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NEC Ch.9
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC Ch.9 reference behind every number.
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Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What is conduit fill?
Conduit fill is the percentage of a conduit's interior cross-sectional area that is taken up by the conductors inside it. The National Electrical Code limits that percentage so wires can dissipate heat, so insulation isn't damaged during the pull, and so future conductors can be added safely. Go over the limit and you risk an inspection failure, overheating, or a jammed pull.
This calculator does the whole check for you: pick a conduit type and size, add your conductors, and it compares the total conductor area against the allowable fill from NEC Chapter 9 — then shows a clear PASS or FAIL with the exact numbers behind it.
NEC Chapter 9 fill limits
The maximum fill depends only on how many conductors share the raceway, per the note to NEC Chapter 9, Table 1:
| Conductors in the conduit | Maximum fill |
|---|---|
| 1 conductor | 53% |
| 2 conductors | 31% |
| 3 or more conductors | 40% |
The conductor areas come from Chapter 9, Table 5 (insulated conductors) and the conduit interior areas from Table 4. The app stores all of these values, so there's nothing to look up by hand.
Worked example
Say you're pulling 9 × #12 THHN in ½″ EMT:
Each #12 THHN conductor = 0.0133 in² (Table 5). Nine conductors = 9 × 0.0133 = 0.1197 in².
½″ EMT interior area = 0.304 in² (Table 4). With 3 or more conductors the limit is 40% → 0.1216 in² allowed.
0.1197 in² ≤ 0.1216 in² → PASS. This matches Annex C, Table C.1, which lists 9 as the maximum number of #12 THHN conductors in ½″ EMT.
Add a tenth conductor and the total jumps to 0.1330 in² — over the 0.1216 in² limit — so the app flags it as a FAIL and you'd step up to ¾″ EMT.
Supported conduit types
- EMT — electrical metallic tubing
- IMC — intermediate metal conduit
- RMC — rigid metal conduit
- PVC Schedule 40 and PVC Schedule 80
All standard trade sizes from ½″ to 4″ are included, and you can combine different wire sizes and insulation types in the same calculation.
Related calculators
Once the conduit passes, confirm the conductors themselves: size ampacity and derating with the wire size calculator, or check circuit length against the 3% target using the voltage drop calculator.
Conduit fill FAQ
What fill percentage does the NEC allow?
Per NEC Chapter 9, Table 1, the maximum conduit fill is 53% for one conductor, 31% for two conductors, and 40% for three or more conductors in a single conduit or tubing.
Which conduit types does the calculator support?
EMT, IMC, RMC (rigid metal), PVC Schedule 40 and PVC Schedule 80 — all standard trade sizes from ½ inch to 4 inch, using the dimensions in NEC Chapter 9 Table 4.
Can I mix different wire sizes in one conduit?
Yes. Add each conductor with its size and insulation type (for example THHN/THWN-2). The app sums the cross-sectional areas from NEC Chapter 9 Table 5 and compares the total to the conduit's allowable fill.
Does the conduit fill calculator work offline?
Yes, it works 100% offline. All NEC tables are built in, so no account or internet connection is required on the job site.
Is the app free?
The calculator is free with rewarded ads. A one-time $2.99 Pro unlock removes ads, saves unlimited calculation history, and enables PDF export. There is no subscription.
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