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Conduit Fill Calculator - NEC Chapter 9 Wire Fill Percentages by Conduit Type & Size

Calculate allowable conductor count and fill percentage for EMT, IMC, RMC, PVC, and flexible conduit

Free NEC-compliant conduit fill calculator based on Chapter 9, Tables 1, 4, and 5. Select conduit type and trade size, then add conductors by wire gauge and insulation type to see total fill area, allowable fill area, and fill percentage. Supports EMT, IMC, rigid metal (RMC), PVC Schedule 40 and 80, LFMC, and FMC conduit types. Calculates fill limits for one conductor (53%), two conductors (31%), and three or more conductors (40%) per NEC Table 1. Includes actual conductor areas from NEC Chapter 9, Table 5 for THHN, THWN, XHHW, and other common insulation types.

Pro Tip: The 40% fill rule for three or more conductors is the maximum, not the target. Pulling wire through a conduit at 40% fill requires significantly more force and risks damaging insulation, especially on long runs with multiple bends. Experienced electricians target 30-35% fill for practical pullability. If your calculated fill is between 35% and 40%, add one trade size to make the pull easier and leave room for future additions. The labor cost of fighting a tight pull always exceeds the material cost of larger conduit.

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NEC Conduit Fill Calculator
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How It Works

  1. Select Conduit Type and Size

    Choose the conduit type (EMT, IMC, RMC, PVC Sch 40, PVC Sch 80, LFMC, or FMC) and the trade size from 1/2" through 6". Each conduit type has a different internal area per NEC Chapter 9, Table 4 due to varying wall thicknesses.

  2. Add Conductors

    Select each conductor's wire gauge (14 AWG through 750 kcmil) and insulation type (THHN/THWN-2, XHHW, USE, RHH, etc.). The calculator uses the actual conductor areas from NEC Chapter 9, Table 5, which vary significantly by insulation type.

  3. Include Equipment Grounding Conductor

    Add the equipment grounding conductor (EGC) sized per NEC 250.122 based on the overcurrent protection device rating. The EGC counts toward conduit fill and is often overlooked during initial sizing.

  4. Review Fill Percentage

    See the total conductor area, maximum allowable fill area, and fill percentage. The calculator applies NEC Table 1 limits: 53% for one conductor, 31% for two conductors, and 40% for three or more. Color-coded warnings flag fills above 40%.

  5. Adjust and Optimize

    If fill exceeds the NEC limit, try upsizing the conduit, splitting into parallel runs, or switching to a compact insulation type like THHN/THWN-2 which has a smaller outside diameter than XHHW or RHH for the same wire gauge.

Built For

  • Electricians sizing conduit for branch circuit and feeder installations in commercial buildings
  • Electrical estimators calculating conduit and wire quantities for bid proposals
  • Engineers designing conduit systems for industrial motor control center feeders
  • Inspectors verifying NEC conduit fill compliance during rough-in inspections
  • Maintenance electricians determining if existing conduit can accommodate additional circuits
  • Solar installers sizing conduit for PV source circuit and inverter output conductors
  • Data center electricians planning high-density power distribution conduit runs

Features & Capabilities

NEC Chapter 9 Compliance

Uses exact conduit internal areas from NEC Table 4 and conductor outside diameters from NEC Table 5. Fill percentages are calculated per NEC Table 1 with correct thresholds for one, two, or three-plus conductors.

Multiple Insulation Types

Supports all common insulation types including THHN/THWN-2, XHHW, XHHW-2, USE-2, RHH, RHW, RHW-2, TW, THW, and THHW. Each type has a different outside diameter that significantly affects fill calculations.

Mixed Conductor Sizes

Handles runs with mixed wire sizes and insulation types in the same conduit. Correctly calculates total fill area from individual conductor areas rather than using the simplified same-size tables in NEC Annex C.

Bare and Covered Conductors

Includes dimensions for bare copper and aluminum equipment grounding conductors from NEC Table 8, which have different areas than insulated conductors of the same gauge.

Jam Ratio Warning

Flags combinations where the conduit inner diameter to conductor outer diameter ratio falls in the 2.5-3.2 range, which creates jamming risk during pulling. This NEC-adjacent concern is critical for successful installations.

Frequently Asked Questions

NEC Chapter 9, Table 1 limits conduit fill to 40% of the conduit's total internal cross-sectional area when three or more conductors are installed. For one conductor the limit is 53%, and for two conductors it is 31%. These percentages ensure adequate space for heat dissipation and practical wire pulling. The fill is calculated using the actual conductor outside diameters from Table 5, not the copper area.
Yes. All conductors in the conduit, including equipment grounding conductors (EGC), count toward the fill calculation. This is a commonly overlooked detail that can push a conduit over the 40% limit, especially on smaller trade sizes. The EGC area is determined by the insulation type if insulated, or from NEC Table 8 if bare. Always include the EGC when calculating fill.
NEC Annex C provides pre-calculated tables showing the maximum number of conductors of the same size and insulation type per conduit size. Chapter 9 Tables 4 and 5 provide the raw area data for calculating fill with mixed conductor sizes. If all conductors are the same size and type, Annex C is a quick lookup. If you have mixed sizes (common in feeders with smaller grounds), you must use the Chapter 9 area-based calculation method.
Different insulation types have different outside diameters for the same wire gauge. For example, a 10 AWG THHN/THWN-2 conductor has an outside diameter of 0.182 inches (area 0.0260 sq in), while a 10 AWG RHH has an outside diameter of 0.242 inches (area 0.0460 sq in) - nearly 77% more area. Choosing compact insulation like THHN/THWN-2 can allow more conductors in the same conduit or permit using a smaller conduit size.
Jam ratio is the conduit inner diameter divided by the conductor outer diameter. When this ratio falls between approximately 2.5 and 3.2 for three conductors, the wires can lock together in a triangular pattern and jam during pulling, making the pull extremely difficult or impossible regardless of fill percentage. This is a practical installation concern not directly addressed in the NEC fill tables. If your configuration falls in this range, upsizing the conduit by one trade size typically eliminates the problem.
NEC 376.22 and the Chapter 9 notes allow conduit nipples not exceeding 24 inches in length to be filled to 60% of their total cross-sectional area. This higher fill allowance exists because short runs do not develop significant heat buildup and pulling friction is minimal. This exception is commonly used for connections between panels, junction boxes, and equipment termination cabinets where space is tight.
NEC 300.3(C)(1) generally permits power conductors of different systems in the same conduit if all conductors are insulated for the maximum voltage present. However, NEC 725.136 restricts mixing Class 1, Class 2, and Class 3 circuits with power circuits in certain configurations. Control wiring for motor starters can typically share conduit with the motor feeder, but low-voltage signal wiring (thermostats, data, fire alarm) generally requires separate raceways. Always check the specific NEC article for the circuit type.
Disclaimer: This calculator provides conduit fill estimates based on NEC Chapter 9 tables. Local codes may impose additional requirements. Actual installation conditions including bends, pull points, and conductor length affect pulling feasibility beyond fill percentage alone. Always verify calculations against the current edition of the NEC and applicable local amendments. ToolGrit is not responsible for electrical design, code compliance, or installation outcomes.

Learn More

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NEC Conduit Fill Rules Explained

How NEC Chapter 9 conduit fill calculations work. Tables 1, 4, and 5 explained with examples for EMT, IMC, RMC, and mixed conductor types.

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