Treat MC as the flexible, widely permitted commercial wiring method whose defining feature is a dedicated grounding conductor, and the rest of the article becomes a series of comparisons rather than a list to memorize cold.

Practice questions on this rule

Real journeyman-exam practice questions from Loomi's bank — answer first, then reveal the explanation.

Q1 NEC 330.80(C)

Four Type MC cables, each containing three current-carrying conductors, are run bundled together in direct contact with attic thermal insulation, with no spacing maintained between the cables. What must be done per 330.80(C)?

  1. A The cables must be derated using the raceway fill percentages in Chapter 9
  2. B No adjustment is needed because MC cable is rated for direct burial in insulation
  3. C The ampacity of each conductor must be adjusted per Table 310.15(C)(1)
  4. D Only the outer two cables need adjustment since the inner cables are already insulated
Show answer & explanation

Correct: C — The ampacity of each conductor must be adjusted per Table 310.15(C)(1)

Per 330.80(C), when more than two Type MC cables with two or more current-carrying conductors each are in contact with thermal insulation without maintained spacing, every conductor's ampacity must be adjusted in accordance with Table 310.15(C)(1). MC cable is not exempt from derating just because it has its own outer sheath, and raceway fill tables (Chapter 9) don't apply to this cable-bundling scenario. The rule applies to all the cables in the group, not just the outer ones.

Q2 NEC 330.17

MC cable is installed through bored holes in wood studs in a dwelling. Which code section sets the physical protection requirements for this installation?

  1. A 330.17, which incorporates 300.4(A), (C), and (D)
  2. B 330.10, which lists permitted uses for MC cable
  3. C 330.30, which covers securing and supporting MC cable
  4. D 300.5, which covers underground installations
Show answer & explanation

Correct: A — 330.17, which incorporates 300.4(A), (C), and (D)

330.17 is the section that requires MC cable through or parallel to framing members to be protected per 300.4(A), (C), and (D). 330.10 addresses where MC cable is permitted to be used, not physical protection through framing. 330.30 addresses securing and supporting the cable, a separate requirement. 300.5 applies to underground wiring, not framing penetrations.

Q3 NEC 330.17

An electrician cannot maintain the required setback from the edge of a joist when routing MC cable through a bored hole. What must be done instead, per 330.17?

  1. A Install a steel plate or sleeve at least 1/16 inch thick over the area
  2. B Switch to a smaller MC cable so it fits within the setback
  3. C Nothing extra is needed since MC cable has an armored jacket
  4. D Wrap the cable in electrical tape at the penetration point
Show answer & explanation

Correct: A — Install a steel plate or sleeve at least 1/16 inch thick over the area

330.17 requires MC cable to comply with 300.4(A), which calls for a steel plate or sleeve at least 1/16 inch thick when the 1¼-inch setback can't be maintained. A smaller cable doesn't address the setback requirement itself, tape offers no equivalent protection, and MC's armor doesn't exempt it from 300.4 — that's the whole point of 330.17.

Q4 NEC 330.30(D)

An installer runs interlocked-armor Type MC cable to a motor that requires flexibility to reduce vibration transmission. What is the maximum unsupported length allowed from the last securely fastened point?

  1. A 1.8 m (6 ft)
  2. B 1.2 m (4 ft)
  3. C There is no limit as long as the armor is interlocked type
  4. D 900 mm (3 ft)
Show answer & explanation

Correct: D — 900 mm (3 ft)

330.30(D)(3) limits this vibration-isolation exception to interlocked-armor MC not exceeding 900 mm (3 ft) from the last securely fastened point. The 1.8 m (6 ft) figure is the separate luminaire-in-accessible-ceiling allowance under 330.30(D)(2), and there is no unlimited-length option under this section.

More wiring-methods practice questions (Articles 305–330)

Frequently asked questions

Which NEC article covers Type MC (metal-clad) cable?
Article 330. It defines MC cable, its permitted and prohibited uses, support and securing rules, and its construction requirements. Its shelf-neighbor AC cable lives one article earlier, in Article 320 — a classic exam trap.
Does MC cable need a separate ground wire?
Yes. Standard MC cable includes a dedicated equipment grounding conductor sized to Table 250.122, and the metal armor is generally not permitted to serve as the equipment grounding conductor by itself. This is the main difference from AC cable, where the armor and an internal bonding strip form the ground-fault path.
What is the support spacing for MC cable?
MC cable is generally secured at intervals not exceeding 6 feet and within 12 inches of each box or termination for cables with four or fewer conductors no larger than 10 AWG. Watch for AC cable, which uses a tighter 4½-foot support interval.
Can MC cable be used outdoors or in wet locations?
Only when the specific cable is listed and identified for that use. MC with a suitable moisture-impervious jacket can be installed in wet locations, direct buried, or in concrete, but ordinary MC is limited to dry locations.
Related articles
NM Cable (Romex) The nonmetallic residential cable MC is most often compared against. Conductor Ampacity How the conductors inside MC cable are sized and derated. Conduit & Raceway Fill The wiring-methods topic hub on the journeyman exam.
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