What is Flexible Metal Conduit (FMC)?
FMC, also called Greenfield, is spiral-wound steel conduit for short flexible connections, governed by NEC Article 348. Limited as EGC over 6 ft.
Flexible Metal Conduit (FMC), commonly called Greenfield, is a spiral-wound interlocked steel raceway used for the final connection to motors, transformers, light fixtures, and equipment where rigid conduit cannot accommodate movement or alignment. It is governed by NEC Article 348. FMC is bendable by hand, allows vibration isolation, and is the standard short-flex connection in commercial work — but its grounding capability is sharply limited.
Construction
- Material: Galvanized steel strip, spiral-wound with interlocking edges
- Trade sizes: 3/8" (limited use) through 4"
- No outer jacket — bare metal coils (compare to LFMC which has a PVC jacket)
- Length: Sold in coils, typically 25 ft, 50 ft, or 100 ft
The interlocked spiral construction gives FMC its flexibility but also makes it electrically less reliable as a grounding path than threaded RMC or IMC.
Interior cross-section (NEC Chapter 9 Table 4)
| Trade size | Interior area in² |
|---|---|
| 3/8" | 0.077 |
| 1/2" | 0.317 |
| 3/4" | 0.533 |
| 1" | 0.817 |
| 1-1/4" | 1.277 |
| 1-1/2" | 1.858 |
| 2" | 3.269 |
| 2-1/2" | 4.909 |
| 3" | 7.069 |
| 3-1/2" | 9.621 |
| 4" | 12.566 |
Apply NEC Chapter 9 Table 1 fill percentages on the flex conduit fill chart. The 3/8" size has its own dedicated fill table — NEC Table 348.22 — for fixture whips.
NEC code reference (Article 348)
- NEC 348.10 — permitted in dry, exposed or concealed locations
- NEC 348.12 — not permitted in wet locations (use LFMC), where exposed to gasoline or oil, in hoistways (other than elevator wiring), in storage battery rooms, or in hazardous locations except as permitted by other articles
- NEC 348.20 — minimum size 3/8" (with restrictions); 1/2" through 4" for general use
- NEC 348.22 — special fill table for 3/8" FMC fixture whips
- NEC 348.30 — supports within 12 inches of every box and every 4-1/2 ft thereafter (with exceptions for fixture whips and fished concealed runs)
- NEC 250.118(5) — FMC as EGC: 6 ft maximum length, 20 A overcurrent maximum, listed fittings
Common applications
- Motor connections (vibration isolation)
- Recessed lighting fixture whips (3/8" or 1/2", 4–6 ft drops)
- HVAC equipment final taps
- Transformer secondary feeders (short run)
- Equipment subject to occasional relocation
- Final connections in tight ceiling cavities
Trade-offs vs LFMC and Rigid
| Property | FMC | LFMC | RMC |
|---|---|---|---|
| Article | NEC 348 | NEC 350 | NEC 344 |
| Jacket | Bare metal | PVC outer | Bare metal |
| Wet locations | No | Yes | Yes |
| EGC qualifier | Limited (6 ft, 20 A) | Limited similarly | Yes |
| Cost per ft | Lowest of flex | Higher | Highest |
| Bend radius | Very tight | Very tight | Mechanical bender |
For most equipment connections, a separate green EGC pulled inside the FMC sidesteps the 6-ft / 20-A grounding restriction. Many specifications require this regardless.
Bonding and grounding details
Per NEC 250.118(5), the conditions for FMC as EGC are cumulative:
- Conduit terminated in listed fittings, AND
- Circuit conductors protected by overcurrent devices rated 20 A or less, AND
- Combined length in any ground return path is 6 ft or less, AND
- The conduit is not installed for flexibility
Fail any one condition and a separate EGC is required.
Quick reference
- Article: NEC 348
- Slang: Greenfield
- Wet locations: no — use LFMC
- EGC qualifier: limited (6 ft, ≤20 A)
- Common use: fixture whips, motor terminations