3-1/2" FMC Conduit Fill Chart
This page shows the maximum number of conductors allowed in a 3-1/2-inch Flexible Metal Conduit (FMC) conduit per the National Electrical Code. The 3-1/2" FMC has an internal cross-sectional area of 11.545 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 3-1/2" FMC
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 3-1/2" FMC 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 | 476 | 4.6172 |
| 12 AWG | 0.0133 | 347 | 4.6151 |
| 10 AWG | 0.0211 | 218 | 4.5998 |
| 8 AWG | 0.0366 | 126 | 4.6116 |
| 6 AWG | 0.0507 | 91 | 4.6137 |
| 4 AWG | 0.0824 | 56 | 4.6144 |
| 3 AWG | 0.0973 | 47 | 4.5731 |
| 2 AWG | 0.1158 | 39 | 4.5162 |
| 1 AWG | 0.1562 | 29 | 4.5298 |
| 1/0 AWG | 0.1855 | 24 | 4.4520 |
| 2/0 AWG | 0.2223 | 20 | 4.4460 |
| 3/0 AWG | 0.2679 | 17 | 4.5543 |
| 4/0 AWG | 0.3237 | 14 | 4.5318 |
| 250 AWG | 0.3970 | 11 | 4.3670 |
| 300 AWG | 0.4608 | 10 | 4.6080 |
| 350 AWG | 0.5242 | 8 | 4.1936 |
| 400 AWG | 0.5863 | 7 | 4.1041 |
| 500 AWG | 0.7073 | 6 | 4.2438 |
About 3-1/2" Flexible Metal Conduit
FMC (also known as Greenfield) is used where rigid conduit would be impractical due to the need for flexibility, vibration isolation, or routing around obstacles. Common applications include connections to motors, transformers, recessed lighting fixtures, and other equipment that vibrates or requires final alignment adjustments. FMC is typically limited to dry locations and short runs. The 3-1/2-inch trade size is one of the standard sizes defined in NEC Chapter 9, Table 4. With an internal area of 11.545 square inches, it provides adequate space for a range of conductor sizes and counts depending on the application requirements.
FMC is manufactured from interlocking spirally-wound steel or aluminum strips, creating a flexible but protective raceway. It does not have a liquid-tight jacket, so it is limited to dry locations. The internal area is similar to EMT since the interlocking design maintains a consistent bore.
NEC Reference: NEC Article 348 governs the installation requirements for FMC 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 3-1/2" FMC
Here is a step-by-step example showing how to calculate conduit fill for 4 conductors of 12 AWG THHN/THWN-2 in this 3-1/2" FMC 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: 3-1/2" FMC = 11.545 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: 11.545 sq.in. x 0.40 = 4.6180 sq.in.
- Compare: Total conductor area (0.0532 sq.in.) is less than allowable fill (4.6180 sq.in.) — COMPLIANT
- Actual fill percentage: 0.0532 / 11.545 = 0.5%
- Maximum conductors: This 3-1/2" FMC can hold up to 347 conductors of 12 AWG THHN/THWN-2 per NEC rules
XHHW Maximum Conductors in 3-1/2" FMC
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 3-1/2" FMC conduit.
| Wire Size | Wire Area (sq.in.) | Max Conductors |
|---|---|---|
| 14 AWG | 0.0139 | 332 |
| 12 AWG | 0.0181 | 255 |
| 10 AWG | 0.0243 | 190 |
| 8 AWG | 0.0437 | 105 |
| 6 AWG | 0.0590 | 78 |
| 4 AWG | 0.0814 | 56 |
| 3 AWG | 0.0962 | 48 |
| 2 AWG | 0.1146 | 40 |
| 1 AWG | 0.1534 | 30 |
| 1/0 AWG | 0.1825 | 25 |
| 2/0 AWG | 0.2190 | 21 |
| 3/0 AWG | 0.2642 | 17 |
| 4/0 AWG | 0.3197 | 14 |
| 250 AWG | 0.3904 | 11 |
| 300 AWG | 0.4536 | 10 |
| 350 AWG | 0.5166 | 8 |
| 400 AWG | 0.5782 | 7 |
| 500 AWG | 0.6984 | 6 |
Installation Advantages of FMC
Flexible routing around obstacles and tight spaces. Vibration isolation for motor and equipment connections. Easy final alignment with equipment connections. No special bending tools required. Available in continuous coils for long runs.
Installation Requirements
FMC uses special flex connectors at termination points. It must be secured within 12 inches of each fitting and at intervals not exceeding 4.5 feet. Runs longer than 6 feet require a separate equipment grounding conductor. The total length between boxes should be minimized to reduce conductor pulling difficulty.
When planning your installation with 3-1/2" FMC 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 FMC Sizes
Browse fill charts for other Flexible Metal Conduit trade sizes. Larger conduit sizes accommodate more conductors and are needed for high-ampacity circuits or runs with many wires.