10.1 Wiring Methods: Cable Systems & Flexible Wiring

From Raceways to Cable Systems

Chapter 9 covered raceway-based wiring methods — rigid and intermediate metal conduit, electrical metallic tubing, and PVC — where conductors are pulled into an empty, already-installed tube. Cable systems take a different approach: the conductors and their protective covering are manufactured together as a single flexible assembly, then run through framing, along surfaces, or through fittings without a separate raceway doing the mechanical-protection work. Cable systems are faster to install in open framing and are the dominant branch-circuit method in much residential and light commercial construction, while flexible wiring methods solve a narrower problem — allowing a short section of a circuit to move, vibrate, or be aligned without disturbing a rigid system feeding it.

Nonmetallic-Sheathed Cable (NM Cable)

Nonmetallic-sheathed cable (NM cable) consists of two or more insulated conductors, plus typically an insulated or bare equipment-grounding conductor, assembled inside a tough nonmetallic (plastic) outer jacket. The jacket holds the conductors together, provides some abrasion resistance, and lets an installer run a complete branch circuit by simply routing and securing one cable rather than installing a raceway and pulling conductors separately.

Where NM Cable Fits

NM cable is generally an interior, dry-location method. It is a practical choice behind finished walls and ceilings where the framing itself shields the cable from impact, and where the space stays dry over the life of the building. It is not the tool for every location — wet locations, areas of significant physical abuse, or spaces requiring extra fire or mechanical performance call for a different wiring method, which is why the electrician's first job on any circuit is matching the wiring method to the location's conditions, not defaulting to whatever is fastest to pull.

Installation Judgment

On the exam and on the job, the underlying logic matters more than memorizing a single rule: NM cable's protection comes almost entirely from where it is installed (concealed, dry, protected by framing) rather than from the cable's own construction. That is the opposite of a raceway system, where the metal or rigid conduit itself is the primary physical shield regardless of what is around it.

Armored Cable (AC) and Metal-Clad Cable (MC)

Armored cable (AC), sometimes called BX in older trade usage, and metal-clad cable (MC) both wrap insulated conductors in a flexible metal jacket — typically a spiral-wound steel or aluminum armor. This construction gives the cable meaningfully more resistance to impact, crushing, and rodent damage than a nonmetallic jacket, without requiring a rigid raceway around it.

AC vs. MC — the Practical Distinction

• AC cable typically includes an internal bonding strip (a thin aluminum or copper strip run alongside the conductors under the armor) that works with the armor itself to provide a grounding/bonding path back to the panel.
• MC cable is a broader category that can include a dedicated equipment-grounding conductor inside the jacket alongside the circuit conductors, and can be built with more conductors, larger conductor sizes, or additional shielding for varied applications (power, control, or even fiber/data in some assemblies).

Both are chosen over NM cable when a project needs the speed and flexibility of a cable system plus more mechanical toughness than a plastic jacket provides — for example, interior spaces with foot traffic, storage, or general wear where a raceway would be overkill but bare NM would be under-protected.

Flexible Wiring Methods: FMC and LFMC

Raceways and cables handle most of a circuit's run, but almost every installation has at least one point where a rigid connection would be a liability rather than an asset. Flexible metal conduit (FMC) — a helically wound metal tube, often called "Greenfield" in trade slang — and liquid-tight flexible metal conduit (LFMC) — the same basic construction with an outer liquid-tight (typically PVC) jacket — exist for exactly that point.

Why Flexibility Is Needed

1. Motor connections. Motors vibrate in normal operation. A rigid conduit connection transmits that vibration directly into the connection point, and over time vibration loosens fittings and fatigues conductors. A short flexible section absorbs the vibration before it reaches the rigid system.
2. Equipment that moves or is adjusted. Some equipment is periodically repositioned, realigned, or requires slight movement for maintenance access. A flexible connection tolerates that movement; a rigid one cracks or pulls apart.
3. Final alignment tolerance. Even non-moving equipment sometimes cannot be positioned with the precision a rigid conduit run demands. A short flexible whip absorbs minor misalignment between a fixed raceway stub-up and the equipment's actual connection point.

LFMC is the choice when the flexible connection is also exposed to oil, coolant, water spray, or outdoor weather — the liquid-tight jacket keeps moisture and liquids away from the conductors inside, which plain FMC's open metal helix cannot do.

The Grounding Question at Flexible Connections

A rigid metal raceway system is often relied upon as part of the equipment-grounding path, since a continuous metal conduit run is electrically continuous back to the panel. A short section of flexible metal conduit is a different story. The helical wound construction of FMC (and LFMC) does not always provide the same low-impedance, reliable path that a solid rigid conduit does — the connections between wraps and the fittings at each end introduce more resistance and more opportunity for a poor bond. For that reason, a flexible connection is typically kept short, and a separate equipment-grounding conductor is run through (or alongside) the flexible conduit unless the specific flexible conduit and fittings are listed and recognized as suitable for grounding on their own. The safe assumption on both the exam and the job site is: do not assume a flexible metal connection grounds itself — verify the grounding path exists independently.

Supporting and Securing Cable and Flexible Conduit

Whether it is NM cable, AC/MC cable, or FMC/LFMC, a flexible wiring method that is left unsecured will eventually sag, chafe against sharp edges or framing members, or place mechanical strain on its termination points. The general principle behind every cable-support requirement is straightforward even without memorizing an exact spacing figure: cable and flexible conduit must be secured close enough to its terminations, and supported at regular intervals along its run, that it cannot sag under its own weight, rub against abrasive surfaces, or transmit pulling/twisting stress into a device or panel connection. Staples, straps, and cable ties rated for the wiring method being used accomplish this. An electrician evaluating an installation should ask: if this cable moved slightly under thermal expansion, vibration, or incidental contact, would it chafe against something sharp, or would it pull on a termination? If the answer is yes, the run needs another support point.

Choosing the Right Method

The recurring theme across NM, AC/MC, and flexible conduit is that the wiring method should match both the electrical need and the physical environment. NM cable trades mechanical toughness for installation speed in protected, dry, concealed spaces. AC/MC cable adds mechanical ruggedness while keeping cable-system speed. FMC and LFMC solve the narrow but universal problem of connecting a rigid system to something that vibrates, moves, or needs final-fit flexibility — with LFMC adding liquid protection where FMC's open helix would let moisture in. None of these methods eliminates the need for a verified equipment-grounding path; they simply change how that path is achieved.