Conduit Fill Calculator
Calculate EMT conduit fill percentage and check it against the NEC 40% limit from conductor count, wire gauge, and conduit size. Shows max conductors allowed.
How this calculator works
Fill percentage is the total area of the wires divided by the internal area of the conduit. The calculator totals your conductors’ THHN area, divides by the EMT internal area, and checks the result against the NEC limit for that conductor count.
Formula: Fill % = (conductor area × count) ÷ conduit internal area × 100. Limit = 40% for 3+ conductors.
Worked example
Six 12 AWG THHN conductors (0.0133 sq in each) in ¾-inch EMT (0.533 sq in internal):
- Wire area: 0.0133 × 6 = 0.0798 sq in
- Fill: 0.0798 ÷ 0.533 ≈ 14.97%
- Under the 40% limit → compliant
- Max 12 AWG conductors in ¾-inch EMT: 16
Reference (12 AWG THHN, 40% limit)
| EMT size | Max conductors |
|---|---|
| ½ in | 9 |
| ¾ in | 16 |
| 1 in | 26 |
| 1¼ in | 45 |
How to use
Enter how many conductors you’re pulling, their gauge, and the conduit trade size. If fill exceeds the limit, step up a conduit size or split the run.
Conduit fill is code-governed. Verify against the current NEC and local amendments; a licensed electrician should confirm the final install.
Frequently asked questions
What is the NEC conduit fill limit?
For three or more conductors, the NEC limits fill to 40% of the conduit's internal cross-sectional area. Two conductors are limited to 31%, and a single conductor to 53%. These limits leave room to dissipate heat and physically pull the wires.
Why can't I just cram wires in?
Overfilled conduit traps heat, which derates the conductors and can damage insulation, and it makes the wires nearly impossible to pull without stripping insulation. The fill limits exist for both safety and installability.
Does this use THHN wire?
Yes — the calculator uses THHN/THWN cross-sectional areas, the most common building wire, and EMT (thin-wall) conduit internal areas from NEC Chapter 9. Other wire types and conduit materials have slightly different areas.
What if I mix wire sizes in one conduit?
This calculator assumes all conductors are the same gauge. For a mixed pull, add each conductor's individual area by hand and compare the total to 40% of the conduit's internal area — or size for the worst case.
How do I fix an overfilled conduit?
Step up to the next conduit trade size, or split the conductors across two conduits. The calculator's 'max conductors this size' output tells you how many of that gauge the conduit legally holds.
Estimates only. Verify quantities with your supplier before purchasing.