Inputs
Example Data Table
| Raceway | Trade Size | Conductors | Qty | Total Area (in^2) | Allowed Area (in^2) |
|---|---|---|---|---|---|
| EMT | 1 | #12 THHN/THWN-2 | 6 | 0.0798 | 0.3456 |
| PVC Sch 40 | 1-1/2 | #4 XHHW/XHHW-2 | 3 | 0.2880 | 0.7944 |
| IMC | 2 | 250 kcmil THHN/THWN-2 | 2 | 0.7940 | 1.0776 |
Formula Used
- Total Conductor Area = Sum(Area per conductor x Quantity)
- Allowed Fill Area = Raceway Internal Area x Allowed Fill Factor
- Fill Percentage = (Total Conductor Area / Raceway Internal Area) x 100
How to Use This Calculator
- Select a raceway type and trade size.
- Add each conductor size, insulation type, and quantity.
- Press Calculate to see pass/fail and fill percent.
- Use CSV or PDF exports for submittals and checklists.
- If results fail, adjust size, count, or routing strategy.
Article
Why raceway fill matters on real projects
Raceway fill is a practical capacity check that supports safer pulls, cleaner workmanship, and predictable installation time. Overfilled raceways increase pulling tension, raise the risk of insulation damage, and often lead to rework when conductors must be removed. A documented fill calculation also helps plan labor and materials, especially when multiple circuits share a route.
Key inputs that influence the result
The calculation depends on raceway internal area, conductor outside diameter, insulation type, and total conductor count. This tool uses representative conductor areas and a standard fill factor selection based on conductor quantity. Consistent input discipline—matching insulation types and sizes to the actual specification—keeps estimates aligned with field conditions.
Interpreting pass or fail outcomes
A PASS indicates the summed conductor area is within the allowed fill area for the selected raceway size. A FAIL indicates the bundle exceeds the allowable space, suggesting a larger raceway, fewer conductors, or a revised routing approach. Even with a pass, consider bend count, pull length, and terminations to maintain install quality.
Using exports for coordination and documentation
CSV exports are useful for takeoffs, submittals, and checklists because they can be merged into broader electrical schedules. PDF exports provide a quick field-ready record for inspections and coordination meetings. Keeping these files alongside shop drawings supports consistent decisions when scope changes occur.
Example data walkthrough
Example scenario: EMT trade size 1 with nine conductors total: eight #12 THHN/THWN-2 and one #10 THHN/THWN-2. Using the built-in conductor areas, the total conductor area is approximately 0.1275 in^2 (8×0.0133 + 1×0.0211). The EMT 1 internal area is 0.864 in^2, and the 3+ conductor factor is 0.40, so the allowed area is 0.3456 in^2. This produces a fill of about 14.76%, which passes.
| Raceway | Trade Size | Conductors | Total Area (in^2) | Allowed Area (in^2) | Fill (%) |
|---|---|---|---|---|---|
| EMT | 1 | 8× #12 THHN/THWN-2 + 1× #10 THHN/THWN-2 | 0.1275 | 0.3456 | 14.76 |
FAQs
1) What does “fill percentage” represent?
It is the total conductor area divided by the raceway internal area, expressed as a percent. It shows how much space the conductors occupy inside the selected raceway.
2) Why does the allowed factor change with conductor count?
Common fill rules apply different limits for one, two, or three-plus conductors to account for installation practicality. More conductors require more free space for pulling and arrangement.
3) Can I mix conductor sizes and insulation types?
Yes. Add a row for each size and insulation combination. The calculator sums the individual areas so mixed bundles are evaluated correctly.
4) What should I do if my result fails?
Increase the raceway size, reduce the number of conductors in that run, or split the circuits into multiple raceways. Shorten pull length or reduce bends when possible.
5) Are the area values official for every jurisdiction?
The tool uses representative area values for common combinations. Always verify against the governing electrical code tables and manufacturer data required by your project specifications.
6) Does this replace derating or voltage drop checks?
No. Raceway fill is one constraint. You still need conductor ampacity adjustments, temperature and bundling considerations, and voltage drop checks where applicable.
7) Why might a “pass” still be difficult to install?
Long pulls, many bends, tight boxes, or stiff cable constructions can make installations challenging even below the fill limit. Treat fill as a baseline and plan for real field conditions.