Romex vs. MC Cable vs. BX: What's the Difference and When to Use Each

Romex (Type NM-B) is the plastic-sheathed budget option: cheapest, easiest to run, allowed in dry residential interiors — but it has zero physical protection and can't go where it might take abuse. MC (metal-clad) cable wraps the same kind of conductors in flexible interlocked metal armor and carries its own green ground wire inside — it's the commercial workhorse, allowed exposed, concealed, in trays, and in more locations than Romex. BX (Type AC) is the older armored cable: steel armor with a thin bonding strip inside it — the armor plus the strip is the ground path, and there's no separate ground wire in the bundle.

If your Chicago-area house was wired before about 1960, what's in the walls is almost certainly old BX — and the pre-1960 stuff often doesn't even have the bonding strip, which means no real equipment ground at all. The diagram below shows the physical difference. Once you see it, you can't unsee it.

Romex is Southwire's brand name for Type NM-B (nonmetallic-sheathed cable), and the name stuck to the whole product family. It's a flat PVC jacket wrapped around two or three insulated conductors plus a bare copper ground — 12-2, for example, is black and white 12 AWG with an uncounted bare ground. The conductors inside are insulated to 90°C, but the cable itself must be sized off the 60°C ampacity column (NEC 334.80) — that's why 12-2 Romex is a 20-amp circuit, full stop. Want the full table? Here's the wire gauge chart — what size wire for every amperage.

Where it's allowed: dry, protected residential interiors — inside walls, above ceilings, through bored holes in studs. Where it's not: anywhere it might take physical damage, in wet or damp locations, or run exposed where the code requires protection. It's the cheapest of the three by a wide margin and the fastest to pull, which is why it's the default house wire everywhere the code allows it.

One Chicago-specific catch: the city of Chicago has long restricted NM cable to temporary and very limited uses in its electrical code amendments, favoring EMT conduit. Out in the suburbs — Rolling Meadows, Schaumburg, Palatine, Elgin — Romex follows the NEC and is the normal residential wiring method.

Metal-clad cable (NEC Article 330) is flexible interlocked metal armor — usually aluminum, sometimes steel — wrapped around THHN-type insulated conductors. The standard 12-2 MC has black, white, and a full-size green insulated ground wire inside the armor. That green wire is the equipment ground. Here's the part people get wrong: on standard interlocked MC, the armor itself is not the grounding path (NEC 250.118(10)) — the green wire is, and the armor is bonded through listed fittings as a supplement.

MC is allowed in far more places than Romex: exposed and concealed work, cable trays, and — with listed PVC-jacketed types — damp and wet locations. It costs more than Romex and takes proper MC connectors and a rotary cutter to terminate right, but on commercial jobs and Chicago-area retrofits where conduit isn't practical, MC is the standard answer. Electricians know it; inspectors know it; it passes without drama.

Type AC — armored cable, NEC Article 320 — is what everyone calls BX. BX was originally a manufacturer's trade name for early armored cable; the name stuck and became generic. Modern AC is interlocked steel armor around two to four conductors (14 AWG up to 1 AWG), with a 16 AWG bonding strip running in constant contact with the inside of the armor for the cable's entire length.

That bonding strip is the whole grounding story: the armor plus the strip is the equipment ground (NEC 250.118(8)) — there is no separate ground wire in the bundle. When you terminate AC, you cut the strip back and bond the armor through a listed connector. And this is the critical part for old buildings: BX made before roughly 1960 often has no bonding strip at all. No strip, no reliable ground path — the armor alone isn't a code-recognized ground. If you're looking at cloth-wrapped conductors inside old BX in a Chicago bungalow, that's what you're dealing with. AC is never permitted in wet or damp locations, and it's limited to four conductors or fewer.

How to tell AC from MC at a glance: read the jacket. Modern AC is marked TYPE AC with the bonding strip inside; MC is marked TYPE MC and carries a green or bare ground wire. Learn to read the jacket printing here — it settles the argument in ten seconds.

Chicago's electrical code has required EMT conduit for most residential wiring for generations — that's why Chicago houses don't have the Romex-in-the-walls look the rest of the country is used to. Open up a pre-1960 Chicago bungalow or two-flat and you'll find old BX everywhere: cloth-wrapped conductors, steel armor, usually no bonding strip, which means no equipment ground — two-prong outlets all the way down.

Out in the suburbs, it's the NEC playbook: Romex in new residential, MC in commercial and retrofit work, old BX still lurking in the older housing stock. If you're pulling old BX out of a rewire — or you bought a job's worth of MC and have reels left — that armored cable has resale value in usable lengths. Pulling out old BX on a rewire or sitting on leftover MC from a job? We buy usable wire rolls — anywhere in the country. Fast, free quote.

← Back to The Wire Guide