Conduit fill equation
Conduit fill compares the total cable area against the internal area of the selected raceway.
where:
- A
- Cross-sectional area of a circular cable or conduit[in2]
- d
- Outside diameter for a cable, or inside diameter for a conduit[in]
- n
- Quantity of cables in the selected cable group
- Acable
- Cross-sectional area of one cable[in2]
- Acables
- Total area occupied by all selected cables[in2]
- Aconduit
- Internal conduit area[in2]
- Fill%
- Calculated conduit fill percentage[%]
The calculator compares the result with common 53%, 31%, 40%, and 60% fill limits, but pulling tension, bends, and current-carrying conductor derating still need separate review.
Assumptions
- Uses typical NEC Chapter 9 Table 4 internal diameters for EMT, RMC, PVC Schedule 40, and PVC Schedule 80
- Cable OD values are planning references; manufacturer OD wins
- Mixed cable fill is calculated by summing each cable group's cross-sectional area
Important Warnings
- Conduit fill is not the same as ampacity derating; current-carrying conductor derating must be checked separately.
- Long pulls, bends, and exactly three equal cables can create pull-tension or cable-jam issues even below the fill limit.
FAQ
Why use outside diameter instead of conductor size?
Conduit fill is based on the physical area occupied by the complete insulated cable or conductor, so the outer jacket diameter is the important dimension.
Can I mix cable types?
Yes. The calculator adds the area of Cable group A and Cable group B before comparing the total against the selected conduit area.