Why a conduit cannot be filled completely, and how full is too full
Conduit fill percentages by conductor count, the reason they exist, and choosing the next conduit when the first does not fit.
The engineering question
The cables fit by area, so why is the conduit rejected? What is the actual fill limit and what does it protect?
Worked example
The figures below were produced by running the Conduit Fill Calculator with the same inputs — during the build, from its authoritative engine.
| Total conductor area | 94.44 mm² |
|---|---|
| Conduit fill | 16.9551 % |
| Permitted fill | 40 % |
| Smallest compliant internal area | 344 mm² |
| Assessment | Fill 17.0% is within the 40% NEC Chapter 9 limit. |
- Fill limits are 53 % for one conductor, 31 % for two and 40 % for three or more.
- Total conductor area is the sum of the insulated areas, not the copper alone.
- The limit exists so cables can still be pulled — a conduit filled to 100 % cannot be.
Sanity check
Bare equipment grounding conductors are counted separately from the fill limit. Excluding them is the most common error.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- NEC Chapter 9 Table 1 — percentage of conduit fill
- NEC Chapter 9 Tables 4 and 5 — conduit and conductor dimensions
Related problems
Sizing a cable for both current and voltage drop
Two independent constraints that must both be satisfied, and recognising which one actually chose the size.
Why the voltage at the end of the cable is lower than at the supply
Calculating voltage drop over the run, and why the cable size that satisfies current may not satisfy voltage.