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Conduit Fill Practice Problems

Original exam-style problems on NEC Chapter 9 conduit fill: how many conductors fit, what percent a raceway is filled, and the smallest conduit for a mix of wires. Every answer shows the Table 4 and Table 5 numbers it comes from.

By Blissom · Updated 2026-11-04

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Rules you need (NEC 2023, Chapter 9)

1 conductor 53% · 2 conductors 31% · over 2 conductors 40%
nipple ≤ 24" between enclosures: 60%
max same-size = raceway area × 40% ÷ conductor area
Note 7: same size, decimal ≥ 0.8 rounds up
percent fill = total conductor area ÷ raceway area

Raceway areas come from Table 4, conductor areas from Table 5. Insulated grounds count.

Maximum number of one size

Same-size conductors: 40% of the raceway area ÷ the area of one conductor (Chapter 9, Tables 4 and 5). Note 7: a result of x.8 or more rounds up; anything less rounds down.

How many #14 THHN conductors are permitted in 3/4" EMT?

  • a. 21
  • b. 22
  • c. 29
  • d. 32

Answer: b. 22.

  1. 3/4" EMT: 0.533 in² × 40% = 0.2132 in².
  2. #14 THHN: 0.0097 in². 0.2132 ÷ 0.0097 = 21.98.
  3. The fraction is .98, at least .8, so Note 7 rounds up to 22. Rounding down to 21 is the usual mistake; 29 and 32 use the 53% and 60% limits, which do not apply here.

Conduit fill chart →

How many #10 THHN conductors are permitted in 3/4" EMT?

Answer: 10.

  1. 0.2132 ÷ 0.0211 = 10.10.
  2. .10 is under .8, so 10.

How many #8 THHN conductors are permitted in 1" EMT?

  • a. 9
  • b. 10
  • c. 12
  • d. 7

Answer: a. 9.

  1. 1" EMT: 0.864 × 40% = 0.3456 in². #8 THHN: 0.0366 in².
  2. 0.3456 ÷ 0.0366 = 9.44. The .44 is under .8: 9.

How many #6 THHN conductors are permitted in 1-1/4" EMT?

  • a. 11
  • b. 12
  • c. 13
  • d. 15

Answer: b. 12.

  1. 1-1/4" EMT: 1.496 × 40% = 0.5984 in². #6 THHN: 0.0507 in².
  2. 0.5984 ÷ 0.0507 = 11.80.
  3. .80 meets the Note 7 threshold, so it rounds up to 12.

How many #12 XHHW conductors are permitted in 1" EMT?

Answer: 19.

  1. 0.3456 ÷ 0.0181 = 19.09, so 19.
  2. XHHW is fatter than THHN: the same conduit takes 26 #12 THHN.

How many #12 THW conductors are permitted in 3/4" EMT?

Answer: 8.

  1. #12 THW: 0.0260 in² (the conservative Table 5 row).
  2. 0.2132 ÷ 0.0260 = 8.20, so 8, about half the #12 THHN count.

How many #4 THHN conductors are permitted in 1" PVC Schedule 40?

Answer: 4.

  1. 1" PVC 40: 0.832 in² × 40% = 0.3328 in². #4 THHN: 0.0824 in².
  2. 0.3328 ÷ 0.0824 = 4.04, so 4.

Percent fill and the 53 / 31 / 40% rule

Chapter 9 Table 1: one conductor 53%, two conductors 31%, more than two 40%. Percent fill = total conductor area ÷ raceway area.

Four #8 THHN and one #10 THHN equipment ground run in 3/4" EMT. What is the percent fill?

  • a. 31.4%
  • b. 27.5%
  • c. 78.6%
  • d. 19.4%

Answer: a. 31.4%.

  1. 4 × 0.0366 + 0.0211 = 0.1675 in².
  2. 0.1675 ÷ 0.533 = 31.4%, under the 40% limit.
  3. 27.5% leaves out the ground; 78.6% divides by the 40% area instead of the full area.

Conduit fill calculator →

Are two #3 THHN conductors permitted in 3/4" EMT?

  • a. Yes, they are under 40%
  • b. No, they are over the 31% allowed for two conductors
  • c. Yes, they are under 53%
  • d. Yes, they are under 60%

Answer: b. No, they are over the 31% allowed for two conductors.

  1. 2 × 0.0973 = 0.1946 in². 0.1946 ÷ 0.533 = 36.5%.
  2. Two conductors are held to 31% (0.533 × 31% = 0.1652 in²). One #3 goes 1.70 times into that, short of the 1.8 that Note 7 would round up to two, so they do not fit. Two round wires can jam in a raceway, which is why the limit is lower.

Is a single #1 THHN permitted in 1/2" EMT?

Answer: Yes: 51.4%, under the 53% allowed for one conductor.

  1. #1 THHN: 0.1562 in². 0.1562 ÷ 0.304 = 51.4%.
  2. One conductor may fill 53%. Whether a #1 can be pulled around the bends in 1/2" EMT is a separate, practical question.

A 1/2" EMT holds one #10 THHN hot, one #10 THHN neutral and one #10 THHN equipment ground. Which fill percentage applies?

  • a. 31%
  • b. 40%
  • c. 53%
  • d. 60%

Answer: b. 40%.

  1. The insulated ground is a conductor in the raceway, so this is three conductors: 40%.
  2. 3 × 0.0211 = 0.0633 in², 20.8% of 0.304 in². It fits.

Minimum conduit size for a mix

Add the area of every conductor, grounds included, and find the smallest raceway whose 40% column is at least that total. Different raceway types have different areas for the same trade size.

Three #4 THHN and one #8 THHN equipment ground in EMT. What is the minimum trade size?

  • a. 1/2"
  • b. 3/4"
  • c. 1"
  • d. 1-1/4"

Answer: c. 1".

  1. 3 × 0.0824 + 0.0366 = 0.2838 in².
  2. 3/4" EMT allows 0.2132 in², too small. 1" EMT allows 0.3456 in²: 1".

Six #12 THHN, four #10 THHN and one #10 THHN ground in EMT. What is the minimum trade size?

Answer: 3/4".

  1. 6 × 0.0133 + 5 × 0.0211 = 0.1853 in² (the ground counts as the fifth #10).
  2. 1/2" allows 0.1216 in²; 3/4" allows 0.2132 in². Minimum 3/4".

Four 3/0 THHN and one #4 THHN equipment ground run in PVC Schedule 40. What is the minimum trade size?

Answer: 2".

  1. 4 × 0.2679 + 0.0824 = 1.1540 in².
  2. 1-1/2" PVC 40: 1.986 × 40% = 0.7944 in², too small. 2": 3.291 × 40% = 1.3164 in². Minimum 2".

The same four 3/0 THHN and #4 THHN ground are run in PVC Schedule 80 instead. Minimum trade size?

  • a. 1-1/2"
  • b. 2"
  • c. 2-1/2"
  • d. 3"

Answer: c. 2-1/2".

  1. Same 1.1540 in² of conductors.
  2. 2" PVC 80 has a thicker wall: 2.874 × 40% = 1.1496 in², just too small. Minimum 2-1/2".

Four #1 XHHW and one #6 XHHW equipment ground in rigid metal conduit. Minimum trade size?

Answer: 1-1/2".

  1. 4 × 0.1534 + 0.0590 = 0.6726 in².
  2. 1-1/4" RMC: 1.526 × 40% = 0.6104 in², too small. 1-1/2": 2.071 × 40% = 0.8284 in². Minimum 1-1/2".

Nipples

A nipple 24" or shorter between boxes or enclosures may be filled to 60% (Chapter 9, Note 4). Over 24", the normal 53/31/40% limits apply.

An 18" piece of 1" EMT connects two enclosures. How many #6 THHN conductors may it hold?

  • a. 7
  • b. 10
  • c. 9
  • d. 11

Answer: b. 10.

  1. 18" is a nipple: 0.864 × 60% = 0.5184 in².
  2. 0.5184 ÷ 0.0507 = 10.22, so 10. 11 would be over 60%.
  3. 7 is the answer for a normal run at 40%.

The same connection is made with a 30" piece of 1" EMT. How many #6 THHN may it hold?

Answer: 7.

  1. 30" is over 24", so it is not a nipple: 40% applies.
  2. 0.3456 ÷ 0.0507 = 6.82. The .82 rounds up under Note 7: 7.

Common traps

  • Forgetting the ground. The insulated EGC is in the raceway; leave it out and the fill looks lower than it is.
  • Using 40% for two wires. Two conductors are held to 31%. Three or more (grounds included) get 40%.
  • Rounding down every time. Same-size conductors at x.8 or more round up under Note 7: 11.80 is 12.
  • Rounding up a mix. Note 7 is for one size only. A mix either fits by area or it does not.
  • Using the THHN count for THW or XHHW. Insulation changes the area: 3/4" EMT takes 16 #12 THHN but only 8 #12 THW.
  • Assuming PVC 80 matches PVC 40. The thicker wall leaves less room; a mix that fits 2" Schedule 40 can need 2-1/2" Schedule 80.
  • Calling any short piece a nipple. Over 24", it is a raceway again and the 40% limit applies.
  • Dividing by the 40% area for percent fill. Percent fill is against the full internal area; compare the result with 40%.

Run your own mix in the conduit fill calculator, or look up the counts in the conduit fill chart. Box fill works differently; see the box fill problems.

Sources: NEC 2023: Chapter 9 Tables 1, 4 and 5 (fill), 314.16 (box fill), 358.24 and Chapter 9 Table 2 (bends); Answers checked against the calculation engine used by the Conduit Bending Calculator app. Method.

Questions electricians ask

Does the ground wire count in conduit fill?

Yes. An insulated equipment grounding conductor counts at its Table 5 area like any other conductor, and it counts toward the number of conductors that decides between 53%, 31% and 40%. Bare conductors use their Table 8 dimensions.

When do you round up in conduit fill?

Only when every conductor is the same size (including insulation) and the division comes out with a decimal of 0.8 or more (Chapter 9, Note 7). 21.98 becomes 22; 9.44 stays 9. For a mix of sizes, compare total areas instead.

Why is the limit for two conductors only 31%?

Two round conductors in a round raceway can wedge across the inside diameter during a pull. The lower limit for two keeps room to prevent jamming. One conductor may fill 53%, more than two 40%.

What counts as a nipple for the 60% rule?

A raceway 24" or shorter between boxes, cabinets or similar enclosures (Chapter 9, Note 4). A longer piece is an ordinary raceway and goes back to the 53%, 31% and 40% limits.

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