Conduit fill rules: how NEC Chapter 9 sizes a raceway

The three percentages

Chapter 9, Table 1 limits the share of a raceway's internal area that conductors may take up:

The two-conductor limit is the strictest because two round wires side by side can jam across the pipe during a pull. A single conductor gets 53% because it cannot jam against anything.

The method

1. Find each conductor's area in Chapter 9, Table 5 for its size and insulation type (THHN/THWN-2, XHHW, RHW and so on). Bare conductors use Table 8.
2. Multiply by the quantity and add everything up, grounds included.
3. Pick the fill percentage from Table 1 for the total number of conductors.
4. Look up the internal area of each trade size in Chapter 9, Table 4 for your raceway type (EMT, rigid, IMC, PVC Schedule 40 or 80, flexible). The smallest size whose area × fill percentage is at least the total conductor area is the minimum.

Required raceway area = total conductor area ÷ fill percentage
Example: 0.1197 sq in ÷ 0.40 = 0.299 sq in

When every conductor is the same size and type, the tables in Annex C have already done this division for every raceway type. Mixed sizes always need the full calculation.

Areas you will use most (THHN/THWN-2 and EMT)

Worked examples

Nine 12 AWG THHN in EMT. Total area 9 × 0.0133 = 0.1197 sq in. At 40%, the raceway needs 0.299 sq in. 1/2" EMT has 0.304 sq in, so 1/2" EMT is the minimum, with almost nothing to spare.

A feeder: three 6 AWG THHN plus a 10 AWG THHN ground. 0.1521 + 0.0211 = 0.1732 sq in. Four conductors, so 40%: required 0.433 sq in, which means 3/4" EMT (0.533 sq in). Leaving out the ground would have been a code violation even though it carries no current in normal operation.

Two versus three conductors. Two 1/0 THHN total 0.3710 sq in; at the 31% limit they need 1.197 sq in, which is 1-1/4" EMT. Three 1/0 THHN total 0.5565 sq in; at 40% they need 1.391 sq in, also 1-1/4". Adding a third conductor did not require a bigger pipe, because the percentage rose from 31% to 40%.

Mixed sizes. Six 12 AWG and three 10 AWG THHN: 6 × 0.0133 + 3 × 0.0211 = 0.1431 sq in, required 0.358 sq in at 40%, so 3/4" EMT. Annex C cannot answer this one because it only covers one size at a time.

The notes that change the answer

• Grounds count (Note 3). Equipment grounding and bonding conductors are included in the fill calculation, insulated or bare, at their actual area.
• Short nipples get 60% (Note 4). A conduit or tubing nipple no longer than 24 inches between boxes, cabinets and similar enclosures may be filled to 60%, and the ampacity adjustment for more than three current-carrying conductors does not apply to it. Example: 1/2" EMT at 60% is 0.182 sq in, enough for 13 × 12 AWG THHN.
• Rounding up at 0.8 (Note 7). When you calculate the maximum number of same-size conductors and the result has a decimal of 0.8 or more, you may use the next whole number. That is why some Annex C entries look one higher than a straight division.
• Cables count as one conductor (Note 9). A multiconductor cable is treated as a single conductor, using its outside diameter; an oval cable is treated as a circle of its larger dimension. One cable in a raceway therefore gets 53%, two get 31%.

Fill is not ampacity

Passing the fill check only means the wires physically fit. If a raceway has more than three current-carrying conductors, their ampacity must also be reduced with the adjustment factors of 310.15(C)(1): 80% for 4 to 6, 70% for 7 to 9, 50% for 10 to 20. Grounds count for fill but not for this adjustment. A 3/4" EMT full of nine 12 AWG current-carrying conductors fits, but each conductor is then derated to 70% of its table ampacity before the terminal limit is checked.

Long runs with many bends can also be hard to pull at legal fill. The fill limit is a maximum, not a target; electricians often upsize one trade size to make the pull easier and leave room for future circuits.

Common mistakes

• Using the trade size as a diameter. A 3/4" EMT is not 0.75 inch inside; use the Table 4 area.
• Using THHN areas for other insulation. XHHW and RHW conductors are larger in Table 5, and a 12 AWG XHHW does not fit where the same count of THHN does.
• Using the EMT column for PVC or rigid conduit. Schedule 80 PVC in particular has noticeably less room than EMT of the same trade size.
• Applying 40% to two conductors. Two wires get 31%.
• Forgetting the ground, or counting it for derating.

Questions people ask