How do you calculate conduit fill under the NEC?

Count the conductors, use Chapter 9 Table 5 for conductor area, use Chapter 9 Table 4 for raceway area, apply the Table 1 fill limit, then compare total conductor area to allowed raceway area.

What conduit fill percentage do I use?

Use 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. Most exam questions use three or more, but the one-conductor and two-conductor cases are where candidates get trapped.

Which NEC tables are used for conduit fill?

Use Chapter 9 Table 1 for the fill percentage, Table 4 for raceway area, and Table 5 for conductor area by size and insulation type.

Do equipment grounding conductors count for conduit fill?

Yes. Conduit fill is about physical space in the raceway, so the equipment grounding conductor takes area. Do not confuse this with ampacity adjustment, where the EGC is not current carrying.

Is conduit fill the same as box fill?

No. Conduit fill compares conductor cross-sectional area to raceway area. Box fill counts cubic-inch allowances inside a box. The names sound similar, but the tables and setup are different.

NEC Chapter 9 | conduit fill

NEC Conduit Fill Calculations

Conduit fill is a table-order problem. If you use the right tables in the wrong order, the answer still looks clean and still fails.

Last reviewed July 2026

The table order

Conduit fill is short when you follow the table order. Count first. Then conductor area. Then raceway area. Then compare.

Chapter 9 Table 1: choose the fill percentage from the conductor count.
Chapter 9 Table 5: find the area for each conductor size and insulation type.
Chapter 9 Table 4: find the raceway area for the raceway type and trade size.
Compare total conductor area to allowed raceway area.

If you want to check the arithmetic after you know the setup, use the free conduit fill calculator. If you are studying for the exam, learn the table order first.

The three fill percentages

One conductor: 53% maximum fill.
Two conductors: 31% maximum fill.
Three or more conductors: 40% maximum fill.

Worked example: 6 x 10 AWG THHN and 2 x 12 AWG THHN

Six 10 AWG THHN conductors and two 12 AWG THHN conductors are run in EMT. What is the smallest EMT trade size that works?

Conductor count: 8 conductors, so use the 40% fill rule.
10 AWG THHN area: 0.0211 square inches x 6 = 0.1266 square inches.
12 AWG THHN area: 0.0133 square inches x 2 = 0.0266 square inches.
Total conductor area: 0.1532 square inches.
1/2 in. EMT at 40% fill is about 0.122 square inches. Too small.
3/4 in. EMT at 40% fill is about 0.213 square inches. It fits.

Answer: 3/4 in. EMT. The miss usually happens when someone uses the wrong raceway row or forgets to add both conductor sizes.

What gets mixed up with conduit fill

Conduit fill versus ampacity derating

Conduit fill answers a space question: does the conductor area fit inside the raceway? Ampacity derating answers a heat question: how much current can those conductors carry after bundling and ambient corrections? Both can apply to the same raceway, but they are not the same calculation.

Conduit fill versus box fill

Conduit fill uses square inches of cross-sectional area. Box fill uses cubic inches of box volume. If the question says raceway, pipe, EMT, PVC, or conduit, think Chapter 9. If it says device box, junction box, yoke, or clamp, think Article 314.

Trade size versus actual inside area

Same trade size does not mean same internal area. EMT, rigid, IMC, and PVC do not all have the same Table 4 area. Read the raceway type before you pick the row.

How to drill this for the exam

Do not memorize one example. Drill the table order until it is automatic. Count. Table 5. Table 4. Table 1. Compare. When that order is automatic, conduit fill stops feeling like a new problem every time.

See whether conduit fill is costing you points

The free diagnostic separates conduit fill, box fill, grounding, motors, and other NEC weak areas so your next practice block has a reason.

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