THE MC CABLE GUIDE

• MC (metal-clad) cable is THHN copper conductors plus a green ground wire inside corrugated aluminum armor — the standard wire of commercial buildings.
• BX is not MC. BX is the old trade name for AC (armored) cable: it has a bonding strip, no ground wire, and uses its armor as the ground path.
• MC installs far faster than conduit but cannot be re-pulled later. Conduit wins where wires change or local code requires it.
• Standard MC is for dry locations; wet or buried use needs cable specifically marked for it.
• A 250-foot coil of 12/2 MC holds on the order of 15 pounds of copper — the armor is light aluminum, not the value.

MC stands for metal-clad cable, and the name describes the construction exactly. Inside are ordinary THHN insulated copper conductors — the same wire electricians pull through conduit — plus one dedicated green insulated equipment ground wire. Around all of them goes a corrugated armor of interlocked aluminum (steel versions exist too), which is the "metal clad" part. The armor is mechanical protection: it lets the cable be run exposed or fished through commercial walls without a separate pipe. The whole product is governed by NEC Article 330. This guide is part of your friendly guide to all things electrical wire — the hub where the full wire family lives.

The green wire is the point.

The single most important thing about MC is the green wire inside it. Standard MC carries a full-size, insulated equipment grounding conductor as part of the cable, and the armor is not part of the grounding path. That is the opposite of its famous cousin, AC cable, and it is the detail that confuses everyone. There are specialty MC variants — hospital-grade and all-purpose types — that add a bonding conductor alongside the ground wire for sensitive installations, but the everyday MC on commercial jobs is simple: conductors, green ground, armor.

Where it lives.

MC is the default wiring method for light commercial work: offices, retail stores, restaurants, strip malls, and anywhere Romex is not allowed but full conduit would be overkill. It fishes through walls and ceilings, runs exposed where the code permits, and handles branch circuits and feeders alike. Standard MC is rated for dry locations; versions with a PVC outer jacket and wet-rated conductors are listed for wet locations and even direct burial — but only if the cable is marked for it. Read the markings, match them to the location, and never assume.

Ask for "BX" at a supply house and you will probably be handed MC. The whole trade does it, and the whole trade is technically wrong. BX is not a product — it is an old trade name for AC (armored) cable, a different cable governed by NEC Article 320. The two look similar from the outside and work nothing alike on the inside, and mixing them up is how grounding mistakes happen.

AC cable, plainly explained.

AC cable wraps its insulated conductors in armor like MC does, but instead of a green ground wire it has a thin aluminum bonding strip running under the armor. The armor plus the bonding strip together serve as the equipment grounding path — there is no separate ground wire. The conductors are typically wrapped in paper, and the cable is limited to the 60°C ampacity column (NEC 320.80), like NM cable. AC is for dry locations, and because the armor is the ground path, the terminations and fittings matter enormously.

The differences that matter.

Grounding is the big one: AC grounds through its armor and bonding strip, while MC grounds through its green wire and its armor is not a grounding conductor. Ampacity follows: MC's THHN conductors use the 75°C column (where terminations allow), while AC is capped at 60°C. Support spacing differs too — MC is secured at intervals up to 6 feet and within 12 inches of boxes, while AC is held to 4.5 feet. And the little red anti-short bushings (the "redheads") that protect conductors from the cut armor edge are required on AC but not on MC. When in doubt, read the cable's own markings: real MC is printed "MC" and real AC is printed "AC." The printing settles every argument.

On commercial work, the real decision is rarely MC versus AC. It is MC versus conduit — cable against pipe — and each wins in different situations. Understanding the tradeoff is what separates someone who knows the products from someone who knows the job.

Where MC wins.

MC installs dramatically faster. There is no bending, no threading, no pulling wire through pipe — you cut it to length, fit the listed connectors, and run it. For straightforward branch-circuit runs in walls and ceilings, the labor savings are real, and on bid work that matters. It also fishes beautifully through existing construction, which is why it dominates commercial remodels. The price is higher per foot than the equivalent THHN, but the total installed cost usually comes out lower.

Where conduit wins.

Conduit wins everywhere the future matters. Wires in conduit can be pulled out and replaced, added to, or upgraded without opening walls — cable is permanent once it is in. Conduit gives superior physical protection, it looks cleaner in exposed industrial work, and some occupancies and cities simply require it. Chicago is the famous example: the city's electrical code has long required conduit (EMT) in most occupancies where the NEC would happily allow cable. Local amendments like that one are exactly why every page in this guide ends with "check your local amendments" — the NEC is the floor, and your city may build higher.

Strip away the armor and MC is the same copper as everything else: THHN conductors sized like any other wire. A 250-foot coil of 12/2 MC holds on the order of 15 pounds of copper between its insulated conductors and the green ground wire — roughly comparable to Romex of the same gauge, because the conductors are the same size. The aluminum armor is light and is a separate, low-value material; the money is in the copper inside.

The demo angle.

Commercial demos and office gut-jobs produce MC by the dumpster-load, and most of it gets tossed with the drywall. The pros I buy from strip the armor and keep the copper conductors — or just coil the cable and sell it as insulated copper wire. Either way, keep it separate from Romex and from bare copper, because each grades differently. When the pile is worth a call, you know the number.