Check NEC conduit fill compliance. Select conduit type and size, add your THHN/THWN-2 wire sizes and quantities, and see whether your fill percentage passes the NEC 40% limit.
✓ NEC 2023 Chapter 9 fill percentages (1 wire 53%, 2 wire 31%, 3+ wire 40%)
✓ EMT, IMC, RMC, and PVC Schedule 40 conduit types
✓ Multi-wire entry with mixed AWG sizes
✓ Shows fill %, pass/fail, and remaining capacity
Enter Conduit and Wire DetailsNEC 2023
Wire Size (THHN/THWN-2)Qty
How Conduit Fill Is Calculated
NEC conduit fill is the ratio of total wire cross-sectional area to the conduit's internal area, compared against the applicable percentage limit based on wire count.
Total wire area = sum(qty × wire area) for each conductor
Max fill area = conduit internal area × fill % limit
Wire areas are from NEC 2023 Table 5 for THHN/THWN-2 conductors. Conduit internal areas are from Table 4. Other insulation types have different areas: check Table 5 for XHHW, RHH, and other types.
Worked Example
Rick needs to run 3 circuits in a single 1" EMT conduit: 3 × #12 AWG for one circuit, 3 × #10 AWG for a second, and 1 × #12 AWG ground. Total: 7 conductors.
1" EMT internal area: 0.864 in²
NEC fill limit (3+ wires): 40% → 0.864 × 0.40 = 0.346 in²
4 × #12 AWG: 4 × 0.0133 = 0.0532 in²
3 × #10 AWG: 3 × 0.0211 = 0.0633 in²
Total wire area: 0.1165 in²
Fill %: 0.1165 / 0.864 = 13.5%
13.5% fill is well under the 40% NEC limit. The 1" EMT passes with 0.229 in² of remaining capacity.
Common Mistakes
!
Not counting the ground wire
The equipment grounding conductor is a physical conductor that occupies conduit space. Every conductor in the conduit, including grounds, must be counted for NEC fill purposes.
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Using wrong wire area table for insulation type
THHN and XHHW have different insulation thicknesses. Using THHN areas for XHHW conductors will understate the actual fill. Always match the wire area to the specific insulation type being installed.
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Applying 40% fill to a 1- or 2-wire conduit
The 40% limit applies to three or more conductors only. A single conductor can fill up to 53%; two conductors up to 31%. Using 40% for a one- or two-wire conduit is incorrect and more restrictive than the NEC requires.
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Using nominal conduit size as the area
A 1" conduit does not have 1 square inch of internal area. EMT 1" has 0.864 in². Always use the table values for internal conduit area, not the trade size number.
NFPA 70. National Electrical Code (NEC) 2023 Edition. Chapter 9, Tables 1, 4, and 5. National Fire Protection Association.
NFPA 70. NEC Annex C: Conduit and Tubing Fill Tables for Conductors and Fixture Wires of the Same Size. 2023 Edition.
HR
Hassaan Rasheed
Developer and Researcher, CalculatorFlux
Researches and verifies the formulas, methodology, and source data behind each calculator on CalculatorFlux. All tools are built and checked against the cited references before publication.
Last updated: May 2026
NEC Fill Limits
1 conductor53%
Chapter 9, Table 1
2 conductors31%
Chapter 9, Table 1
3+ conductors40%
Chapter 9, Table 1
Percentages apply to the ratio of total wire area to conduit internal area.
Conduit Type Guide
EMT
Commercial interiors, accessible locations
IMC
Industrial, moderate mechanical risk
RMC
Outdoor exposed, high-risk areas
PVC Sch 40
Underground, corrosive environments
Pro Tip
When fill is close to the limit, switch to the next trade size. Going from 3/4" to 1" EMT adds 0.331 in² of internal area at minimal material cost, far cheaper than a failed inspection and conduit replacement.