Conduit Fill Calculations (NEC Chapter 9), Step by Step

Conduit fill is the percentage of a conduit's internal area taken up by the conductors inside it, and the NEC caps it at 53% for one conductor, 31% for two, and 40% for three or more (Chapter 9, Table 1). Get those three numbers down and conduit-fill questions become free points on the exam.

The two tables you need

Conduit fill lives almost entirely in NEC Chapter 9:

• Table 5 — the cross-sectional area (in²) of each conductor, by insulation type. For THHN, a #12 is 0.0133 in² and a #10 is 0.0211 in².
• Table 4 — the total internal area (in²) of each conduit, by type and trade size. For EMT, 1/2" is 0.304 in² and 3/4" is 0.533 in².

Table 1 sets the maximum fill percentages. That's the whole toolkit.

The method

1. Add up conductor area. Multiply each conductor's area (Table 5) by how many of that size you have, then sum.
2. Find the conduit area. Look up the conduit's internal area in Table 4.
3. Divide and compare. Fill % = conductor area ÷ conduit area × 100. Compare to the Table 1 limit for your conductor count.

A worked example

Say you're pulling four #12 THHN in 1/2" EMT:

• Conductor area: 4 × 0.0133 = 0.0532 in²
• Conduit area (1/2" EMT): 0.304 in²
• Fill: 0.0532 ÷ 0.304 × 100 = 17.5%

Four conductors → the limit is 40%. At 17.5% you're well under — compliant. If you bumped to a dozen #12, you'd be at 52.5%, over the 40% limit, and you'd size up to 3/4" EMT.

Why the limits exist

The percentages aren't arbitrary. They leave room to dissipate heat and to pull conductors without scraping insulation off against the others. Inspectors hold you to them, and so does the exam.

Practice it instantly

You can run any combination on our free Conduit Fill Calculator — it shows the worked steps and the NEC limit so you learn the method, not just the answer. When you're ready to drill it under exam conditions, the free diagnostic shows whether conduit fill is one of your weak spots before test day.