Conduit fill (NEC 2026 Chapter 9, Tables 1, 4, and 5)

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In 50 minutes, your students learn to find the smallest conduit that holds a set of wires: read the percent limit from Table 1, the pipe's room from Table 4, and each wire's area from Table 5, then check their answer against Annex C.

At a glance

Level: First-year apprentices
Time: 50 minutes, with a 90-minute variant
Sections: 300.19(A), 358.22, Chapter 9, Table 1, Chapter 9, Table 4, Chapter 9, Table 5, Chapter 9, Table 8, Chapter 9, Note 1, Chapter 9, Note 2, Chapter 9, Note 3, Chapter 9, Note 4, Chapter 9, Note 5, Chapter 9, Note 6, Chapter 9, Note 7, Chapter 9, Note 8, Chapter 9, Note 9, Chapter 9, Note 10, Chapter 9, Informational Note 1, Chapter 9, Informational Note 2, Annex C, Table C.1
Students need: Their own 2026 code book, open to Chapter 9, and a calculator
Edition: 2026 NEC
Last checked: October 2026

Class plan by minutes

0 to 5 minWarm-upAsk: why can't you stuff a conduit full? Then ask how many 12 AWG wires they think fit in 1/2 inch EMT. Take guesses and leave them up.
5 to 20 minVisualPlay the Crowded conduit visual. It is about heat, not fill: a pipe can pass fill and still need its wires derated. Then teach the three steps on the board, through a pull with mixed wire sizes. Stop after the three steps and have students total the example themselves before you show the answer.
20 to 35 minSimulationPairs play Conduit Fill jobs 1 and 2 (Garage circuit, Three circuits), then job 4 (Panel nipple). Each pair says the total area before they pull.
35 to 45 minQuestionsAsk the three quiz questions below, one at a time. Students write an answer alone, then you give the answer and its section. They can use the code book.
45 to 50 minWrap-upCheck the warm-up guesses against Annex C. Ask: when does Annex C stop working?

For a 90-minute block, give the simulation 30 minutes so pairs finish jobs 1 to 4, and add a 10-minute talk where each pair sizes a mixed pull on the board with Tables 4 and 5, then checks a same-size version against Annex C.

What students learn

By the end of the period, each student can total the area of the wires in a pull, ground included, find the smallest pipe whose limit holds that total, say when a short nipple earns the higher limit, and use Annex C to check a same-size pull.

300.19(A): The wires in a complete raceway must stay under the fill percentage in Chapter 9, Table 1.
358.22: EMT sends you to the same Table 1 percentages.
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, worked out at each fill limit.
Chapter 9, Table 5: The area of each insulated wire, insulation included.
Chapter 9, Table 8: The area of a bare wire, such as a bare ground.
Chapter 9, Note 1: Annex C lists the count for wires that are all the same size.
Chapter 9, Note 2: Table 1 is for finished conduit systems, not for a pipe that only guards wiring in the open.
Chapter 9, Note 3: Ground and bonding wires count toward fill at their real size.
Chapter 9, Note 4: Between two enclosures, a piece of pipe 24 in. or less may fill to 60%, and the ampacity cut for bundled wires does not apply.
Chapter 9, Note 5: A cable that Chapter 9 does not list is counted at its real dimensions.
Chapter 9, Note 6: For mixed sizes, add up each wire's area from Table 5 and compare the total with Table 4.
Chapter 9, Note 7: With wires of one size, a result of .8 or more rounds up to the next whole wire.
Chapter 9, Note 8: Bare wires may use the dimensions in Table 8.
Chapter 9, Note 9: A multiconductor cable counts as one wire. Single wires bundled with no jacket count one by one.
Chapter 9, Note 10: Table 5 areas are worst-case values. A wire's known real area may be used.
Chapter 9, Informational Note 1: A long pull or many bends can call for a bigger pipe or less fill than the table allows.
Chapter 9, Informational Note 2: Three wires can jam when the pipe is 2.8 to 3.2 times as wide as a wire.
Annex C, Table C.1: Informative shortcut: the most same-size wires that fit each trade size of EMT.

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

Quiz questions

1. A pull has three 10 AWG THHN, three 12 AWG THHN, and one 12 AWG THHN ground. The total area is 0.1165 square inch. Does it fit in 1/2 inch EMT?
Answer. Yes, by a small margin. 1/2 inch EMT allows 0.122 square inch at 40 percent. (Chapter 9, Table 4)
2. Three 4 AWG THHN and one 8 AWG THHN ground go in an 18 inch nipple between two panels. The total area is 0.2838 square inch. What is the smallest EMT size?
Answer. 3/4 inch. A nipple may fill to 60 percent, which is 0.320 square inch for 3/4 inch EMT. (Chapter 9, Note 4)
3. A pull has three 10 AWG THHN wires and a 12 AWG THHN ground. Can you size the pipe from Annex C?
Answer. No. Annex C works only when every wire is the same size, so add the areas from Table 5 and compare them with Table 4. (Chapter 9, Note 1)

Teaching notes

Before class

Students need their own 2026 code book open to Chapter 9, Tables 1, 4, and 5, and a calculator. Open this lesson and the conduit fill calculator on your teaching computer before class, and project the lesson. Step through the visual and the Conduit Fill simulation with the arrow keys. Students can play the simulation on a phone or a laptop. If they have neither, you can project the simulation and have the class call each answer. This lesson uses EMT and the areas for THHN wire. Schedule 80 PVC and cable fill are left for a later class.

How to use the simulation in class

Run Conduit Fill job 1 (Garage circuit) after you cover the percent limits. Students see the 53, 31, and 40 percent lines as each wire goes in.
Then pairs take job 2 (Three circuits). Each pair adds up the areas, grounds included, before they pull.
Job 4 (Panel nipple) shows the 60 percent rule. Ask which pipe size the same wires need in a long run.
Early finishers play job 3 (Subpanel feeder) or job 6 (Basement run). Job 5 (PVC feeder) is a good homework job for a later class.
When a pull fails, ask whether it broke the fill limit or just would not pull easily. Those are different problems.

Two discussion questions

1. A helper says the ground wire is bare and carries no current, so it does not count toward fill. What do you say?
Look for: The ground counts at its real size (Chapter 9, Note 3). A bare one is smaller than an insulated one, and Table 8 gives its area (Chapter 9, Note 8).
2. A pull fits 1/2 inch EMT with 0.0055 square inch to spare, and the run has four bends. What do you do?
Look for: Look for students who say a bigger pipe is a good call even though the math passes (Chapter 9, Informational Note 1). The table sets a ceiling, not a promise that the pull will be easy.

Mistakes to watch for

Leaving the ground wire out of the total.
Using the copper size instead of the area with insulation from Table 5.
Reading the THHN row for a wire with thicker insulation, like THW.
Missing the 60 percent allowance on a short nipple, or taking it on a pipe longer than 24 inches.
Using Annex C for a pull that mixes sizes.
Rounding down when the division ends in .8 or more.
Treating a loose bundle of single wires as one cable.

If a student’s book is a different edition

This lesson follows the 2026 NEC. The fill percentages did not change: 53, 31, and 40 percent. The general fill rule now sits in 300.19(A), which was 300.17 in the 2023 book, and it now points straight to the Table 1 percentages. Chapter 9, Informational Note 1 now says raceway instead of conduit, Note 7 applies the .8 round-up to cables as well as wires, and Note 8 lets bare wires use Table 8 dimensions wherever they are installed. If a student has an older book, tell them to find the same topic and compare, and to check which edition your authority having jurisdiction enforces.

Questions people ask

How full can a conduit be?

It depends on how many wires are in it: 53 percent of the inside area for one wire, 31 percent for two, and 40 percent for three or more (Chapter 9, Table 1). The rule that sends you there is 300.19(A).

Do ground wires count toward conduit fill?

Yes. Equipment grounding and bonding wires count at their real size, bare or insulated (Chapter 9, Note 3). A bare wire may use the dimensions in Table 8 (Chapter 9, Note 8).

How do you size conduit for wires of different sizes?

Look up each wire's area in Table 5, add them up, and pick the smallest trade size whose limit in Table 4 is at least that total (Chapter 9, Note 6).

When can a conduit fill to 60 percent?

Between two boxes or panels, a piece of pipe that is 24 inches or less may fill to 60 percent, and the ampacity cut for bundled wires does not apply to it (Chapter 9, Note 4).

Can you use Annex C instead of doing the math?

Only when every wire is the same size and type (Chapter 9, Note 1). Annex C is informative and is built from Tables 1, 4, and 5, so it is a way to check your math.

Ask Shorty is independent and is not affiliated with NFPA. Check every answer against your adopted code and your authority having jurisdiction. This lesson follows the 2026 NEC and was last checked October 2026. See something wrong? Tell us and we will check it against the code.

The visuals on their own

Open this one full size, outside the lesson.

Crowded conduit