Journeyman
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Conduit fill calculation

Updated · Written for the 2023 NEC

Short answer

Conduit fill is the share of a raceway's internal area that conductors are allowed to occupy. NEC Chapter 9, Table 1 sets the limit at 53% for one conductor, 31% for two, and 40% for three or more. To check a raceway, add up each conductor's area from Chapter 9, Table 5 and compare the total to the allowable area for that raceway in Chapter 9, Table 4.

What conduit fill is and why it matters on the exam

Conduit fill limits conductors in a raceway so they can be pulled without damage and can shed heat. Each raceway article points to Chapter 9 for its fill limit, for example 358.22 for EMT. Exams ask for the percentages, the smallest trade size, or the maximum number of wires.

Conduit fill is one of the standard journeyman electrician exam calculations, and it uses the same habits as box fill: count everything, then compare against the space available.

The key rules (NEC Chapter 9)

The fill percentage in Chapter 9, Table 1 depends on how many conductors are in the raceway. One conductor may fill 53% of the internal area, two conductors 31%, and three or more 40%.

The two-conductor limit is lower than the one-conductor limit because two round conductors in a round raceway are more likely to jam during a pull.

The notes to the Chapter 9 tables add rules that show up on the exam:

  • EGCs count. Equipment grounding and bonding conductors take up space and are included in the fill calculation (Chapter 9, Note 3). Bare conductors use the dimensions in Chapter 9, Table 8.
  • Nipples get 60%. A nipple 24 in or shorter between boxes or enclosures may be filled to 60% of its total area, and the ampacity adjustment factors of 310.15(C)(1) do not apply to it (Chapter 9, Note 4).
  • The 0.8 rule. When every conductor is the same size and the calculation ends in a decimal of 0.8 or more, you may round up to the next whole number (Chapter 9, Note 7). Any lower decimal rounds down.
  • Cables count as one. A multiconductor cable in a raceway is treated as a single conductor, using its overall diameter. An elliptical cable is figured as a circle using its major diameter.
  • Wireways are different. Metal wireways are limited to 20% fill under 376.22(A), not the Chapter 9 percentages.

Using Table 4 and Table 5

You need two numbers for every calculation, and they come from two tables:

  • Chapter 9, Table 4 lists the internal area of each raceway type and trade size. It already shows the area at each fill percentage, so you read the 40% column for three or more wires instead of multiplying yourself. Each raceway type has its own section because inside diameters differ.
  • Chapter 9, Table 5 lists the approximate area of each conductor, including insulation, by insulation type and size. THHN and THWN share a row. Compact aluminum conductors use Table 5A instead.
Total conductor area = sum of (number of each size × area from Table 5) Raceway OK if total conductor area ≤ allowable area from Table 4 Maximum conductors = allowable area ÷ area of one conductor

Using Annex C

Informative Annex C gives a ready count of the maximum number of conductors of one size and one insulation type for each raceway type and trade size. Table C.1, for example, covers EMT. Annex C is the fast route when every conductor is the same size and type. As soon as the sizes or insulation types are mixed, Annex C no longer applies and you must calculate with Tables 4 and 5.

Annex C is built from the same areas and the 0.8 rule, so a correct calculation matches it.

Worked examples

Example 1: mixed sizes, smallest EMT

A circuit in EMT has four 6 AWG THHN conductors (three ungrounded and a neutral) and one 10 AWG THHN equipment grounding conductor. What is the smallest EMT trade size?

  1. Conductor areas from Chapter 9, Table 5: 6 AWG THHN is 0.0507 sq in, and 10 AWG THHN is 0.0211 sq in.
  2. 6 AWG total: 4 × 0.0507 = 0.2028 sq in.
  3. Add the EGC: 0.2028 + 0.0211 = 0.2239 sq in.
  4. Five conductors means the 40% limit applies.
  5. Trade size 3/4 EMT: Chapter 9, Table 4 gives 0.213 sq in at 40%. The conductors need 0.2239, which is 0.0109 sq in too much.
  6. Trade size 1 EMT: Table 4 gives 0.346 sq in at 40%. That leaves 0.1221 sq in to spare.

The answer is trade size 1 EMT. Notice that leaving out the EGC would have made 3/4 look acceptable, which is the classic trap.

Example 2: same size, maximum number

How many 14 AWG THHN conductors fit in trade size 3/4 EMT?

  1. Allowable area: 0.213 sq in at 40% (Chapter 9, Table 4).
  2. One 14 AWG THHN: 0.0097 sq in (Chapter 9, Table 5).
  3. Divide: 0.213 ÷ 0.0097 = 21.96.
  4. Apply the 0.8 rule: every conductor is the same size and the decimal is 0.96, so round up to 22.

Annex C, Table C.1 gives the same 22. In 1/2 EMT, 0.122 ÷ 0.0097 = 12.58, which rounds down to 12.

Example 3: a nipple

A 1/2 in EMT nipple 18 in long joins two panels. Its total internal area is 0.304 sq in (Chapter 9, Table 4). How many 10 AWG THHN conductors can it hold?

  1. Nipple fill is 60%: 0.304 × 0.60 = 0.1824 sq in.
  2. Divide by 0.0211: 0.1824 ÷ 0.0211 = 8.64.
  3. Decimal is under 0.8, so round down to 8 conductors.

Common exam mistakes

  • Leaving out the EGC. Every conductor in the raceway counts, including grounds and bonding jumpers.
  • Using 40% for one or two wires. One conductor gets 53% and two get 31%.
  • Multiplying the wrong column. Table 4 already lists the 40% area. Do not take 40% of the 40% value.
  • Using Annex C for mixed sizes. Annex C only works when all conductors are the same size and insulation type.
  • Rounding up without the 0.8 rule. Round up only when all conductors match and the decimal is 0.8 or more.
  • Applying 60% to long runs. The nipple allowance stops at 24 in.

Practice questions

Question 1 · Box and conduit fillWhat is the maximum fill for a conduit containing only one conductor?
Show answer

Answer: C. 53%. NEC Chapter 9, Table 1

A single conductor can occupy up to 53% of the conduit's internal area. Two conductors are limited to 31% because they tend to jam.

Question 2 · Box and conduit fillA conduit nipple not exceeding 24 in long connects two enclosures. What is the maximum allowable fill?
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Answer: A. 60%. NEC Chapter 9, Table 1

Nipples 24 in or shorter may be filled to 60% of their total area, and the ampacity adjustment factors do not apply to them.

Question 3 · Box and conduit fillHow many 8 AWG THHN conductors are permitted in trade size 3/4 EMT, applying the Chapter 9 rule for conductors of the same size?
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Answer: B. 6. NEC Chapter 9, Table 1

0.213 ÷ 0.0366 = 5.82. When all conductors are the same size and the result has a decimal of 0.8 or more, the next higher whole number is permitted, so 6 conductors may be installed.

  1. 3/4 EMT, 40% = 0.213 in²; 8 AWG THHN = 0.0366 in²
  2. 0.213 / 0.0366 = 5.82
  3. Decimal ≥ 0.8 → round up to 6 (Chapter 9, Note 7)
Question 4 · Box and conduit fillFour 1/0 THHN conductors (0.1855 sq in each) and one 6 AWG THHN EGC (0.0507 sq in) are installed in EMT. What is the smallest trade size? (40% areas: 1-1/4 = 0.598 sq in, 1-1/2 = 0.814 sq in, 2 = 1.342 sq in)
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Answer: D. 1-1/2. NEC Chapter 9, Table 4

Total area is 4 × 0.1855 + 0.0507 = 0.7927 sq in. It is too large for 1-1/4 EMT (0.598) but fits 1-1/2 EMT (0.814).

  1. 1/0: 4 × 0.1855 = 0.742 in²
  2. Total = 0.742 + 0.0507 = 0.7927 in²
  3. 1-1/4 (0.598) too small; 1-1/2 (0.814) OK → 1-1/2
Question 5 · Box and conduit fillWhen may the tables in Informative Annex C be used to determine conduit fill?
Show answer

Answer: C. When all conductors in the raceway are the same size and insulation type. NEC Informative Annex C

Annex C tables give the maximum number of conductors of one size and insulation type for each raceway. Mixed sizes require calculating total area with Chapter 9, Tables 4 and 5.

Frequently asked questions

What is the 40% rule for conduit fill?

When a raceway holds three or more conductors, their total cross-sectional area cannot exceed 40% of the raceway's internal area, per Chapter 9, Table 1. One conductor may fill 53% and two may fill 31%.

Do ground wires count in conduit fill?

Yes. Equipment grounding and bonding conductors are included in the calculation. An insulated EGC uses its Table 5 area, and a bare one uses Table 8.

How many 12 AWG THHN wires fit in 1/2 inch EMT?

Nine. Trade size 1/2 EMT has 0.122 sq in available at 40%, and 12 AWG THHN is 0.0133 sq in, so 0.122 ÷ 0.0133 = 9.17, which rounds down to 9. Annex C gives the same number.

When can I use Annex C instead of calculating?

Use Annex C when every conductor in the raceway is the same size and insulation type. For mixed sizes, add up the areas from Chapter 9, Table 5 and compare to Table 4.

Does conduit fill change ampacity?

Fill does not, but more than three current-carrying conductors in a raceway triggers the 310.15(C)(1) adjustment factors. See conductor ampacity for how that works, then test yourself on a journeyman practice test.

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