NEC conduit fill: Chapter 9 tables and Annex C

Where to turn

• Chapter 9, Table 1 gives the maximum percent of a raceway's cross section that conductors may fill, by number of conductors. Read the notes to the tables with it.
• Chapter 9, Table 4 gives raceway dimensions by type and trade size, including the area at each fill percentage. Pick the table for your raceway: EMT, rigid metal, PVC and others each have one.
• Chapter 9, Table 5 gives the area of each insulated conductor by type and size. Table 8 covers bare conductors.
• Annex C has ready-made tables of the maximum number of same-size conductors for a given raceway. It is informational, but the Chapter 9 notes point to it, so mark it too.

The method

1. Choose the raceway type and the conductor insulation type.
2. From Table 5, take each conductor's area and add them. Count insulated equipment grounding conductors too.
3. Use Table 1 to find the fill percentage for the number of conductors. Table 1 gives 53% for one conductor, 31% for two and 40% for more than two.
4. In the Table 4 section for your raceway, find the smallest trade size whose area at that percentage is at least the total.
5. If all conductors are the same size and type, check Annex C to confirm your answer.

Worked example

A run of EMT longer than 24 inches carries four 6 AWG THHN, two 8 AWG THHN and one 10 AWG THHN equipment grounding conductor.

• Table 5 areas, square inches: 6 AWG is 0.0507 each, 8 AWG is 0.0366 each, 10 AWG is 0.0211.
• Total: 4 × 0.0507 = 0.2028, plus 2 × 0.0366 = 0.0732, plus 0.0211. The sum is 0.2971 square inches.
• Seven conductors means the over-two percentage, 40%.
• Table 4 for EMT: 3/4 inch allows 0.213 square inches at 40%, which is too small. 1 inch allows 0.346, which is enough.
• Answer: 1-inch EMT.

The mixed sizes mean Annex C is no help here, since it only covers one size at a time. That is why you need Table 5.

Common traps

• Using the wrong table for the raceway. Table 4 has a separate section for each raceway type, and the areas differ.
• Applying the 40% to everything. One conductor and two conductors use other percentages, so count first.
• Forgetting grounding conductors. An insulated equipment grounding conductor is included in fill. Take its area from Table 5. A bare one uses Table 8.
• Short nipples. The notes to the tables give different treatment for runs of 24 inches or less. Read the note, then apply it.
• Using Annex C for mixed sizes. It lists same-size cases only.
• Reading the wrong insulation row. THHN, THWN and XHHW have different areas. Match the type in the problem.
• Conduit fill versus derating. Fitting the conductors does not mean the ampacity is unchanged. Adjustment for more than three current-carrying conductors is a separate step in 310.15 (2023 numbering).

Working it with the book open

Tab the Table 1 page, the Table 4 page for each raceway type you expect, and Table 5. Write each conductor's area on scratch paper, add the column once and compare with the Table 4 column for the right percentage. Put a finger on the percentage column before you read across, because the neighboring columns hold other fill percentages and are easy to misread. Recheck the count of conductors before you pick the percentage.

How to practice

Invent a dozen conductor sets, some with a ground and some with different sizes. Work each through Tables 5, 1 and 4, then check the same-size ones against Annex C. Aim for about two minutes each.

Related guides: conductor ampacity and box fill.

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Sources

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• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• Electrician exam information, Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.