Electrical Reference Library · Technical Reference

Metal-Clad (MC) and Armored (AC) Cable

MC and AC are both metal-armored cables and look alike, but they ground differently. This sheet explains the armor types, how each provides an equipment grounding path, and the MC-AP and health-care variants.

Ref EL-0009NEC 330NEC 320UL 1569UL 4

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Type AC (armored cable, often called BX) is listed to UL 4 and installed under NEC Article 320. It has thermoplastic insulated conductors, each wrapped in paper, inside an interlocked metal armor with an internal bonding strip. Type MC (metal-clad cable) is listed to UL 1569 and installed under NEC Article 330. Standard interlocked MC has a polyester tape over the conductors and includes an insulated green equipment grounding conductor. MC is also made with corrugated and smooth sheaths.

How to identify

Strip back the armor. If you find a thin bare aluminum strip running against the inside of the armor and paper-wrapped conductors, it is AC. If you find a clear or white tape and a green insulated conductor, it is standard MC. If you find tape plus a bare aluminum conductor touching the armor, it is MC-AP or a similar "all purpose" cable. The jacket or tag also names the type. Some MC cables have a PVC outer jacket for wet or direct-burial use.

Grounding paths

Cable	Equipment grounding path	Notes
AC	Armor plus internal bonding strip together	Bonding strip is not an insulated conductor
Standard MC (interlocked)	Green insulated grounding conductor	Armor alone is not the grounding conductor
MC-AP (all purpose)	Armor plus bare aluminum grounding/bonding conductor, as listed	Can also carry an insulated ground; read the listing
Health-care AC or MC (often marked HCF)	Insulated green conductor plus a listed armor path	Built for the redundant grounding required in NEC 517.13

Key points

Use an insulating anti-short bushing ("red head") at cut ends of AC cable. Many MC makers also call for them.
Use fittings listed for the cable type. MC connectors and AC connectors are not always the same.
In patient care spaces, NEC 517.13 requires two grounding paths. Only cables listed for that use qualify.
Permitted and prohibited uses differ. AC is limited to dry locations. MC uses depend on the sheath and jacket. See NEC 320.10, 320.12, 330.10 and 330.12.
Ampacity is governed by NEC 320.80, 330.80 and the code tables.

Common mistakes

Treating interlocked MC armor as the ground and leaving the green conductor unconnected.
Cutting off the AC bonding strip instead of folding it back against the armor.
Calling all armored cable "BX" and shipping AC where the job needs MC.
Using standard MC in a patient care area that needs a health-care listed cable.

Governing standards: NEC Article 320 (AC); NEC Article 330 (MC); NEC 517.13; UL 4 (armored cable); UL 1569 (metal-clad cables).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

More in Technical Reference

← All Electrical sheets