MC vs AC cable: grounding, where each is allowed and fittings
Published Sep 30, 2026
MC and AC cable both come in a spiral metal armor and both get cut with the same rotary cutter, but they are separate wiring methods under separate NEC articles. In short, AC is a dry-location cable whose armor is the ground, while MC carries its own grounding conductor and is permitted in far more places, including wet locations and large feeders.
How they are built
AC cable (NEC Article 320), often called BX, has insulated conductors wrapped in paper inside a flexible metal armor. Running under the armor, in contact with it for the whole length, is a thin bonding strip of copper or aluminum. There is no separate green grounding conductor in standard AC. The armor and the bonding strip together are the equipment grounding path (320.108).
MC cable (NEC Article 330) has insulated conductors, usually including an insulated green equipment grounding conductor, under interlocked, corrugated or smooth metal armor. The common branch-circuit product is interlocked aluminum armor over THHN-type conductors. Ordinary interlocked MC armor does not count as a grounding conductor on its own. Some MC types are listed so that the armor, alone or combined with a bare bonding conductor, does qualify; 330.108 points to 250.118 for which ones.
Side by side
| AC (Article 320) | MC (Article 330) | |
|---|---|---|
| Equipment ground | Armor plus internal bonding strip | Insulated green conductor in most types |
| Armor | Interlocked metal tape | Interlocked, corrugated or smooth |
| Dry locations | Yes | Yes |
| Damp and wet locations | No (320.12) | Yes, under the conditions in 330.10 |
| Direct burial and concrete | No | Only where the cable is identified for it |
| Support interval (branch sizes) | 4 1/2 ft, and within 12 in. of boxes (320.30) | 6 ft, and within 12 in. of boxes (330.30) |
| Minimum bend radius | 5 times the cable diameter (320.24) | 7 times for interlocked or corrugated (330.24) |
| Anti-short bushing | Required at every termination (320.40) | Not required by Article 330; many installers use one |
| Size range | Branch-circuit sizes | Branch circuits through large feeders |
Both support sections have exceptions for fished cable and short luminaire whips.
The bonding strip and the grounding path
The bonding strip in AC is not a conductor you land on a screw. Cut the armor, fold the strip back over the outside of the armor and let the connector clamp it. That contact is what makes the armor a low-impedance fault path. Cut the strip off flush and you lose it.
With MC, the green conductor lands on the ground screw or bar like any other grounding conductor. The inspector can see it.
Where each one fits
AC fits dry commercial and residential branch circuits where the local practice or the spec asks for it. Some installers like it because there is one fewer conductor to strip and terminate.
MC goes nearly everywhere AC goes, plus places AC cannot:
- Wet locations, when the cable meets one of the conditions in 330.10 (a sheath or jacket that keeps moisture out, with wet-rated conductors where the section asks for them). A PVC-jacketed MC is the usual pick outdoors.
- Large feeders. MC is made up into the hundreds of kcmil, while AC is sold in branch-circuit sizes.
- Health care patient spaces. 517.13 wants two grounding paths, which health care MC provides: a listed armor path plus an insulated copper ground.
Fittings
AC terminations need a fitting that protects the conductors from the cut armor plus an insulating (anti-short) bushing between the conductors and the armor (320.40). MC fittings must be listed and identified for MC (330.40). Many connectors are listed for both and say so on the carton. Match the connector to the armor type and the cable diameter.
Ampacity and cost
Both are sized from NEC Table 310.16 through 310.14 (310.15 in the 2017 edition), limited by the terminal ratings in 110.14(C). AC installed in thermal insulation must have 90 °C conductors but is held to the 60 °C ampacity (320.80(A)). In the common 14 through 10 AWG branch sizes, both cables end up at 15, 20 and 30 A under 240.4(D).
On price, AC saves the copper of a grounding conductor. For branch circuits the per-foot numbers land close together, so the spec and the labor decide.
Common mistakes
- Running AC in a wet or damp location, such as an exterior wall cavity that takes water or an underground raceway.
- Snipping the bonding strip off instead of folding it back.
- Treating standard interlocked MC armor as the ground and skipping the green conductor.
- Leaving out the anti-short bushing on AC.
Shop MC cable
- 12/2 MC stranded, 12/3 MC stranded and 10/2 MC stranded
- 12/2 MC for health care facilities
- 6/3 MC with a PVC jacket for wet and outdoor runs
- Feeder sizes in copper building wire and aluminum building wire
Verify with your local code. Some jurisdictions and project specs limit or ban one of these cables, and support, grounding and fitting rules shift between NEC editions. Ask your inspector before you rough in.