12/3 MC cable carries three 12-gauge conductors plus a ground inside flexible metal armor, rated 600V for branch circuits.
The gap comes down to armor material, jacket type, and strand count — not to any difference in what the cable does. Underneath the armor, every one of them is the same basic build: three 12-gauge copper conductors, one green ground, and an interlocked metal jacket that stands in for conduit.
That 600V assembly covers most commercial branch circuits and a growing share of residential homeruns. The sections below break down what’s inside the cable, where it belongs, and which specs move the price.
What Does 12/3 MC Cable Have Inside It?
A standard 12/3 MC cable holds three current-carrying 12 AWG copper conductors plus a green grounding conductor, all wrapped in interlocked aluminum or galvanized steel armor. The “12/3” label counts the current-carrying conductors; the ground rides along without appearing in that number.
Southwire notes on its Armorlite Type MC listings that the conductor number covers phase conductors and leaves out the green ground. Insulation on the copper is THHN/THWN, and one manufacturer listing puts the dry-location temperature rating at 90°C. Conductors in 120/208V sets are colored black, red, and white, with green for the ground; neutral-per-phase versions add extra white-striped neutrals.
Two builds sell side by side. Solid 12 AWG copper measures about 0.514 in over the armor and weighs roughly 132 lb per 1,000 ft on one Southwire listing. A 19-strand class C stranded version with PVC jacketing runs about 0.648 in and 215 lb per 1,000 ft.
Dimensions, weight, and armor options for a specific SKU are laid out on Southwire’s Armorlite Type MC product page.
| Cable Detail | Typical Rating Or Size | What It Changes |
|---|---|---|
| Conductors | Three 12 AWG copper plus ground | Feeds a 120/208V circuit that needs a neutral |
| Voltage rating | 600V | Rated above typical branch-circuit voltage |
| Insulation | THHN/THWN, 90°C dry | Standard building-wire insulation type |
| Armor | Interlocked aluminum or galvanized steel | Flexible jacket that replaces conduit |
| Outside diameter | About 0.51 in to 0.65 in | Stranding and jacketing widen the cable |
| Weight | Roughly 132 to 215 lb per 1,000 ft | Heavier coils pull harder |
| Common lengths | 250 ft coil, 1,000 ft and 5,000 ft reels | 250 ft is the usual retail size |
Where 12/3 MC Cable Gets Used
12/3 MC cable feeds 20-amp branch circuits and multi-wire homeruns in commercial buildings, and it turns up in residential work wherever a metal-clad wiring method is required. Retail listings pair the cable with a 20-ampere THHN rating.
The 120/208V color set makes it a natural fit for panel-fed circuits that share a neutral. A PVC-jacketed version exists for runs where bare interlocked armor won’t do, and that jacketed build also comes in stranded copper.
Compliance claims deserve a careful read. One retailer listing states the product meets UL 83, 1479, 1569, 1581, and 2556, plus a long list of NEC articles and OSHA and HUD requirements. Those references attach to that specific SKU, not to every 12/3 MC cable sold. Local code and your inspector decide what’s allowed on a given job.
Buying 12/3 MC Cable: What Decides The Order
Four things drive the cost of a 12/3 MC cable order: armor material, jacket type, strand count, and coil length. Armor moves the number first, since aluminum costs less than galvanized steel.
That’s a retailer figure, not a manufacturer list price, and it shifts with copper markets. Southwire identifiers worth having on hand include 68583455, 56727402, and 68583409. The 250 ft coil is the length most supply houses stock, alongside 1,000 ft and 5,000 ft reels.
Before you order, it helps to see how the brands compare on armor, stranding, and price per foot — our roundup of 12/3 MC cable options lines them up side by side.
For a typical 120/208V job, a 250 ft coil of solid 12/3 with a green ground covers most runs at the lowest cost per foot. Step up to stranded copper when the pull has bends, and to a PVC-jacketed build when the run needs protection over the armor.
FAQs
Is 12/3 MC cable the same as 12/2 MC?
No. The number after the slash counts current-carrying conductors, so 12/3 carries three 12-gauge conductors and 12/2 carries two. Both include a separate green grounding conductor that isn’t part of that count. The third conductor lets one cable serve a circuit that needs a neutral, such as a 120/208V multi-wire run.
What size breaker does 12-gauge MC cable take?
Most 12 AWG copper branch circuits are protected by a 20-amp breaker. Insulation on 12/3 MC cable is typically THHN/THWN, which carries a higher temperature rating than the 60°C column, but breaker size follows the circuit design and termination ratings rather than the cable’s maximum. Check the panel schedule and local code before changing a breaker.
Can 12/3 MC cable be run outdoors?
It depends on the version. Standard interlocked armor without a jacket is made for dry indoor locations, while a PVC-jacketed 12/3 MC cable is available for runs that need protection over the armor. Whether a given cable can sit outdoors or in a wet spot comes down to its listing and your local code.
References & Sources
- Southwire. “Armorlite Type MC THHN/THWN Circuit-Size Copper Conductor, 120/208V Colors” Source for 12/3 solid copper construction, armor diameter, conductor weight, and color coding.
- Southwire. “Armorlite Type MC THHN/THWN 600V PVC-Jacketed, 120/208V Colors” Covers the jacketed stranded 12/3 build and its larger outside diameter and weight.
- Southwire. “Armorlite Type MC THHN/THWN Circuit-Size Copper Conductor, 120/208V Colors” Additional 12/3 listing used to confirm conductor counts and 600V rating.
