Conduit Fill Calculator
This conduit fill calculator finds how full a conduit is from its inner diameter, the wire outer diameter, and the number of wires, then checks the result against the NEC fill limits of 53, 31, and 40 percent. As an electrical conduit fill calculator it also tells you the most wires of that size the conduit can take, which makes conduit sizing quick.
Results
The Conduit Fill Calculator is part of our Cable & Protection collection. Browse more cable & protection to size, convert, and check your work alongside this tool.
What the conduit fill calculator shows
Wires packed too tightly into a conduit are hard to pull and run hot. The conduit fill calculator compares the cross-section of the wires with the inside of the pipe and gives the fill as a percentage. Used as an electrical pipe fill calculator or a raceway fill calculator, it tells you at once whether a run is within the allowed fill or needs a bigger conduit.
How the conduit fill percentage calculator works
The conduit fill percentage calculator compares areas. Because both the wire and the conduit are circles, the area ratio reduces to a ratio of squared diameters:
n is the number of wires, d the wire outer diameter, and D the conduit inner diameter. NEC Chapter 9 Table 1 limits fill to 53 percent for one wire, 31 percent for two, and 40 percent for three or more. This is the rule any nec pipe fill calculator applies.
Conduit sizing example
Six wires of 4.0 mm outer diameter in a conduit with a 21 mm inner diameter. The conduit fill calculator gives:
The run is well within the limit, and the same conduit could take up to 11 of these wires at 40 percent.
How to use the electrical conduit fill calculator
Enter the conduit inner diameter in millimeters, the outer diameter of one wire including insulation, and the number of wires. The electrical conduit fill calculator returns the fill percentage, the allowed limit for that wire count, a pass or over-limit result, and the maximum number of those wires at 40 percent. Take the inner diameter from the conduit table for your type, since EMT, PVC, and rigid differ.
Reading the pipe fill calculator result
If the fill is at or below the allowed percentage, the conduit is acceptable for fill. If it is over, step up a trade size or split the run. Staying well under the limit makes long pulls and pulls with several bends much easier. The pipe fill calculator checks geometry only; it does not size the wire for current or apply derating for many conductors in one conduit.
When to use the conduit size calculator
- Choosing a trade size with the conduit size calculator
- Checking a PVC run with a pvc conduit fill calculator
- Adding circuits to an existing conduit
- Planning raceway fill on a new installation
Common conduit fill calculator mistakes
- Using the nominal trade size. Enter the true inner diameter for the conduit type.
- Ignoring insulation. Use the full outer diameter of each wire.
- Treating fill as ampacity. Passing fill does not mean the wire is sized for the current.
Tips for accurate results
- Stay well below the limit for long runs with bends.
- Count the ground wire as a conductor for fill.
- Check the conduit table for your exact type and schedule.
Electrical conduit sizing step by step
Electrical conduit sizing starts with the conductors, not the pipe. First size each wire for its load, then look up its outer diameter or area for the insulation type, add the areas of all wires, and pick the smallest conduit whose allowed area is at least that total. Conduit sizing for wire this way keeps fill legal and pulls manageable. The breaker on the circuit sets the current the wire must carry, which the breaker size calculator works out.
Raceway fill calculator and derating
A raceway fill calculator only answers whether the wires fit. When more than three current-carrying conductors share a raceway, their ampacity must also be derated because they heat each other. That is a separate step from fill, so a conduit can pass fill and still need larger wire for current. Keep both checks on your list for every run.
Conduit fill calculator limits (NEC Chapter 9)
| Number of wires | Maximum fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
| Nipple under 600 mm (24 in) | 60% |
Conduit Fill Calculator FAQ
What is the conduit fill limit?
The NEC allows 53 percent for one wire, 31 percent for two wires, and 40 percent for three or more. Short nipples under 24 inches may be filled to 60 percent.
Why is two-wire fill only 31 percent?
Two round wires can jam side by side across the conduit during a pull, so the code sets a lower limit for that case.
Does conduit fill include insulation?
Yes. Use the overall outer diameter of each wire, insulation included, since that is the space it takes inside the pipe.
Does the calculator size wire for current?
No. It checks fill only. Size the conductor for current and apply derating separately with a wire size calculation.