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.
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 (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.
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.
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:
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.
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