Safety guide for running BX cable

What BX actually is, and where it belongs

BX is the trade name still used for armored cable (Type AC) under NEC Article 320. Don't confuse it with Type MC (Article 330). They look similar in the bundle but the bonding strip, fittings, and listed uses differ. Type AC has a thin aluminum bonding strip in continuous contact with the armor; the armor itself is the equipment grounding path. Type MC does not, and relies on a separate insulated EGC.

Article 320.10 permits AC cable in dry locations for branch circuits and feeders, in both exposed and concealed work, and inside air voids of masonry block where not exposed to excessive moisture. Article 320.12 prohibits it in wet locations, where exposed to corrosive fumes or vapors, embedded in plaster on walls exposed to weather, or in storage battery rooms. If the jobsite is damp, switch to MC with an appropriate jacket or go to conduit.

Verify the cable is listed for the application before it leaves the gang box. A roll of mystery armor on the truck is not a substitute for reading the print on the jacket.

Cutting and prepping without wrecking the conductors

The fastest way to fail an inspection with BX is a sloppy cut. Nick the conductor insulation and the circuit is compromised before the breaker is even on. Use a rotary armor cutter (Roto-Split or equivalent), not a hacksaw freehand. Set the depth so the blade scores the armor without biting through to the THHN underneath.

After the cut, twist the armor off, inspect the conductors, and trim the bonding strip. NEC 320.40 requires an anti-short bushing (the red plastic insert, often called a redhead) at every termination where the cable enters a box or fitting. The bushing protects the conductors from the sharp armor edge. Inspectors look for it. No redhead, no pass.

Field tip: bend the bonding strip back over the outside of the armor, then slide the connector on. The strip gets clamped between the connector and the armor, completing the ground path. Cutting it flush is a common rookie mistake.

Support, securing, and bend radius

Article 320.30 sets the support rules. AC cable must be secured within 12 inches of every box, cabinet, or fitting, and at intervals not exceeding 4.5 feet. There are exceptions for fished cable in finished walls and for short lengths at terminations of luminaires (up to 6 feet for whips per 320.30(D)).

Bend radius matters. NEC 320.24 requires bends to be made so the cable is not damaged, with a radius of the inner edge of any bend at least 5 times the cable diameter. Kink it and you crush the conductors inside.

• Staple within 12 inches of every box.
• Staple every 4.5 feet on runs.
• Use listed AC cable staples, not romex staples crushed flat.
• Keep bends loose, 5x diameter minimum.
• Protect cable run through framing per 300.4 (bored holes 1.25 inches from edge or steel plates).

Boxes, fittings, and the bonding question

Use connectors listed for Type AC. Many MC connectors are dual-listed, but check the carton. The connector locknut must be tight against the box, and the cable must be secured in the connector before the locknut is set. A loose connector is a bonding failure waiting to happen.

Box fill counts under 314.16. Each conductor entering the box counts, the EGC counts as one (collectively), each device counts as two, and internal clamps count as one. BX armor and the bonding strip together act as the EGC. When you mix Type AC with conduit or other wiring methods in the same box, all EGCs must be bonded together per 250.148.

Field tip: if a junction box feels crowded with three AC cables and a switch, do the math before you fight the cover. A 4 inch square 1.5 inch deep box gives you 21 cubic inches. A switch and three 14/2 AC runs eat it fast.

Common code violations to avoid

Inspectors see the same mistakes over and over. None of these are obscure. They are listed in plain language in Article 320 and the general installation rules of Article 300.

1. Missing anti-short bushings at terminations (320.40).
2. Bonding strip cut flush instead of folded back.
3. Staples missed within 12 inches of a box.
4. AC cable used in a wet location, garage slab, or buried (320.12).
5. Sharp bends that flatten the armor.
6. Cable run across the top of ceiling joists in an accessible attic without guard strips (320.23).
7. Mixing up Type AC and Type MC connectors and assuming the bond is intact.

Document the cable type on your rough-in walkthrough. If the GC swaps materials mid-job, you want a record of what you actually installed where.

When to reach for something else

BX is excellent for retrofit work in old commercial buildings, exposed runs in unfinished basements, and any application where you need flexibility plus mechanical protection. It is not the right pick for damp locations, direct burial, or runs subject to physical abuse beyond what the armor can absorb.

For wet or damp environments, specify Type MC with a PVC jacket, or run EMT. For high-abuse industrial environments, rigid or IMC. For residential concealed work where local code permits, NM cable is faster and cheaper, though many jurisdictions (Chicago, parts of New York) still require AC or conduit throughout.

Know your local amendments. The NEC is the floor, not the ceiling, and the AHJ writes the final check.