4" RMC Conduit Fill Chart
This page shows the maximum number of conductors allowed in a 4-inch Rigid Metal Conduit (RMC) conduit per the National Electrical Code. The 4" RMC has an internal cross-sectional area of 13.631 square inches, which determines how many wires of each size can legally fit inside. Use the tables below to quickly look up conductor capacity for your installation, or use the calculator for custom configurations.
THHN/THWN-2 Maximum Conductors in 4" RMC
THHN/THWN-2 is the most commonly used conductor type in commercial and residential wiring. The table below shows the maximum number of THHN/THWN-2 conductors allowed in this 4" RMC conduit based on NEC Chapter 9 fill rules. For 3 or more conductors the 40% fill rule applies. Values of 0 mean the conductor is too large to fit even a single wire in this conduit size.
| Wire Size | Wire Area (sq.in.) | Max Conductors | Fill at Max (sq.in.) |
|---|---|---|---|
| 14 AWG | 0.0097 | 562 | 5.4514 |
| 12 AWG | 0.0133 | 409 | 5.4397 |
| 10 AWG | 0.0211 | 258 | 5.4438 |
| 8 AWG | 0.0366 | 148 | 5.4168 |
| 6 AWG | 0.0507 | 107 | 5.4249 |
| 4 AWG | 0.0824 | 66 | 5.4384 |
| 3 AWG | 0.0973 | 56 | 5.4488 |
| 2 AWG | 0.1158 | 47 | 5.4426 |
| 1 AWG | 0.1562 | 34 | 5.3108 |
| 1/0 AWG | 0.1855 | 29 | 5.3795 |
| 2/0 AWG | 0.2223 | 24 | 5.3352 |
| 3/0 AWG | 0.2679 | 20 | 5.3580 |
| 4/0 AWG | 0.3237 | 16 | 5.1792 |
| 250 AWG | 0.3970 | 13 | 5.1610 |
| 300 AWG | 0.4608 | 11 | 5.0688 |
| 350 AWG | 0.5242 | 10 | 5.2420 |
| 400 AWG | 0.5863 | 9 | 5.2767 |
| 500 AWG | 0.7073 | 7 | 4.9511 |
About 4" Rigid Metal Conduit
RMC is the heaviest and most durable metal conduit, providing maximum physical protection for enclosed conductors. It is the standard choice for outdoor installations, hazardous locations (Class I and II), direct burial, and anywhere conductors need protection from severe physical damage. RMC is also commonly used for service entrance raceways and industrial applications where impact or crushing is a concern. The 4-inch trade size is one of the standard sizes defined in NEC Chapter 9, Table 4. With an internal area of 13.631 square inches, it provides adequate space for a range of conductor sizes and counts depending on the application requirements.
RMC is made from heavy-wall galvanized steel or aluminum. Its thick walls provide excellent protection but result in a smaller internal area than EMT or IMC of the same trade size. RMC uses threaded connections, providing a mechanically strong and electrically continuous raceway.
NEC Reference: NEC Article 344 governs the installation requirements for RMC conduit. Conduit fill is calculated using NEC Chapter 9, Table 1 (fill percentages), Table 4 (conduit areas), and Table 5 (conductor areas). This calculator uses the exact values from these tables for accurate results.
Worked Example: 12 AWG THHN in 4" RMC
Here is a step-by-step example showing how to calculate conduit fill for 4 conductors of 12 AWG THHN/THWN-2 in this 4" RMC conduit. This is a common scenario for a 20-amp branch circuit requiring two hots, a neutral, and a ground.
- Identify the conduit internal area: 4" RMC = 13.631 sq.in. (from NEC Table 4)
- Identify the conductor area: 12 AWG THHN/THWN-2 = 0.0133 sq.in. per conductor (from NEC Table 5)
- Calculate total conductor area: 4 conductors x 0.0133 sq.in. = 0.0532 sq.in.
- Determine the fill percentage rule: 4 conductors uses the 40% fill rule (NEC Table 1)
- Calculate allowable fill area: 13.631 sq.in. x 0.40 = 5.4524 sq.in.
- Compare: Total conductor area (0.0532 sq.in.) is less than allowable fill (5.4524 sq.in.) — COMPLIANT
- Actual fill percentage: 0.0532 / 13.631 = 0.4%
- Maximum conductors: This 4" RMC can hold up to 409 conductors of 12 AWG THHN/THWN-2 per NEC rules
XHHW Maximum Conductors in 4" RMC
XHHW conductors use cross-linked polyethylene insulation, which provides excellent moisture and heat resistance. XHHW has slightly different insulation dimensions than THHN, resulting in different fill counts. The table below shows the maximum XHHW conductors allowed in this 4" RMC conduit.
| Wire Size | Wire Area (sq.in.) | Max Conductors |
|---|---|---|
| 14 AWG | 0.0139 | 392 |
| 12 AWG | 0.0181 | 301 |
| 10 AWG | 0.0243 | 224 |
| 8 AWG | 0.0437 | 124 |
| 6 AWG | 0.0590 | 92 |
| 4 AWG | 0.0814 | 66 |
| 3 AWG | 0.0962 | 56 |
| 2 AWG | 0.1146 | 47 |
| 1 AWG | 0.1534 | 35 |
| 1/0 AWG | 0.1825 | 29 |
| 2/0 AWG | 0.2190 | 24 |
| 3/0 AWG | 0.2642 | 20 |
| 4/0 AWG | 0.3197 | 17 |
| 250 AWG | 0.3904 | 13 |
| 300 AWG | 0.4536 | 12 |
| 350 AWG | 0.5166 | 10 |
| 400 AWG | 0.5782 | 9 |
| 500 AWG | 0.6984 | 7 |
Installation Advantages of RMC
Maximum physical protection for conductors. Suitable for hazardous locations and severe environments. Can serve as an equipment grounding conductor. Approved for direct burial and concrete encasement. Fire-resistant for extended periods.
Installation Requirements
RMC requires threaded connections using standard pipe threads. Factory-cut threads are standard, and field threading is common. Supports must be placed within 10 feet of each box and at 10-foot intervals. Total bends between pull points cannot exceed 360 degrees.
When planning your installation with 4" RMC conduit, remember that the fill calculation only addresses the straight sections. The number of bends, total conduit length, and pulling tension also affect whether the installation is practical. The NEC limits total bends to 360 degrees (four 90-degree bends) between pull points. For long runs or runs with many bends, consider using the next larger conduit size even if the fill calculation passes, as this will reduce pulling tension and make future maintenance easier.
Other RMC Sizes
Browse fill charts for other Rigid Metal Conduit trade sizes. Larger conduit sizes accommodate more conductors and are needed for high-ampacity circuits or runs with many wires.