Electricians

Conduit Fill Calculator

Check NEC conduit fill percentage for your conduit size and wire configuration.

Count all conductors including neutral and ground
7.5%
Fill Percentage
40%
NEC Max Fill
0.533
Conduit Area (sq in)
0.0399
Wire Area (sq in)
OK - 32.5% to spare
Status

NEC Chapter 9 allows 40% fill for 3 wires in conduit. EMT 3/4″ has 0.533 sq in of internal area. 3 × 12 AWG THHN = 0.0399 sq in (7.5% fill).

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What is conduit fill?

Conduit fill refers to how much of a conduit's internal area is taken up by the wires running through it. The NEC limits how full you can pack a conduit because wires generate heat and they need room for that heat to dissipate. Overfilling conduit is a code violation and a safety hazard. This calculator tells you the maximum number of wires you can run in a given conduit size.

NEC conduit fill rules

For one wire in a conduit, you can fill up to 53 percent of the conduit area. For two wires, 31 percent. For three or more wires, the limit drops to 40 percent. These percentages exist because more wires means more heat and less room for the wires to be pulled through without damaging insulation. You also need to account for the wire insulation type since different types have different outer diameters.

EMT vs PVC vs rigid conduit

EMT (electrical metallic tubing) is the most common for indoor commercial and residential work. It is lightweight and easy to bend. PVC conduit is used for underground and outdoor runs. Rigid conduit is heavier duty and used where physical protection is needed. Each type has slightly different internal dimensions even at the same trade size, which affects fill calculations.

Common Questions

Can I run Romex through conduit?

Technically yes, but it is not a great idea. Romex counts as a single cable and fills up conduit fast. Most pros pull individual THHN wires through conduit instead.

What size conduit do I need for 3 wires?

It depends on the wire gauge. Three 12 AWG THHN wires fit in 1/2 inch EMT. Three 6 AWG wires need 1 inch EMT. This calculator tells you the exact size based on your wire count and gauge.

Do I need conduit in a garage?

It depends on local code. Many areas require conduit or armored cable in garages since exposed Romex can get damaged. Check with your local inspector before starting the job.

Can you mix different wire sizes in the same conduit?

Yes, you can mix wire sizes in the same conduit. Just make sure you add up the total area of all wires and stay under the NEC fill limit for that conduit size.

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