IMPORTANT
Quick Summary: Under the National Electrical Code (NEC Article 344 and Annex C Table C.8), Rigid Metal Conduit (RMC / GRC) provides the highest level of mechanical strength and physical protection in the electrical industry. When pulling three or more conductors of the same size and insulation type (THHN/THWN-2), the standard 40% fill capacities are:
- 1/2″ RMC: 12 wires of 14 AWG | 9 wires of 12 AWG | 5 wires of 10 AWG | 2 wires of 6 AWG
- 3/4″ RMC: 22 wires of 14 AWG | 16 wires of 12 AWG | 10 wires of 10 AWG | 4 wires of 6 AWG
- 1″ RMC: 36 wires of 14 AWG | 26 wires of 12 AWG | 16 wires of 10 AWG | 7 wires of 6 AWG
- 1-1/4″ RMC: 62 wires of 14 AWG | 45 wires of 12 AWG | 28 wires of 10 AWG | 12 wires of 6 AWG
Key Takeaways
- Maximum Physical and Impact Protection (NEC Article 344): RMC is the heaviest wall-thickness steel conduit manufactured, approved for severe physical damage, industrial processing plants, and hazardous (classified) locations.
- The 40 Percent Standard (NEC Chapter 9 Table 1): For runs containing three or more conductors, total wire cross-sectional area cannot exceed 40% of the RMC conduit interior area.
- Equipment Grounding Conductor Path (NEC 250.118(2)): Threaded RMC conduit is legally recognized as an approved equipment grounding conductor (EGC), frequently eliminating the need to pull a separate green ground wire.
- Shallow 6-Inch Burial Advantage (NEC Table 300.5): While PVC requires an 18-inch trench, threaded RMC only requires a 6-inch trench burial depth under general dirt and rock, saving massive excavation costs.
RMC Rigid Steel Fill Limits At-A-Glance
NEC Chapter 9, Table 1 Allowable Cross-Sectional Fill Ratios
Master RMC Conduit Fill Chart (NEC Annex C Table C.8)
The table below provides the maximum allowable number of conductors for compact copper THHN/THWN-2 conductors inside Rigid Metal Conduit (RMC) from 1/2 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.) | 1/2″ RMC (0.125 sq. in.) | 3/4″ RMC (0.219 sq. in.) | 1″ RMC (0.355 sq. in.) | 1-1/4″ RMC (0.610 sq. in.) | 1-1/2″ RMC (0.828 sq. in.) | 2″ RMC (1.363 sq. in.) | 2-1/2″ RMC (1.946 sq. in.) | 3″ RMC (3.000 sq. in.) | 3-1/2″ RMC (4.004 sq. in.) | 4″ RMC (5.153 sq. in.) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 14 AWG THHN | 0.0097 | 12 | 22 | 36 | 62 | 85 | 140 | 200 | 309 | 412 | 531 |
| 12 AWG THHN | 0.0133 | 9 | 16 | 26 | 45 | 62 | 102 | 146 | 225 | 301 | 387 |
| 10 AWG THHN | 0.0211 | 5 | 10 | 16 | 28 | 39 | 64 | 92 | 142 | 189 | 244 |
| 8 AWG THHN | 0.0366 | 3 | 5 | 9 | 16 | 22 | 37 | 53 | 81 | 109 | 140 |
| 6 AWG THHN | 0.0507 | 2 | 4 | 7 | 12 | 16 | 26 | 38 | 59 | 78 | 101 |
| 4 AWG THHN | 0.0824 | 1 | 2 | 4 | 7 | 10 | 16 | 23 | 36 | 48 | 62 |
| 3 AWG THHN | 0.0973 | 1 | 2 | 3 | 6 | 8 | 14 | 20 | 30 | 41 | 52 |
| 2 AWG THHN | 0.1159 | 1 | 1 | 3 | 5 | 7 | 11 | 16 | 25 | 34 | 44 |
| 1 AWG THHN | 0.1339 | 0 | 1 | 2 | 4 | 6 | 10 | 14 | 22 | 29 | 38 |
| 1/0 AWG THHN | 0.1562 | 0 | 1 | 2 | 3 | 5 | 8 | 12 | 19 | 25 | 32 |
| 2/0 AWG THHN | 0.1863 | 0 | 0 | 1 | 3 | 4 | 7 | 10 | 16 | 21 | 27 |
| 3/0 AWG THHN | 0.2223 | 0 | 0 | 1 | 2 | 3 | 6 | 8 | 13 | 18 | 23 |
| 4/0 AWG THHN | 0.2679 | 0 | 0 | 0 | 2 | 3 | 5 | 7 | 11 | 14 | 19 |
| 250 kcmil (MCM) | 0.3970 | 0 | 0 | 0 | 1 | 2 | 3 | 4 | 7 | 10 | 12 |
| 300 kcmil (MCM) | 0.4608 | 0 | 0 | 0 | 0 | 1 | 2 | 4 | 6 | 8 | 11 |
| 350 kcmil (MCM) | 0.5242 | 0 | 0 | 0 | 0 | 1 | 2 | 3 | 5 | 7 | 9 |
| 400 kcmil (MCM) | 0.5863 | 0 | 0 | 0 | 0 | 1 | 2 | 3 | 5 | 6 | 8 |
| 500 kcmil (MCM) | 0.7073 | 0 | 0 | 0 | 0 | 0 | 1 | 2 | 4 | 5 | 7 |
[!PRO TIP] Custom Industrial Calculations: Designing complex industrial feeders or hazardous classified raceways? Calculate custom wire mixtures with our Interactive Conduit Fill Calculator.
RMC Dimensions and Internal Area Reference (NEC Table 4)
Rigid Metal Conduit is hot-dip galvanized steel with threaded ends. The table below outlines internal dimensions and legal fill limits across all trade sizes.
| RMC 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) |
|---|---|---|---|---|---|---|
| 1/2″ | 0.632 in. | 0.314 sq. in. | 0.166 sq. in. | 0.097 sq. in. | 0.125 sq. in. | 0.188 sq. in. |
| 3/4″ | 0.836 in. | 0.549 sq. in. | 0.291 sq. in. | 0.170 sq. in. | 0.219 sq. in. | 0.329 sq. in. |
| 1″ | 1.063 in. | 0.887 sq. in. | 0.470 sq. in. | 0.275 sq. in. | 0.355 sq. in. | 0.532 sq. in. |
| 1-1/4″ | 1.394 in. | 1.526 sq. in. | 0.809 sq. in. | 0.473 sq. in. | 0.610 sq. in. | 0.916 sq. in. |
| 1-1/2″ | 1.624 in. | 2.071 sq. in. | 1.098 sq. in. | 0.642 sq. in. | 0.828 sq. in. | 1.243 sq. in. |
| 2″ | 2.083 in. | 3.408 sq. in. | 1.806 sq. in. | 1.056 sq. in. | 1.363 sq. in. | 2.045 sq. in. |
| 2-1/2″ | 2.489 in. | 4.866 sq. in. | 2.579 sq. in. | 1.508 sq. in. | 1.946 sq. in. | 2.920 sq. in. |
| 3″ | 3.090 in. | 7.499 sq. in. | 3.974 sq. in. | 2.325 sq. in. | 3.000 sq. in. | 4.499 sq. in. |
| 3-1/2″ | 3.570 in. | 10.010 sq. in. | 5.305 sq. in. | 3.103 sq. in. | 4.004 sq. in. | 6.006 sq. in. |
| 4″ | 4.050 in. | 12.882 sq. in. | 6.827 sq. in. | 3.993 sq. in. | 5.153 sq. in. | 7.729 sq. in. |
In rocky terrain where digging deep trenches is cost-prohibitive, Rigid Metal Conduit only requires 6 inches of cover compared to 18 inches for PVC conduit.
Important NEC Installation Rules for RMC Conduit
When engineering and installing heavy-wall Rigid Metal Conduit, adhere to these key trade codes:
- Approved as Equipment Grounding Conductor (NEC 250.118(2)): Properly made-up threaded RMC conduit joints and locknuts form an approved low-impedance grounding path. In many commercial branch installations, this frees up conduit fill space because an equipment grounding wire does not need to be pulled.
- Hazardous Location Mandate (NEC Article 500): Threaded RMC is the standard raceway for Class I, Division 1 and 2 hazardous locations (such as gasoline dispensing facilities, grain silos, and chemical processing units).
- Threading and Reaming Requirements (NEC 344.28): All cut ends of RMC must be reamed with a pipe reamer to remove sharp metal burrs that could slice conductor insulation during heavy cable pulls.
- Bend Radius and Support Spacing (NEC 344.30): RMC must be securely fastened within 3 feet of each box and supported every 10 to 20 feet depending on conduit trade size. Total bends between pull boxes cannot exceed 360 degrees per our 360-degree conduit bend limit guide.
Frequently Asked Questions (FAQs)
Under NEC Annex C Table C.8, you can install up to 9 conductors of 12 AWG THHN in 1/2-inch RMC conduit under the 40% fill limit.
Generally, no. Under NEC 250.118(2), threaded Rigid Metal Conduit is an approved equipment grounding conductor. However, some engineering specifications or wet corrosive environments may still require an insulated copper ground wire.
RMC (Rigid Metal Conduit) and GRC (Galvanized Rigid Conduit) refer to the exact same heavy-wall steel raceway. GRC specifically highlights that the steel has been hot-dip galvanized for corrosion resistance.
Under NEC Table 300.5, RMC conduit only requires a minimum burial depth of 6 inches under general dirt or rock (compared to 18 inches for PVC).
Dedicated RMC 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 RMC conduit for industrial lighting circuits.
- View capacity limits for 12 AWG in RMC conduit for 20A plant power circuits.
- View capacity limits for 10 AWG in RMC conduit for 30A industrial machinery.
Medium Feeder & Subpanel Gauges:
- View capacity limits for 8 AWG in RMC conduit for heavy motor circuits.
- View capacity limits for 6 AWG in RMC conduit for hazardous location equipment.
- View capacity limits for 4 AWG in RMC conduit for 70A and 80A industrial feeders.
- View capacity limits for 3 AWG in RMC conduit for 100A distribution panels.
- View capacity limits for 2 AWG in RMC conduit for 100A and 115A feeders.
- View capacity limits for 1 AWG in RMC conduit for 125A commercial feeds.
Heavy Service Entrance & Commercial Cables:
- View capacity limits for 1/0 AWG in RMC conduit for 150A services.
- View capacity limits for 2/0 AWG in RMC conduit for 175A feeders.
- View capacity limits for 3/0 AWG in RMC conduit for 200A main services.
- View capacity limits for 4/0 AWG in RMC conduit for 225A services.
- View capacity limits for 250 MCM in RMC conduit for heavy industrial distribution.
- View capacity limits for 300 MCM in RMC conduit for 300A main feeders.
- View capacity limits for 350 MCM in RMC conduit for parallel switchgear feeds.
- View capacity limits for 400 MCM in RMC conduit for petrochemical plant mains.
- View capacity limits for 500 MCM in RMC conduit for utility transformer feeds.