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
Fill Result: PASSNEC 40% limit
Fill used3.4%NEC limit: 40%
3
Total wires
0.0291
Total wire area (in²)
0.3456
Max fill area (in²)
Remaining fill capacity: 0.3165 in². You can add more conductors without exceeding the NEC limit.
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.
Who Uses This Calculator
ElectriciansVerify fill compliance before rough-in inspection and avoid failed inspections that require conduit upsizing.
Electrical EngineersConfirm conduit sizing during design without manually referencing NEC tables during every calculation.
Electrical ContractorsProduce accurate material take-offs by confirming minimum conduit sizes before ordering materials.
Apprentices and StudentsLearn NEC fill rules by testing real scenarios and understanding how wire count changes the applicable fill limit.
How to Use This Calculator
1
Select conduit type
Choose EMT, IMC, RMC, or PVC Schedule 40. EMT is standard for most commercial interior applications.
2
Select conduit size
Choose the trade size you are planning to use. The internal area for that size is shown in the dropdown.
3
Enter your wire sizes and quantities
For each wire size in the conduit, select the AWG or kcmil size from the dropdown and enter the number of conductors of that size. Include all conductors, including the ground wire.
4
Add more wire sizes if needed
Click Add Wire Size to add a row for a different AWG size. Conduits often contain multiple circuit conductors of different sizes.
5
Read the fill result
The result shows fill percentage, whether it passes the NEC limit, the total and maximum wire area, and how much remaining capacity is available.
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.
!
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.
!
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.
!
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.
About the Author
RN
Rick Navarro, Master Electrician
Licensed master electrician and electrical contractor with 20 years of commercial and industrial installation experience. Rick has trained apprentice electricians in NEC code compliance and conduit installation methodology on projects ranging from commercial buildouts to data center infrastructure.
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.