IMPORTANT
Quick Summary: Under the National Electrical Code (NEC Article 348 and Annex C Table C.3), Flexible Metal Conduit (FMC), commonly known as “Greenfield”, is a spiral-wound metal raceway engineered for flexibility, equipment vibration absorption, and fixture connections. When pulling three or more conductors of the same size (THHN/THWN-2 at 40% fill), the standard capacities are:
- 3/8″ FMC (Fixture Whips): 4 wires of 14 AWG | 3 wires of 12 AWG | 2 wires of 10 AWG (NEC Table 348.22)
- 1/2″ FMC: 12 wires of 14 AWG | 9 wires of 12 AWG | 5 wires of 10 AWG | 2 wires of 6 AWG
- 3/4″ FMC: 21 wires of 14 AWG | 15 wires of 12 AWG | 10 wires of 10 AWG | 4 wires of 6 AWG
- 1″ FMC: 33 wires of 14 AWG | 24 wires of 12 AWG | 15 wires of 10 AWG | 6 wires of 6 AWG
- 1-1/4″ FMC: 53 wires of 14 AWG | 38 wires of 12 AWG | 24 wires of 10 AWG | 10 wires of 6 AWG
Key Takeaways
- The 40 Percent Standard (NEC Chapter 9 Table 1): For standard conduit runs containing three or more conductors, total wire cross-sectional area cannot exceed 40% of the FMC conduit interior area.
- 3/8-Inch Trade Size Allowance (NEC 348.20(B)): While 1/2-inch is the general minimum conduit size under the NEC, 3/8-inch FMC is legally permitted in lengths up to 6 feet for lighting fixture taps and motor connections per NEC Table 348.22.
- Equipment Grounding Mandate (NEC 348.60): FMC may only serve as an equipment grounding conductor if the run is 6 feet or less and the circuit breaker does not exceed 20 amps. For longer runs or higher amperage, a separate green equipment grounding conductor is mandatory.
- Dry Locations Only (NEC 348.12): Standard FMC is not liquidtight and cannot be installed in wet or damp locations, underground in direct burial trenches, or outdoors exposed to weather.
FMC Greenfield Fill Rules At-A-Glance
NEC Article 348 Flexible Metal Conduit Standards
Master FMC Conduit Fill Chart (NEC Annex C Table C.3)
The table below provides the maximum allowable number of conductors for compact copper THHN/THWN-2 conductors inside Flexible Metal Conduit (FMC) from 3/8 inch to 4 inches based on NEC Chapter 9 Table 4 (40% fill limit for 3 or more wires).
| Conductor Size (AWG/kcmil) | Wire Area (sq. in.) | 3/8″ FMC (Table 348.22) | 1/2″ FMC (0.125 sq. in.) | 3/4″ FMC (0.211 sq. in.) | 1″ FMC (0.327 sq. in.) | 1-1/4″ FMC (0.515 sq. in.) | 1-1/2″ FMC (0.745 sq. in.) | 2″ FMC (1.307 sq. in.) | 2-1/2″ FMC (1.963 sq. in.) | 3″ FMC (2.827 sq. in.) | 3-1/2″ FMC (3.848 sq. in.) | 4″ FMC (5.027 sq. in.) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 14 AWG THHN | 0.0097 | 4 | 12 | 21 | 33 | 53 | 76 | 134 | 202 | 291 | 396 | 518 |
| 12 AWG THHN | 0.0133 | 3 | 9 | 15 | 24 | 38 | 56 | 98 | 147 | 212 | 289 | 377 |
| 10 AWG THHN | 0.0211 | 2 | 5 | 10 | 15 | 24 | 35 | 61 | 93 | 133 | 182 | 238 |
| 8 AWG THHN | 0.0366 | 0 | 3 | 5 | 8 | 14 | 20 | 35 | 53 | 77 | 105 | 137 |
| 6 AWG THHN | 0.0507 | 0 | 2 | 4 | 6 | 10 | 14 | 25 | 38 | 55 | 75 | 99 |
| 4 AWG THHN | 0.0824 | 0 | 1 | 2 | 3 | 6 | 9 | 15 | 23 | 34 | 46 | 60 |
| 3 AWG THHN | 0.0973 | 0 | 1 | 2 | 3 | 5 | 7 | 13 | 20 | 29 | 39 | 51 |
| 2 AWG THHN | 0.1159 | 0 | 1 | 1 | 2 | 4 | 6 | 11 | 16 | 24 | 33 | 43 |
| 1 AWG THHN | 0.1339 | 0 | 0 | 1 | 2 | 3 | 5 | 9 | 14 | 21 | 28 | 37 |
| 1/0 AWG THHN | 0.1562 | 0 | 0 | 1 | 2 | 3 | 4 | 8 | 12 | 18 | 24 | 32 |
| 2/0 AWG THHN | 0.1863 | 0 | 0 | 0 | 1 | 2 | 4 | 7 | 10 | 15 | 20 | 26 |
| 3/0 AWG THHN | 0.2223 | 0 | 0 | 0 | 1 | 2 | 3 | 5 | 8 | 12 | 17 | 22 |
| 4/0 AWG THHN | 0.2679 | 0 | 0 | 0 | 0 | 1 | 2 | 4 | 7 | 10 | 14 | 18 |
| 250 kcmil (MCM) | 0.3970 | 0 | 0 | 0 | 0 | 1 | 1 | 3 | 4 | 7 | 9 | 12 |
| 300 kcmil (MCM) | 0.4608 | 0 | 0 | 0 | 0 | 0 | 1 | 2 | 4 | 6 | 8 | 10 |
| 350 kcmil (MCM) | 0.5242 | 0 | 0 | 0 | 0 | 0 | 1 | 2 | 3 | 5 | 7 | 9 |
| 400 kcmil (MCM) | 0.5863 | 0 | 0 | 0 | 0 | 0 | 1 | 2 | 3 | 4 | 6 | 8 |
| 500 kcmil (MCM) | 0.7073 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 2 | 4 | 5 | 7 |
[!PRO TIP] Custom Flexible Whip Calculations: Sizing custom transformer taps or multi-wire lighting fixture whips? Calculate exact multi-conductor combinations instantly with our Interactive Conduit Fill Calculator.
FMC Dimensions and Internal Area Reference (NEC Table 4)
Flexible Metal Conduit is manufactured by helically coiling a continuous galvanized steel or aluminum strip. The table below outlines internal dimensions and legal fill limits across all trade sizes.
| FMC Trade Size | Internal Diameter (in.) | Total Internal Area (100%) | 1 Wire (53% Fill Limit) | 2 Wires (31% Fill Limit) | 3+ Wires (40% Fill Limit) | 24″ Nipple (60% Fill Limit) |
|---|---|---|---|---|---|---|
| 3/8″ | 0.384 in. | 0.116 sq. in. | 0.061 sq. in. | 0.036 sq. in. | 0.046 sq. in. | 0.070 sq. in. |
| 1/2″ | 0.630 in. | 0.312 sq. in. | 0.165 sq. in. | 0.097 sq. in. | 0.125 sq. in. | 0.187 sq. in. |
| 3/4″ | 0.820 in. | 0.528 sq. in. | 0.280 sq. in. | 0.164 sq. in. | 0.211 sq. in. | 0.317 sq. in. |
| 1″ | 1.020 in. | 0.817 sq. in. | 0.433 sq. in. | 0.253 sq. in. | 0.327 sq. in. | 0.490 sq. in. |
| 1-1/4″ | 1.280 in. | 1.287 sq. in. | 0.682 sq. in. | 0.399 sq. in. | 0.515 sq. in. | 0.772 sq. in. |
| 1-1/2″ | 1.540 in. | 1.863 sq. in. | 0.987 sq. in. | 0.577 sq. in. | 0.745 sq. in. | 1.118 sq. in. |
| 2″ | 2.040 in. | 3.269 sq. in. | 1.732 sq. in. | 1.013 sq. in. | 1.307 sq. in. | 1.961 sq. in. |
| 2-1/2″ | 2.500 in. | 4.909 sq. in. | 2.602 sq. in. | 1.522 sq. in. | 1.963 sq. in. | 2.945 sq. in. |
| 3″ | 3.000 in. | 7.069 sq. in. | 3.746 sq. in. | 2.191 sq. in. | 2.827 sq. in. | 4.241 sq. in. |
| 3-1/2″ | 3.500 in. | 9.621 sq. in. | 5.099 sq. in. | 2.983 sq. in. | 3.848 sq. in. | 5.773 sq. in. |
| 4″ | 4.000 in. | 12.566 sq. in. | 6.660 sq. in. | 3.896 sq. in. | 5.027 sq. in. | 7.540 sq. in. |
FMC is permitted as an equipment grounding conductor ONLY IF ALL of the following conditions are met:
- The total length of the FMC in the ground return path does not exceed 6 feet (1.8 m).
- The circuit is protected by an overcurrent device rated at 20 amps or less.
- FMC fittings are listed for grounding.
- If flexibility is required after installation (e.g. vibrating equipment), a separate equipment grounding conductor is always required.
Important NEC Installation Rules for FMC Conduit
When engineering and installing Flexible Metal Conduit systems, keep these code requirements in mind:
- Permitted Uses (NEC 348.10): FMC is permitted in exposed and concealed dry locations, equipment whips, and where flexibility is required to connect motors, transformers, and luminaires.
- Prohibited Uses (NEC 348.12): FMC is strictly prohibited in wet or damp locations, underground in direct contact with earth, embedded in poured concrete, or in hazardous locations (except where explicitly permitted by Article 501).
- Secure Support Requirements (NEC 348.30): FMC must be securely fastened within 12 inches of each box or fitting, and supported at intervals not exceeding 4.5 feet. Exceptions apply for fixture whips up to 6 feet in length.
- Bend Limits (NEC 348.26): Total bends between pull points cannot exceed 360 degrees. Review our 360-degree conduit bend limit guide to avoid tight spiral kinking.
Frequently Asked Questions (FAQs)
No. Under NEC 348.20(B), 3/8-inch FMC cannot be used for general branch runs. It is only permitted in lengths up to 6 feet for lighting fixture taps, motor connections, or manufactured wiring assemblies.
Under NEC Table 348.22, you can install up to 3 conductors of 12 AWG THHN with an external ground, or 2 conductors of 12 AWG with an internal grounding wire in 3/8-inch FMC.
No. Standard FMC is not waterproof. For outdoor wet locations, you must use Liquidtight Flexible Metal Conduit (LFMC) or Liquidtight Nonmetallic Conduit (LFNC).
Under NEC Annex C Table C.3, you can install up to 5 conductors of 10 AWG THHN in 1/2-inch FMC conduit under the 40% fill limit.
Dedicated FMC Wire Sizing Guides
Explore individual wire gauge fill capacities and specific trade size breakdown tables:
Small Branch Circuit Gauges:
- View capacity limits for 14 AWG in FMC conduit for 15A lighting fixture whips.
- View capacity limits for 12 AWG in FMC conduit for 20A commercial receptacle feeds.
- View capacity limits for 10 AWG in FMC conduit for 30A motor whips.
Medium Feeder & Subpanel Gauges:
- View capacity limits for 8 AWG in FMC conduit for 40A commercial equipment.
- View capacity limits for 6 AWG in FMC conduit for 50A transformer connections.
- View capacity limits for 4 AWG in FMC conduit for 70A and 80A feeder drops.
- View capacity limits for 2 AWG in FMC conduit for 100A and 115A equipment flex.
- View capacity limits for 1 AWG in FMC conduit for 125A commercial feeds.
Heavy Service Entrance & Commercial Cables:
- View capacity limits for 1/0 AWG in FMC conduit for 150A flexible transformer taps.
- View capacity limits for 2/0 AWG in FMC conduit for 175A distribution taps.
- View capacity limits for 3/0 AWG in FMC conduit for 200A vibration isolations.
- View capacity limits for 4/0 AWG in FMC conduit for 225A services.
- View capacity limits for 250 MCM in FMC conduit for heavy commercial mains.
- View capacity limits for 350 MCM in FMC conduit for large transformer secondary taps.
- View capacity limits for 500 MCM in FMC conduit for heavy switchboard flex isolation.