Electrical reference chart
Conduit Fill Chart
Use this conduit fill chart after the calculator result to document the conductor set, raceway selection, fill percentage, and field pull concerns before approving a raceway size.
Quick reference table
Conduit fill compares the finished outside area of conductors or cables with the internal area of the selected raceway. Use the calculator worksheet because a passing fill percentage is only one screen; pulling tension, bend count, derating, box sizing, spare capacity, manufacturer dimensions, adopted NEC rules, and AHJ review still matter.
Raceway fill worksheet inputs
| Input | Record from the job | Why it changes the result |
|---|---|---|
| Conductor set | Each size, insulation family, cable type, and count | Determines total occupied area |
| Raceway type | EMT, RMC, IMC, PVC, flexible raceway, or listed system | Changes internal area for the same trade size |
| Routing conditions | Bends, offsets, pull points, and vertical runs | Can make a code-passing fill impractical to pull |
| Derating follow-up | Current-carrying conductor count and load mix | Fill and ampacity adjustment are separate reviews |
| Future space | Spare conductors, controls, or planned circuit additions | May justify a larger raceway than the minimum screen |
Conduit fill result interpretation
| Calculator result | Field meaning | Next check |
|---|---|---|
| Low fill percentage | Physical space is likely not the controlling limit | Check derating, pull route, and box entries |
| Near fill limit | Raceway may be difficult to pull or future-proof | Review bends, spare capacity, and installation sequence |
| Different result by raceway type | Internal area changed with material or schedule | Confirm the exact raceway product and trade size |
| Cable assembly included | Manufacturer outside diameter controls the area | Use product data rather than bare conductor dimensions |
How to use this chart
Start with finished dimensions
Use insulated conductor or cable outside area, not bare conductor diameter, when preparing the conductor list for the calculator.
Match the exact raceway
Record raceway type, schedule where applicable, and trade size before comparing fill results because internal area changes by product family.
Treat fill as one screen
After the calculator result, also review conductor derating, pulling tension, bend count, box entries, and spare capacity before keeping the raceway size.
Formula basis
Fill percentage = total finished conductor or cable area / raceway internal area x 100.
- Total conductor area uses the outside area of insulated conductors or cables, not bare conductor diameter.
- Raceway internal area depends on conduit type, schedule where applicable, and trade size.
- The fill limit depends on conductor count and the adopted raceway fill path used for the project.
Worked examples
Controls added to a crowded raceway
A few small control conductors can push an already tight raceway closer to its fill limit and can also change conductor-count adjustment, so both the fill calculator and ampacity review should be updated.
Same trade size, different raceway
A conductor set may pass in one raceway type and fail in another because the internal area is not identical. The worksheet should name the actual raceway type before the result is used.
Related calculators
Conduit Fill Calculator
Calculate conduit fill percentages, maximum wire counts, and conduit sizing per NEC requirements
NEC Chapter 9 Raceway Fill Calculator
Calculate conduit fill percentage and maximum conductors per NEC Chapter 9
Box Fill Calculator
Calculate electrical box fill requirements per NEC 314.16. Determine if your outlet box, junction box, or switch box has sufficient volume for conductors, devices, and fittings.
Related charts
Wire Gauge Diameter Chart
Look up nominal bare AWG diameter and separate it from insulated conductor, cable, conduit, and lug-fit dimensions.
Box Fill Chart
Plan box fill from conductors, devices, internal clamps, equipment grounds, fittings, pigtails, and marked box volume.
Wire Size Chart
Screen conductor size from calculated load, copper or aluminum material, terminal rating, derating, voltage drop, and equipment notes.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.