MC Cable Explained: A Practical Guide to NEC Article 330

The Most Versatile Wiring Method — and the Most Misunderstood

Metal-clad cable is one of the most popular wiring methods in the world, and it is the everyday workhorse on countless job sites. But as Paul Abernathy of Master The NEC points out, just because it is wrapped in metal, people make a lot of assumptions about what it can and cannot do.

NEC Article 330 covers the use, installation, construction and specifications of MC cable. Get familiar with how that article is built and most of the confusion disappears. Let’s walk through it the way a working electrician actually needs to understand it.

Stranded vs Solid — Stop Going Down the Rabbit Hole

Before the lesson even started, the conversation drifted to a classic social-media argument: does stranded or solid wire carry more current? From a code standpoint, the answer is simple.

“A 14-gauge stranded is the same amount of current that the NEC says… same with a 12, same with a 10.”

Yes, Chapter 9, Table 8 lists slightly different DC resistance values for stranded versus solid, but those fractions of an ohm mean nothing to an electrician sizing conductors. If you need ampacity, you go to Table 310.16. Skin effect is negligible in 60 Hz systems — it doesn’t become a factor until you’re well up into the hundreds of hertz. Learn the code first; leave the engineering theatre alone.

The Three Flavours of MC

MC cable isn’t one product. It comes in three constructions, and the type matters:

• Interlocked — a strip of aluminium or steel formed around the conductors. This is roughly 90% of the market and what you handle every day. It cannot be used as an equipment ground on ordinary MC.
• Smooth — a continuous extruded tube that looks a bit like EMT. It is impervious to moisture and qualifies as an equipment grounding conductor.
• Corrugated / continuous weld — like an interlock but each convolution is welded. Common in hazardous locations.

All of it must be listed to UL 1569, and the fittings and connectors must be listed too. That’s why anti-short bushings aren’t required on MC — the listed fittings are evaluated to work without them. Use one if it helps you sleep, but it isn’t part of the listing.

Uses Permitted — 330.10

Per 330.10, MC cable is permitted for services, feeders and branch circuits; for power, lighting, control and signalling; indoors and outdoors; exposed or concealed. You can run it above a suspended ceiling, even in an environmental airspace acting as a plenum without any additional plenum rating.

Here’s the single biggest point people miss:

“MC cable is a dry-location product. It is not designed to be installed in a wet location inherently if you just have the armour.”

To go into a damp or wet location, interlocked MC needs a corrosion-resistant jacket (PVC) extruded over the armour and wet-location-rated conductors inside. Notice the power of the word and in 330.10 — it’s not a pick-one list. The PVC coating plus wet-rated conductors together is what earns the wet/damp rating.

MC can also be:

• Direct buried, where identified for such (watch your depth of cover in 300.5)
• Installed in cable tray, where marked for CT use
• Run as aerial cable on a messenger (follow 396 for messenger rules)
• Used in hazardous (classified) locations where specifically permitted — often requiring an MC-HL corrugated type per Articles 500–503

Supporting Cables Above a Ceiling — 300.11(B)

A great viewer question: can you tie MC to drop-ceiling hanger wires with batwing clips? Not the wires that support the ceiling itself. Under 300.11(B), cables in a non-fire-rated assembly cavity shall not be secured to or supported by the ceiling support wires. You run your own independent support wires, and they must be secured at both ends — no trapeze, no swinging. The only exception is a fire-rated assembly where the grid manufacturer has listed its wires to carry the extra load — so contact the manufacturer, not the installer.

Uses Not Permitted — 330.12

MC cable shall not be used:

1. Where subject to physical damage. Aluminium or steel, both armour types must pass the same crush and impact test — the metal doesn’t make it rated for abuse. Sleeve it in EMT where damage is a concern.
2. Where exposed to destructive corrosive conditions, unless the sheath is resistant or protected. PVC-jacketed MC is highly resistant to chlorides, caustic alkalis and chlorine vapours — Paul ran an 18-month hydrochloric-acid vapour test and the jacketed cable looked brand new while plain MC was destroyed. That’s why properly jacketed MC holds up in pool rooms and direct burial.

Bending Radius — 330.24

Bend it too tight and you blow out the back of the armour, exposing the conductors — and there is no market-available repair for damaged MC. The minimum inner bend radius is based on the cable’s published outside diameter (OD):

• Smooth sheath: 10× OD (≤ 3/4 in), 12× OD (3/4–1½ in), 15× OD (> 1½ in)
• Interlocked / corrugated: 7× the external diameter
• Shielded conductors: 12× the individual conductor diameter or 7× the cable diameter, whichever is greater

A practical tip: don’t create big loops to feed back into a box. Come straight in — loops put unnecessary tension on the assembly.

Securing and Supporting — 330.30

The numbers here are easy to remember because they match:

• Secured every 6 ft, and supported every 6 ft
• Cables with four or fewer conductors no larger than 10 AWG must be secured within 12 in of every box, cabinet, fitting or termination
• Larger MC follows the general 6 ft rule even at the first support
• Vertical runs with 250 kcmil and larger ungrounded conductors may be secured every 10 ft

A cable fitting counts as a means of support. That matters for whips: an unsupported, unsecured length is permitted up to 6 ft from the last point of support to a luminaire or equipment within an accessible ceiling (added clearly in the 2023 NEC), or 3 ft for interlocked MC connecting equipment where flexibility is needed to limit vibration.

Ampacity and Equipment Grounding

Ampacity comes from Table 310.16 for most installations, with adjustment and correction factors from 310.15(C)(1) when bundling. Even though MC conductors are often 90°C rated, you’re still limited by the terminal rating — usually 60°C or 75°C.

For grounding, every MC cable requires an equipment ground. 250.118(A)(10) spells out three ways MC qualifies as an effective ground-fault current path: a wire-type EGC inside, the interlocked armour combined with an uninsulated bonding conductor (the “smart ground” / MCAP products), or the metal sheath of smooth/corrugated MC. On traditional MC you do not rely on the armour as the path — you bond it at each end and run a separate EGC sized per 250.122.

The Bottom Line

MC cable is flexible and forgiving, but the details in Article 330 are what separate a clean install from a callback:

• It’s a dry-location product unless jacketed
• It’s not rated for physical damage
• Secure and support every 6 ft, within 12 in of boxes for small cables
• Don’t over-bend it — there’s no repair
• Always include the right equipment ground

“I can teach a monkey to run cable, but I can’t teach a monkey to understand the nuances of securing, supporting and installation. Stop being a monkey.”

How NEC Mastery Fits Into This

Article 330 is a perfect example of why navigation beats memorisation — the answers are all there if you know how the article is organised and how the words like “and” change the rule. NEC Mastery is built to drill exactly that kind of code reasoning.

• 8,000+ exam-style questions give you endless repetition on wiring methods like MC, AC and NM cable so the uses-permitted and uses-not-permitted distinctions become second nature
• Detailed explanations referencing specific NEC articles — 330.10, 330.12, 330.24, 330.30 and 250.118 — reinforce where each rule lives and why it reads the way it does
• Timed mock exams weighted to your exam type let you practise finding answers under pressure, so on test day flipping to the right article feels routine

You don’t need to memorise every section of Article 330. You need to know where to look and how to read it — and that’s exactly the skill NEC Mastery builds, one practice question at a time.