What Is Armored Cable (AC/BX) and Where Is It Still Used

Type AC cable is four things wrapped together, and each layer has a job:

1. The conductors. Modern AC carries THHN/THWN copper singles — usually black and white, sometimes with a red — each individually wrapped in a moisture-resistant, flame-retardant paper. Old BX-era cable used rubber-insulated conductors; the construction has improved, the concept hasn't changed.
2. The bonding strip. This is the part most people have never heard of, and it's the whole story. A bare 16 AWG aluminum strip runs the full length of the cable, tucked inside the armor in intimate contact with it (required by NEC 320.100). It shorts out the spiral of the armor so fault current flows instead of choking on the coil's inductance.
3. The armor. A flexible steel (or aluminum) tape wound in an interlocking spiral. It takes the physical abuse — nails, abrasion, being fished through old plaster walls.
4. The anti-short bushing. A little plastic "redhead" slipped over the conductor ends where they exit the cut armor, required by NEC 320.40 so the sharp edge of the tape can't slice into the insulation.

Here's why the bonding strip matters: the armor of Type AC cable is a recognized equipment grounding conductor (NEC 250.118) — but only the armor plus the bonding strip working together. The strip by itself is not a ground wire, and the armor by itself (on old BX that predates the strip) isn't a listed ground path either. That thin aluminum ribbon is what makes the whole assembly legal as a grounding method. When you terminate AC, you can cut the strip off or fold it back against the armor — you don't land it on a ground terminal like a wire.

"BX" is the old trade name for what's technically Type AC cable. Electricians have called it BX for a century; the code calls it AC. The meaningful difference isn't the name — it's the bonding strip. Cable made before the strip became standard (roughly pre-1960s BX) has armor but no bonding strip, and that armor alone doesn't qualify as an equipment grounding conductor under today's code. So when someone says "old BX has no ground," this is what they mean — not that the armor vanished, but that the armor never had its bonding partner.

How to tell what you're holding: cut the armor back (carefully — it's sharp) and look just under it. A thin bare aluminum strip running lengthwise means modern Type AC. No strip means old BX. That one detail changes how an electrician treats the circuit — and it's the kind of thing worth knowing before anyone quotes you a rewire. For the side-by-side with Romex and MC cable, see Romex vs. MC cable vs. BX: what's the difference and when to use each.

AC never went away — it just stopped being the default. It's still specified and installed today for:

• Commercial and institutional branch circuits — offices, schools, retail, where a little physical protection beats Romex but full conduit is overkill.
• Healthcare facilities — patient-care areas require wiring methods whose outer margin qualifies as an equipment grounding path, and AC inherently qualifies.
• Exposed work and fished retrofits — the armor survives being pulled through old plaster and lath walls, which is exactly why Chicago's rehab electricians reach for it.
• Dry locations, concealed or exposed — AC is a dry-location cable. Wet or damp locations need something else.

The Chicago angle: walk through any pre-1960 bungalow, two-flat, greystone, or vintage commercial building in the city and you're likely looking at AC/BX in the walls — miles of it, most of it installed between the 1920s and the 1960s. When those buildings get gutted and rewired, that old armored cable comes out by the dumpster-load. Most of it is scrap. But the unused coils and partial reels the electricians leave behind are a different story.

Pulling old BX out of a rehab — or sitting on unused AC cable? We buy usable leftover wire — anywhere in the country. Fast, free quote.