MC cable is a factory-assembled wiring method with insulated conductors inside flexible metal armor, and 12/3 MC carries three 12 AWG conductors plus a ground at a 600 V rating.
A coil of 12/3 MC bends by hand, feeds through a knockout, and needs no conduit. MC (metal-clad) is its own code-recognized wiring method under NEC Article 330, installable as a complete system on services, feeders, and branch circuits, indoors or outdoors, exposed or concealed, where code conditions are met. The tricky parts are grounding, ampacity, and that two spools of “12/3 MC” can be built differently.
What’s Inside A 12/3 MC Cable Assembly?
Inside the armor: three current-carrying 12 AWG conductors with THHN/THWN insulation plus a green equipment grounding conductor, assembled at the factory. Southwire’s Armorlite Type MC listing for 12-3 solid copper confirms this build: 12 AWG, three conductors, green grounding conductor, 600 V rating. Its MC/AP line (“12-3 STR CU THHN BLACK/WHITE/RED/AL GRND MCAP AIA 250 FT”) uses stranded copper instead of solid.
Colors follow circuit voltage. Southwire’s 12-3 Armorlite page lists black, red, white, and green for 120/208 V circuits; the 277/480 V convention — brown, orange, gray — appears in Southwire’s Duraclad Type MC family, which offers 12 AWG options.
The armor is usually aluminum interlocking strip or steel. Dimensions shift by construction: Southwire’s 12-3 solid Armorlite is 0.514 in over the armor at about 132 lb per 1000 ft; its MC/AP version is 0.528 in at about 122 lb; a 120/208 V colors MC/AP listing is 0.506 in and roughly 117 lb; another 12/3 solid MC listing shows 0.530 in minimum armor O.D. and about 135 lb per 1000 ft.
Does The Armor Ground The Circuit?
Usually not by itself. For standard interlocked-armor MC, the armor is not the equipment grounding conductor — a wire-type grounding conductor must be included unless that construction qualifies under NEC 250.118. That’s the most common wrong assumption, and the key difference from metal-jacketed cable.
Code also sets physical limits: support every 6 ft and within 12 in of each box under NEC 330.30; bend radius at least seven times the cable diameter under NEC 330.24. For 0.514 in cable, that’s about 3.6 in — tighter stresses armor and insulation.
- Support: every 6 ft, plus within 12 in of every box.
- Bend radius: at least 7x the cable diameter.
- Grounding: include the wire-type conductor unless NEC 250.118 says otherwise.
Spools, Ampacity, And What Changes Between Listings
12/3 MC ships on spools of 10, 25, 50, 100, and 250 ft; some solid constructions also come in 1000 ft lengths.
Not every 12/3 MC product is interchangeable, so compare listings before buying. Solid versus stranded copper, aluminum versus steel armor, colors matched to 120/208 V or 277/480 V systems, and diameter and weight all vary by line — Southwire alone sells 12/3 MC under Armorlite, MC/AP, and Duraclad names. Seller naming adds another layer: “12/3,” “12-3,” “3×12,” and “with ground” don’t always describe identical construction, and hazardous-location MC-HL is a separate family, not a stand-in for standard building wire.
For a small job, comparing the top 12/3 MC and BX cable options in 25 ft lengths side by side is faster than checking each spec sheet.
| Southwire 12/3 MC Listing | Outside Diameter | Weight Per 1000 Ft |
|---|---|---|
| Armorlite, solid copper, 120/208 V colors | 0.514 in | 132 lb |
| MC/AP, stranded copper, 250 ft | 0.528 in | 122 lb |
| MC/AP, solid, 120/208 V colors | 0.506 in | 117 lb |
| Solid MC, 250 ft and 1000 ft spools | 0.530 in min armor O.D. | 135 lb |
Southwire publishes full spec sheets for its Armorlite Type MC 12-3 copper construction, the fastest way to confirm a part number.
What To Confirm Before You Install
Match three things to the job. First, confirm the grounding path: if the assembly doesn’t qualify under NEC 250.118, run the wire-type equipment grounding conductor. Second, check conductor colors against system voltage so 120/208 V and 277/480 V codes don’t mix. Third, verify ampacity and termination temperature against the breaker.
Then respect two physical rules: support every 6 ft and within 12 in of each box, and keep bends at seven times the cable diameter or wider.
FAQs
Can 12/3 MC cable be used outdoors?
Yes, in most cases. MC cable is permitted indoors or outdoors, exposed or concealed, subject to NEC Article 330 conditions. The armor resists physical damage without a raceway, which is why it’s common on exterior runs. Check the product listing for any wet-location or sunlight-resistance rating before committing.
How many wires are in 12/3 MC cable?
A 12/3 MC assembly has three current-carrying 12 AWG conductors plus a green equipment grounding conductor — four insulated wires total. The “3” refers to current-carrying conductors only; the ground is separate and always present in standard construction.
What ampacity does 12 AWG MC cable carry?
Twelve-gauge copper in MC cable is rated 20 A for standard branch-circuit and breaker sizing, matching 12 AWG THHN/THWN insulation in typical 60 °C terminations. Some listings quote 25 A at 86 °F for the conductor itself, but breaker and termination ratings govern the circuit.
References & Sources
- Southwire. “Armorlite Type MC, 12-3 Solid Copper, 120/208V Colors.” Confirms 12 AWG conductors, green grounding conductor, 600 V rating, and 0.514 in diameter at 132 lb/1000 ft.
- Southwire. “MC/AP Type MC All-Purpose, 12-3 Stranded Copper, 120/208V Colors.” Source for the stranded 12-3 assembly, 0.528 in diameter, and 122 lb/1000 ft weight.
- Southwire. “Duraclad Type MC, 277/480V Colors.” Shows the brown/orange/gray color convention and 12 AWG options in the Duraclad family.
