The electrical code limits how much of a conduit's inside the wires may take up, so they can be pulled without damage and stay cool. The limit depends on the number of wires, and the fill is the wires' total cross-section divided by the conduit's inside area.
How to use this calculator
- Choose the conduit type and trade size.
- Choose the wire size and enter how many you are pulling, counting every hot, neutral and ground.
- If the ground or other wires are a different size, add them under More options, along with a short nipple.
Worked examples
Two kitchen circuits in 1/2 in EMT
Conduit: EMT · Trade size: 1/2 in · Wire: 12 AWG · Number of wires: 5
- Conduit fill
- 21.9%
- Allowed
- 40%
- Most 12 AWG wires that fit
- 9 wires
- Smallest trade size that fits
- 1/2 in
A full run of 3/4 in EMT
Conduit: EMT · Trade size: 3/4 in · Wire: 12 AWG · Number of wires: 12
- Conduit fill
- 29.9%
- Allowed
- 40%
- Most 12 AWG wires that fit
- 16 wires
- Smallest trade size that fits
- 3/4 in
A dryer circuit in PVC
Conduit: PVC Schedule 40 · Trade size: 3/4 in · Wire: 10 AWG · Number of wires: 3 · Second wire size: 10 AWG · Number of those: 1
- Conduit fill
- 16.6%
- Allowed
- 40%
- Most 10 AWG wires that fit
- 8 wires
- Smallest trade size that fits
- 1/2 in
A subpanel feeder
Conduit: PVC Schedule 40 · Trade size: 1-1/4 in · Wire: 2 AWG · Number of wires: 3 · Second wire size: 6 AWG · Number of those: 1
- Conduit fill
- 27.4%
- Allowed
- 40%
- Most 2 AWG wires that fit
- 4 wires
- Smallest trade size that fits
- 1-1/4 in
A short nipple between boxes
Conduit: EMT · Trade size: 1/2 in · Wire: 12 AWG · Number of wires: 10 · Run length: A nipple of 24 in or less
- Conduit fill
- 43.8%
- Allowed
- 60%
- Most 12 AWG wires that fit
- 13 wires
- Smallest trade size that fits
- 1/2 in
The fill limits
The National Electrical Code sets the share of a conduit's inside area that wires may fill. It depends only on how many wires there are:
- One wire: 53 percent.
- Two wires: 31 percent, lower because two round wires side by side can jam in a bend.
- Three or more wires: 40 percent, the limit for almost every real run.
- A nipple, a short piece of conduit no longer than 24 inches between boxes or enclosures: 60 percent.
Wire and conduit areas
Fill is worked out from areas, not diameters. A 12 AWG THHN wire takes up 0.013 sq in and a 10 AWG one 0.021 sq in, while the inside of 1/2 in EMT is 0.304 sq in and of 3/4 in EMT 0.533 sq in. PVC has thicker walls than EMT of the same trade size, so it holds less: 3/4 in PVC Schedule 40 is 0.508 sq in inside, and PVC Schedule 80 even less, 0.409 sq in.
Add up the areas of all the wires, divide by the conduit's inside area, and compare the result with the limit. The calculator does the same for every trade size of the conduit you pick, so you can see the next size up at a glance.
What else limits a conduit
- Count every conductor: hots, neutrals and the equipment ground. A bare or green ground counts too.
- More than three current-carrying wires in one conduit run warmer, and the code lowers how many amps each may carry; the wire size calculator covers the ampacity side.
- Bends add friction. Long runs with several bends pull far easier with room to spare, so many electricians stay well under the limit.
- These areas are for THHN, the usual building wire, which is also rated THWN. XHHW and other insulations have different areas.
Common questions
How many 12 AWG wires can go in 1/2 in EMT?
Pick EMT, 1/2 in and 12 AWG in the calculator and read the most wires that fit: it divides 40 percent of the conduit's inside area, 0.304 sq in, by the area of one wire, 0.013 sq in.
What is the forty percent rule for conduit?
With three or more wires, their total cross-section may fill no more than 40 percent of the conduit's inside area. One wire may fill 53 percent and two wires 31 percent.
Does the ground wire count toward conduit fill?
Yes. Every conductor in the conduit counts, including the equipment ground, bare or insulated.
Can a short piece of conduit be filled more?
Yes. A nipple no longer than 24 inches between boxes may be filled to 60 percent.