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Conduit fill simulation

In Conduit Fill, you drop wires into a pipe drawn to scale and pick the smallest trade size that stays under the fill limit. Each wire's area comes from Chapter 9, Table 5, and each pipe's limit comes from Table 4. It builds the skill of sizing EMT and PVC by hand, with mixed sizes, grounds, and short nipples.

Play Conduit FillFree, no sign-up. Works on a phone, tablet, or computer.
  • The 53, 31, and 40 percent limits
  • Mixed sizes and grounds
  • Short nipples and Schedule 80
  • Over fill versus jamming
The Conduit Fill simulation on a laptop screen, at the start of the first job.

How it works

Each job gives you a tray of wires and a shelf of pipe sizes. Drag each wire into the pipe, or click it to drop it in. On a phone, tap a wire to see what it is, then tap it again to drop it in. On the first jobs, a meter shows the fill and the limit line as each wire goes in. On later jobs, you add up the areas yourself and check them against each size's limit on the shelf.

Pick the smallest trade size that holds the wires, then pull. If it's over the limit, pick another size and pull again. You earn up to three stars: one for a legal pull, one for no mistakes, and one for the smallest size on the first pull. The simulation runs in a browser on a phone, tablet, or computer, with no sign-up.

The jobs

  1. 1Garage circuit. Run three 12 AWG wires in EMT, and learn the 53%, 31%, and 40% limits.
  2. 2Three circuits. Add up the area of each wire in a run that mixes 10 and 12 AWG circuits with one shared ground.
  3. 3Subpanel feeder. See how fast big wire fills a pipe: four feeder wires push 1½ in. EMT past 40%.
  4. 4Panel nipple. Use the 60% limit on a nipple 24 in. or shorter to save a pipe size.
  5. 5PVC feeder. Size PVC where forklifts can hit it, and see why Schedule 80 holds less than Schedule 40.
  6. 6Basement run. Count every wire, ground included, in a run with a feeder, a multiwire circuit, and a 20 A circuit.

NEC 2026 sections it checks

Chapter 9, Table 1
Fill limits by wire count: 53% for one wire, 31% for two, 40% for three or more.
Chapter 9, Table 4
The inside area of each pipe type and trade size.
Chapter 9, Table 5
The area of each wire, insulation included.
Chapter 9, Note 3
Grounds count toward fill at their real size.
Chapter 9, Note 4
A nipple 24 in. or shorter between enclosures may fill to 60%.
Chapter 9, Note 7
With one wire size, round up to the next whole wire at .8 or more.
352.10(K)
Use Schedule 80 PVC where the pipe can get hit.
Table 250.122, 250.122(E)
Size the ground from the breaker. One ground sized for the biggest circuit can serve every circuit in the pipe.

Section numbers only. Read the full text in your code book.

Questions apprentices ask

How many 12 AWG THHN wires fit in ½ in. EMT?

Annex C, Table C.1 lists up to 9 of 12 AWG THHN/THWN-2 in ½ in. EMT. Annex C is informative, so use it to check your own math from Chapter 9, Tables 4 and 5. With three or more wires, the limit is 40% of the pipe's inside area (Chapter 9, Table 1).

What is the conduit fill limit?

It depends on the number of wires (Chapter 9, Table 1). One wire may fill 53% of the pipe, two wires 31%, and three or more 40%. A nipple 24 in. or shorter between boxes or panels may fill to 60% (Chapter 9, Note 4).

Do ground wires count in conduit fill?

Yes. Ground and bonding wires count at their real size, bare or insulated (Chapter 9, Note 3). In a mixed run, add each wire's area from Table 5, ground included, and compare the total with the pipe's limit from Table 4.

Why does Schedule 80 PVC need a bigger pipe?

Schedule 80 has thicker walls, so it has less room inside than Schedule 40 of the same trade size (Chapter 9, Table 4). Where PVC can get hit, it must be Schedule 80 (352.10(K)). The same wires can need the next trade size up.

How this simulation was checked

Every rule the inspector applies was checked against the 2026 NEC text in a code review, and the game was play-tested for ease of use. It is a study aid. It doesn’t replace the code book, your instructor, or your local inspector.

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Teach it in one class

This simulation is part of a free lesson that plays in your browser, with a class plan and quiz questions: Conduit fill (NEC 2026 Chapter 9, Tables 1, 4, and 5).

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See how it works

This interactive picture shows what happens inside the wiring.

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