The calculation, without hidden assumptions
Check how much of a circular raceway is occupied before a pull. Enter actual insulated conductor or cable outside diameters, so mixed sizes work without pretending conductor metal area equals cable area. Use the NEC Chapter 9 count preset only where that rule applies, or enter a project-specific limit.
How to use this calculator
Choose inches or millimetres and keep every diameter in that unit.
Enter the actual raceway inside diameter, not nominal trade size or outside diameter.
Add up to three conductor/cable groups using finished outside diameter including insulation or sheath.
Use the conductor-count preset only when NEC Chapter 9 Table 1 applies; otherwise choose a custom project/code limit.
Treat the reverse ID as a geometry threshold, then select a real listed raceway from current manufacturer/code dimensions.
Where people use it
- •Mixed-size conductor raceway fill checks.
- •Cable OD based fill when a table entry is unavailable.
- •Compare existing conduit ID against a planned pull.
- •Reverse-solve the minimum theoretical circular ID for a fill target.
- •Check short nipple geometry separately from normal raceway fill.
Example: three 0.20 in cables in a 1.00 in ID raceway
Each cable area is π×0.20²/4 = 0.03142 in². Three occupy 0.09425 in²; the 1.00 in ID raceway area is 0.78540 in², so fill is 12.00%. For three conductors the 40% preset is below its limit; the ideal minimum ID at 40% is about 0.548 in.
What the result does not assume
- •This owner intentionally does not embed proprietary/current conductor and raceway dimension tables. Use actual dimensions or verified current table/manufacturer data.
- •NEC Chapter 9 Table 1 uses 53% for one conductor/cable, 31% for two and 40% for over two; a qualifying conduit/tubing nipple not over 24 in may use 60%. Verify the adopted code edition and all notes/exceptions.
- •Equipment grounding or bonding conductors installed in the raceway are included in fill calculations under the cited NEC table notes.
- •Fill compliance does not establish ampacity, adjustment/derating, pulling tension, bend radius, box fill, fault protection or installation approval.
- •Three equal cables can have jamming risk when raceway ID to cable OD is near 2.8–3.2; the calculator flags that geometry but does not model pulling mechanics.
- •Nominal conduit trade size is not internal diameter. Schedule/material/type change the usable area.
Frequently asked questions
What conduit fill percentage should I use?+
For NEC Chapter 9 Table 1 applications, the general count limits are 53% for one conductor/cable, 31% for two and 40% for over two. A qualifying conduit or tubing nipple up to 24 inches has a 60% provision. Verify your adopted edition and installation conditions.
Do I enter wire conductor diameter or insulated outside diameter?+
Use the finished outside diameter or verified cross-sectional area that the applicable rule requires. Bare copper area is not a substitute for insulated cable area.
Can I mix cable sizes?+
Yes. Enter separate OD and quantity groups; the calculator sums their circular areas before dividing by raceway internal area.
Why does the calculator ask for conduit inside diameter?+
Fill is based on usable internal cross-sectional area. Nominal trade size and outside diameter do not uniquely determine that area.
What is the minimum conduit size result?+
It is the minimum ideal circular inside diameter at the selected fill percentage, not a trade-size recommendation. Choose an actual listed raceway using verified current dimensions.
Does passing conduit fill mean the wiring design is compliant?+
No. Ampacity, conductor adjustment, temperature, grounding, bend radius, pulling tension, local amendments and other requirements remain separate checks.
Semantic next steps
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