How to Terminate MC Cable

1. Pick a listed connector that matches the cable and the box

NEC 330.40 requires fittings used with MC cable to be listed and identified for the cable. That means a real MC connector — not an NM (Romex) clamp, not whatever's loose in the gang box. Connectors are sized by cable diameter; a connector marked for 14/2–10/2 MC won't properly grip 8/3, and an oversized connector won't grip at all. Check the connector's marking or the box label against your cable.

Style is a productivity call. Set-screw connectors are cheap and universal — one screw bears on the armor, and you spin the locknut inside the box. Snap-in (push-in) connectors install into the KO by hand with no locknut and cut termination time roughly in half on high-count jobs like lighting whips; the trade-off is they're essentially one-time-use and cost more per hole. Two-screw saddle connectors grip the armor hardest and are the right pick for larger MC and anywhere the cable sees strain. Whatever you use, follow the manufacturer's instructions — 110.3(B) makes them enforceable code.

2. Cut the armor with a rotary cutter, not dikes

Measure how much free conductor you need in the box — figure 6 inches minimum of free conductor per 300.14, measured from where the conductors emerge from the cable sheath in the box, with at least 3 inches extending outside the box opening where the opening is under 8 inches in any dimension. Mark the armor there.

Set the rotary cutter's depth so the blade scores the armor without touching the conductors, clamp it square on the cable at your mark, and crank until you feel it break through one convolution. Open the tool, grab the short end, and twist against the spiral — the armor separates clean at the score line. Slide the cut section off the conductors.

Dikes work in a pinch: bend the cable sharply at your mark until a convolution of armor opens up, snip the exposed strip, then unwind and twist off. It's slower and it's where most nicked conductors come from, because the cutter jaw rides against the insulation. If you cut MC every day, the rotary cutter pays for itself the first week. Either way, inspect every conductor after the cut — any nick or shiner in the insulation means cut it back and do it again.

3. Install the anti-short bushing — and know what the code actually says

Slip a red head (anti-short bushing) over the conductors and seat it inside the cut end of the armor so it separates the conductors from the sharp edge. Here's the code reality: NEC 320.40 requires an insulating bushing at every AC cable termination — that's Article 320, armored cable. Article 330 for MC cable has no equivalent blanket requirement. For MC, red heads are required only where the cable's listing or the fitting manufacturer's instructions call for them, which 110.3(B) then makes mandatory.

In practice: the bushings come in the carton, they cost nothing, and plenty of inspectors expect to see them regardless of the fine print. Some MC connectors are listed with an insulated throat that does the same job. Install the red head anyway unless the fitting instructions say otherwise — it's a two-second insurance policy on the exact spot where armor edges eat insulation.

4. Land the connector and make it mechanically sound

Slide the connector over the cable end so the armor bottoms out against the stop and the conductors pass through the throat. On a set-screw connector, tighten the screw onto the armor — snug, so the cable can't pull out or rotate, without crushing the armor oval. On a saddle type, tighten both screws evenly.

Remove the correct-size knockout at the box, feed the conductors through, seat the connector, and run the locknut down tight — set it with a flat screwdriver and hammer tap or locknut pliers so the teeth bite the box. A loose locknut is a loose fitting and a bad bond path. Snap-in connectors just press into the KO until they click; give the cable a firm pull test to confirm the tangs set. Only one cable per connector unless the fitting is listed for multiple cables — duplex connectors exist and are marked for it.

5. Secure within 12 inches, support every 6 feet — 330.30

NEC 330.30(B) requires MC cable containing four or fewer conductors sized no larger than 10 AWG — which covers typical branch-circuit MC — to be secured within 12 inches of every box, cabinet, fitting, or other termination point, and cables must be secured and supported at intervals not exceeding 6 feet per 330.30(B) and (C). Securing means the cable can't move at the termination — a strap, listed clip, or cable tie to structure. Support means the run's weight is carried — bridle rings, batwing clips on ceiling wire, J-hooks, or straps. Ceiling grid itself isn't support unless you're using clips listed for the purpose and the AHJ allows independent support wires per local practice.

There are real exceptions in 330.30(D): cable fished through concealed finished spaces doesn't need securing where impractical, and lengths of not more than 6 feet from the last point of cable support to a luminaire or equipment within an accessible ceiling are permitted unsupported — that's your fixture whip allowance. It is not a license to leave 20 feet of MC lying on the grid. Whips draped across ceiling tiles are the single most common MC violation on commercial punch lists.

6. Check box fill and bundling before you stuff the box

Box fill is 314.16. Count each insulated conductor that terminates or is spliced in the box at the volume from Table 314.16(B)(1) — 2.25 cubic inches each for 12 AWG, 2.50 for 10 AWG. Add one allowance for all internal clamps together (if any), one allowance covering up to four EGCs plus a quarter allowance for each EGC beyond four (a 2020 change), and two allowances per device yoke at the largest conductor connected to it. A 4-inch square 1-1/2 deep box is 21 cubic inches — it fills up fast with two 12/2 MCs and a mud ring full of devices, so do the math before rough-in, not at trim.

Bundling matters on homerun racks. Where more than three current-carrying conductors are bundled longer than 24 inches without maintaining spacing, the ampacity adjustment factors of Table 310.15(C)(1) apply — 4 to 6 current-carrying conductors get derated to 80 percent. Stacking a dozen MC homeruns in one tight bundle above the panel can quietly derate your 12 AWG below 20 amps. Spread the runs or do the derate math from the 90°C column.

7. Terminate the conductors and torque to spec

Strip conductors to the device or lug's strip gauge, land the EGC first, and terminate per 110.14. Termination temperature limits are 110.14(C) — for circuits 100 amps or less, use the 60°C ampacity unless the termination is marked 75°C, even though MC conductors are typically THHN rated 90°C. The 90°C column is for derating headroom, not final ampacity at the lug.

NEC 110.14(D) requires terminations to be tightened to the manufacturer's marked torque using an appropriate tool. Breaker lugs, panel neutral bars, and device screws all have published values — use a torque screwdriver, not your wrist calibration. Before you close up, give every cable one last look: red head seated, connector locknut tight, armor secured within 12 inches, conductors free of nicks. That thirty seconds is your whole quality program.

Does the NEC require anti-short bushings (red heads) on MC cable?

Not directly. NEC 320.40 requires insulating bushings for AC cable, but Article 330 for MC has no equivalent rule. For MC, red heads are required only where the cable's listing or the fitting manufacturer's instructions call for them, which NEC 110.3(B) makes enforceable. Most electricians install them on every termination anyway.

How close to the box does MC cable have to be secured?

NEC 330.30(B) requires MC cable containing four or fewer conductors sized no larger than 10 AWG — typical branch-circuit MC — to be secured within 12 inches of every box, cabinet, fitting, or other termination, and the run must be secured and supported at intervals not exceeding 6 feet. Fished cable and fixture whips up to 6 feet within an accessible ceiling have specific exceptions in 330.30(D).

Can I use the MC armor as the equipment grounding conductor?

Standard interlocked-armor MC is not listed as an equipment grounding conductor by itself, so you must use the green insulated EGC inside the cable. Certain products such as MC-AP style cable with a bare aluminum bonding conductor under the armor are listed so the armor assembly qualifies as the EGC; check the cable's marking and listing.

What's the best tool for cutting MC cable armor?

A rotary MC cutter, often called a Roto-Split, scores one convolution of armor at a set depth so the armor twists off clean without touching the conductors. Dikes work by bending the cable and snipping the armor, but they are slower and cause most nicked-conductor damage. For daily MC work the rotary cutter is the standard.

Are snap-in MC connectors code compliant?

Yes, as long as they are listed for MC cable and installed per the manufacturer's instructions, which satisfies NEC 330.40 and 110.3(B). They press into the knockout without a locknut and save significant time on high-volume work. Always pull-test the cable after installation to confirm the retention tangs have set.

How many current-carrying conductors can I bundle before derating?

Once more than three current-carrying conductors are bundled together for longer than 24 inches, the adjustment factors of NEC Table 310.15(C)(1) apply. Four to six current-carrying conductors are derated to 80 percent of ampacity, and the percentage drops as the count grows. Start the derate math from the conductor's 90°C rating if the insulation allows.