Internal area: 0.533 sq in
Conductors
Passes NEC
3 conductors — max fill 40% per NEC Table 1
Fill Analysis
Wire Breakdown
NEC Fill Rules (Chapter 9, Table 1)
Based on NEC Chapter 9, Tables 1, 4, and 5. Always verify with your local AHJ and follow the most current edition of the NEC adopted in your jurisdiction.
Last updated: 2026-03-31
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How to Use This Calculator
Select your conduit type and size
Choose the conduit type (EMT, PVC Schedule 40, RMC, or FMC) and the trade size from 1/2 inch through 4 inches. The calculator uses the internal area from NEC Chapter 9, Table 4 for your selection.
Add your conductors
Select the wire gauge (14 AWG through 500 kcmil) and enter the quantity for each conductor group. Add multiple groups if your conduit run carries different wire sizes. The default insulation type is THHN/THWN-2, which covers most residential and commercial work.
Check the fill percentage
The calculator shows the fill percentage and whether your conduit run passes NEC Chapter 9, Table 1 limits. One conductor allows 53% fill, two allow 31%, and three or more allow 40%. Green means compliant, red means you need a larger conduit.
Review the recommendation
If your conduit is over the NEC limit, the calculator recommends the minimum trade size that will pass. Use the wire breakdown table to see exactly how much area each conductor group uses inside the conduit.
NEC Conduit Fill Formulas
Fill % = (Total Wire Area / Conduit Internal Area) x 100
Max Fill: 1 wire = 53%, 2 wires = 31%, 3+ wires = 40%
Wire Area = per NEC Chapter 9, Table 5 (includes insulation)
Conduit Area = per NEC Chapter 9, Table 4 (by type and trade size)Where:
- Fill %
- = Percentage of conduit cross-section occupied by conductors
- Total Wire Area
- = Sum of all conductor cross-sectional areas including insulation (sq in)
- Conduit Internal Area
- = Internal cross-sectional area of the conduit by type and trade size (sq in)
- Max Fill
- = Maximum allowable fill percentage per NEC Chapter 9, Table 1
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Frequently Asked Questions
What is conduit fill and why does it matter?
Conduit fill is the percentage of the conduit's internal cross-sectional area that is occupied by conductors. The NEC limits conduit fill to prevent overheating and make it possible to pull wires without damaging insulation. Overfilling a conduit is a code violation that can cause failed inspections, overheated conductors, and difficulty pulling wire. Every commercial and residential electrical installation must comply with NEC Chapter 9 fill limits.
What are the NEC conduit fill percentages?
NEC Chapter 9, Table 1 sets three fill limits based on the number of conductors: 1 conductor: 53% fill, 2 conductors: 31% fill, and 3 or more conductors: 40% fill. The 2-conductor limit is lower because two round conductors side by side leave more dead space than three or more conductors that can nest together. These percentages apply to all raceway types including EMT, PVC, RMC, and FMC.
What is the difference between EMT, PVC, and RMC conduit?
EMT (Electrical Metallic Tubing) is the most common conduit for indoor commercial work. It is thin-walled, lightweight, and connects with set-screw or compression fittings. PVC Schedule 40 is plastic conduit used for underground, wet, and corrosive environments. It is cheaper than metal but cannot be used where physical protection is needed above ground without proper support. RMC (Rigid Metal Conduit) is the heaviest and strongest option, used where maximum physical protection is required. Each type has different internal dimensions for the same trade size, so fill calculations vary by conduit type.
Does the ground wire count toward conduit fill?
Yes, equipment grounding conductors count toward conduit fill per NEC 300.17. When calculating fill, include every conductor in the raceway: hot conductors, neutral conductors, and equipment grounding conductors. The only exception is short lengths of conduit used as sleeves (NEC 300.17 Exception). Always include the ground wire in your fill calculation to avoid a failed inspection.
How do I pair this calculator with the wire size calculator?
Use the wire size calculator first to determine the correct AWG gauge for each circuit based on amperage, voltage, and distance. Then use this conduit fill calculator to verify that all the conductors for those circuits fit within your conduit without exceeding NEC fill limits. This two-step process ensures both proper wire sizing and code-compliant conduit fill.
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