Conduit Fill Calculator
Check NEC conduit fill (total conductor area versus the allowed percentage for EMT) from your wire gauges and counts, with a pass or fail. Calculates in your browser.
Conduit fill
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Show the math
Total conductor area must stay under the NEC fill percentage:
fill% = Σ(conductor area) ÷ conduit area × 100
limit: 53% (1 wire) · 31% (2) · 40% (3 or more)
Areas use NEC Chapter 9 THHN conductor and EMT internal dimensions. This is an educational aid, confirm against the current code and have an electrician verify any install. All math runs locally in your browser.
What this does
A conduit fill calculator checks NEC fill (total conductor area versus the allowed percentage for EMT) from your wire gauges and counts, with a pass or fail.
How to use it
- Choose the conduit size.
- Add your wire gauges and counts.
- See the fill percentage.
- Read the pass or fail result.
How it works
NEC Chapter 9 sets the fill limit by conductor count. The tool uses THHN and EMT areas from the NEC tables.
limits: 53% (1), 31% (2), 40% (3+)
Understanding your result
The pass or fail is against the NEC fill limit set by how many conductors share the conduit, and staying under it is not just code: it leaves room to pull wires without damaging insulation and lets heat escape. Note that derating for more than three current-carrying conductors is a separate check, so passing fill alone does not finish the job.
Example
Three 12 AWG THHN in 1/2-inch EMT is well under the 40% limit.
Sources & methodology
Last updated .
Frequently asked questions
What is the NEC conduit fill limit?
NEC Chapter 9 allows 53% fill for one conductor, 31% for two, and 40% for three or more. The tool applies the right limit automatically.
Which wire and conduit are supported?
It uses THHN conductor areas and EMT internal areas from the NEC tables. Other insulations and conduit types have different areas.
Should I verify with code?
Yes. This is an educational aid, confirm against the current NEC and have an electrician verify any installation.
Is anything stored?
No. Everything is calculated locally in your browser.
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