Electrical
Conduit Fill Calculator
Check conduit fill against NEC 2026 with power, data, and custom cable rows.
Rows
40% fill chart
Maximum same-size cable counts by conduit size.
| Cable | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" |
|---|---|---|---|---|---|---|---|---|
| 14 AWG THHN | 12 | 21 | 35 | 61 | 83 | 138 | 197 | 304 |
| 12 AWG THHN | 9 | 16 | 25 | 44 | 61 | 100 | 143 | 222 |
| 10 AWG THHN | 5 | 10 | 16 | 28 | 38 | 63 | 90 | 140 |
| 8 AWG THHN | 3 | 5 | 9 | 16 | 22 | 36 | 52 | 80 |
| 6 AWG THHN | 2 | 4 | 6 | 11 | 16 | 26 | 37 | 58 |
| 4 AWG THHN | 1 | 2 | 4 | 7 | 9 | 16 | 23 | 35 |
| 2 AWG THHN | 1 | 1 | 2 | 5 | 7 | 11 | 16 | 25 |
Need a change for Conduit Fill Calculator?
About this calculator
Method, formulas, and limits.What this does
Checks conduit fill for power, data, and custom cables against NEC 2026 conduit fill tables. Supports EMT, PVC Schedule 40, and RMC conduit with fill limits of 53% (single conductor), 31% (two conductors), and 40% (three or more).
Who it is for
Electricians, engineers, and DIY builders who need to verify that the cables they plan to pull will fit inside a conduit under the NEC fill rules before buying materials.
How it works
Each cable row contributes its cross-sectional area. The calculator sums the areas, applies the fill limit for the total conductor count, and compares the used area to the conduit's allowed area. A status shows pass, near limit, over limit, or input errors.
Limitations
Uses conductor cross-sectional area only. It does not model bend radius, pulling tension, derating, wet locations, or local amendments. Always verify with the current local code edition before permit work.
Key calculations
- Fill Limit
- The maximum percent of conduit area that can be occupied: 53% for 1 conductor, 31% for 2 conductors, and 40% for 3 or more conductors per NEC Table 1.
- Used Area
- Sum the cross-sectional area of each cable: usedArea = sum(cableArea × quantity). Area for round cables is π × (diameter/2)².
- Fill Percentage
- fillPercent = usedArea / conduitArea × 100. The result passes when fillPercent is at or below the limit for the conductor count.
Reference ranges
- Fill Limits
- NEC 2026 Table 1: 53% for one conductor, 31% for two conductors, 40% for three or more. These limits protect conductors from insulation damage during pulling.
- Common Conduit
- 1/2-inch EMT internal area is about 0.30 sq in. 3/4-inch EMT is about 0.53 sq in. 1-inch EMT is about 0.86 sq in.
How to use it
- 1.Choose the modeSelect Power for electrical cables, Data for low-voltage cables, or Custom OD to enter a cable's outside diameter directly.
- 2.Pick conduit type and sizeChoose EMT, PVC Schedule 40, or RMC, then the trade size. The calculator uses the conduit's internal area for the fill check.
- 3.Add cable rowsAdd each cable with its quantity. For custom rows, enter the outside diameter in inches or millimeters.
- 4.Review the statusThe result shows pass, near limit, or over limit, plus the fill percentage and the smallest conduit that fits.
Conduit fill is the percentage of a conduit's internal cross-sectional area occupied by conductors. NEC limits this to prevent damage during pulling and overheating in service.
More conductors make pulling harder and heat dissipation worse. NEC Table 1 sets 53% for one conductor, 31% for two, and 40% for three or more.
No. This calculator checks fill only. Bend radius, pulling tension, ampacity derating, and local amendments must be checked separately.
Yes. Data mode uses typical low-voltage cable diameters. Data installations may also need bend-radius and pulling-tension checks.
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