Conduit Fill Calculations: NEC Chapter 9 Tables Explained

The fill percentages

From NEC Chapter 9, Table 1:

• One conductor → 53% max fill
• Two conductors → 31% max fill
• More than two conductors → 40% max fill

Memorize these three numbers. The whole exam topic flows from them.

The 5-step calculation

Every conduit fill problem follows the same five steps:

1. Identify the conductor sizes and types. Different insulation = different area.
2. Look up the area per conductor in Chapter 9 Table 5 (square inches).
3. Multiply by the count of each type. Sum to get total conductor area.
4. Identify the raceway type (EMT, RMC, IMC, ENT, PVC) and the fill percentage rule (40% for 3+ conductors).
5. Look up the raceway internal area in Chapter 9 Table 4 at the chosen fill percentage. Pick the raceway size whose listed area is greater than or equal to your conductor sum.

A worked example

Six 10 AWG THHN conductors plus two 12 AWG THHN, all in a single EMT raceway.

• 10 AWG THHN area: 0.0211 sq in × 6 = 0.1266 sq in
• 12 AWG THHN area: 0.0133 sq in × 2 = 0.0266 sq in
• Total: 0.1532 sq in
• 8 conductors → 40% fill rule
• EMT 1/2 in (40% fill) = 0.122 sq in. Too small.
• EMT 3/4 in (40% fill) = 0.213 sq in. Fits.

Answer: 3/4 in EMT. The 1/2 in EMT only handles the conductors if you cut the count down or use smaller AWG.

Three things candidates miss

1. Using the wrong insulation type

THHN, THWN, XHHW, RHW: different areas for the same AWG. The tables in Chapter 9 list them separately. Don’t use a generic "10 AWG" area; use the specific insulation row.

2. Forgetting derating

Conduit fill and ampacity derating are separate calculations but interact. A raceway that fits 9 current-carrying conductors per Chapter 9 may require derating per Table 310.15(C)(1). The fill calc says they fit; the ampacity calc says they can’t carry rated current. Both have to pass.

3. The "1, 2, more-than-2" rule applies per cable, not per conductor

For Type AC, MC, MI, NM, and similar cable assemblies, count cables, not the conductors inside them. A single 12-2 NM cable is one cable for fill purposes (when run inside a raceway).