NEC Article 348 — Flexible Metal Conduit (FMC) for HVAC, Motors, and Final Connections

NEC 348 governs Flexible Metal Conduit (FMC) — what's called "Greenfield" or just "flex." Used for the final 6 ft to vibrating equipment (HVAC condensers, well pumps, motor-driven equipment) and as luminaire whips above suspended ceilings.

Where FMC is used:

• HVAC condenser whip — from a building disconnect or junction box, FMC for the last 3-6 ft to the unit so vibration doesn't transfer to rigid conduit
• Motor connections — same vibration-isolation reason
• Luminaire whips — 6 ft maximum per 410.117 connecting an outlet box to a luminaire above a suspended ceiling
• Equipment with movement — rolling racks, lift gates, etc.

FMC as ground path (348.60): FMC IS allowed as the equipment grounding conductor (EGC) when:

• The total length of FMC and LFMC (liquidtight) in the ground path is no more than 6 ft total
• The conduit fittings are listed for grounding
• The branch circuit is 20 A or less

For runs over 6 ft total or circuits over 20 A, you MUST run a separate green or bare EGC inside the FMC. This is the rule most often missed on EV charger installs and water heater connections.

Cable type usually inside FMC:

• THHN/THWN-2 for residential branch circuits and feeders
• MTW (machine tool wire) for some industrial

Length and securing (348.30): FMC must be supported within 12 inches of every box, fitting, or termination, and at intervals of 4.5 ft. Exceptions for whips and where structural conditions don't allow it.

LFMC (Article 350) — Liquidtight Flexible Metal Conduit — is the same idea but waterproof. Used outdoors for HVAC condenser whips and well pump connections. Same length-as-EGC rules apply.

For a typical NJ HVAC condenser disconnect installation:

• Disconnect mounted to the wall near the condenser
• LFMC whip from disconnect to condenser unit (3-4 ft typical)
• Inside the LFMC: 2 phase conductors + EGC if over 6 ft or over 20 A

For full load calc on HVAC additions, the Malfettone Load Calculator at /tools/load-calculator has central A/C and heat pump presets that apply the larger-of-A/C-or-heat rule per NEC 220.82(C).