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Boxes and racewaysJourneymanMasterResidential wiremanNEC 2026

Conduit fill calculations (NEC Chapter 9)

Short answer

Conduit fill limits how much of a raceway's cross section the conductors may take up: 40% for more than two conductors. Add up the area of every conductor from Chapter 9 Table 5, then pick the smallest trade size in Table 4 whose 40% area is at least that total.

Updated Original problems, new numbers every time

Live practice

Practice: conduit fill

Each problem uses new numbers, so you practice the method rather than memorizing an answer.

Formula

Total conductor area (in²) = Σ (number of conductors × area of one, Table 5)
Pick the smallest trade size whose allowed area (Table 4, 40% column) ≥ total conductor area
Max conductors of one size = allowed area ÷ area of one, round up only at 0.8 or more
SymbolMeaningUnit
Conductor areaArea of one insulated conductor, Chapter 9 Table 5in²
Allowed areaRaceway area at the Table 1 fill percent, Chapter 9 Table 4in²
Fill %53% for one conductor, 31% for two, 40% for more than two%

Worked example

A run of Schedule 40 PVC will hold THHN/THWN-2 copper conductors: 6 of 14 AWG and 4 of 8 AWG. Using Chapter 9 Tables 1, 4 and 5, what is the minimum trade size of the raceway?

  1. Look up one 14 AWG THHN in Chapter 9 Table 5: 0.0097 in².
  2. 6 × 0.0097 = 0.0582 in².
  3. Look up one 8 AWG THHN in Chapter 9 Table 5: 0.0366 in².
  4. 4 × 0.0366 = 0.1464 in².
  5. Total conductor area: 0.0582 + 0.1464 = 0.2046 in².
  6. There are 10 conductors, more than two, so Chapter 9 Table 1 allows 40% fill.
  7. In Chapter 9 Table 4, Schedule 40 PVC, 40% column, the smallest trade size with at least 0.2046 in² is trade size 1 (0.333 in²).

Answer: 1

Conduit fill, explained

Conductors packed too tight are hard to pull and run hot. The NEC limits fill by area: the insulated conductors may take up only a set share of the raceway's inside area. Chapter 9 Table 1 gives that share. For the usual case of more than two conductors it is 40%.

Every exam fill problem uses two tables. Table 5 gives the area of one conductor by insulation type and size. Table 4 gives the inside area of each raceway type by trade size, with a column already worked out for 40%. Each raceway type has its own section in Table 4, so EMT, rigid and PVC of the same trade size have different areas.

Equipment grounding conductors count toward fill (Chapter 9 Note 3). A nipple 24 in. long or less between boxes may be filled to 60% (Note 4). When all conductors are the same size, Note 7 lets you round up to the next whole number if the decimal is 0.8 or more. Informative Annex C has tables built from these same rules for conductors that are all one size and type.

How to solve it on the exam

  1. Find each conductor's area in Chapter 9 Table 5 for its insulation type and size, including any insulated equipment grounding conductor.
  2. Multiply each area by the number of conductors of that size and add the results.
  3. Count the conductors and pick the fill percent from Chapter 9 Table 1: 40% for more than two.
  4. Go to the section of Chapter 9 Table 4 for the raceway type and find the smallest trade size whose 40% column is at least your total.
  5. For the most conductors of one size, divide the 40% area by one conductor's area. Round down, unless the decimal is 0.8 or more (Note 7).

Common mistakes

  • Reading the wrong raceway section of Table 4, such as EMT values for a PVC run.
  • Using the total (100%) column of Table 4 instead of the 40% column.
  • Leaving the equipment grounding conductor out of the fill.
  • Using the 60% nipple fill for a full-length raceway.
  • Using THHN areas when the question says XHHW-2 or THW-2.
  • Always rounding down the count of same-size conductors, even when the decimal is 0.8 or more.

Exam tips

  • Tab Chapter 9 Tables 4 and 5 in your code book. Most fill questions need nothing else.
  • If every conductor is the same size and type, Informative Annex C gives the answer directly. Use it to check your math.
  • Keep four decimals for conductor areas until the end, so rounding does not move you to the wrong trade size.

Questions people ask

What is the maximum conduit fill percentage?

For more than two conductors the maximum fill is 40% of the raceway's inside area. One conductor may fill 53% and two conductors 31%, from Chapter 9 Table 1.

Do ground wires count in conduit fill?

Yes. Chapter 9 Note 3 says equipment grounding and bonding conductors count toward fill. Use the Table 5 area for an insulated EGC and the Table 8 area for a bare one.

Why is a nipple allowed 60% fill?

A nipple 24 in. or shorter between boxes or enclosures may be filled to 60% under Chapter 9 Note 4, because it is too short for heat or pulling to be a problem. The 60% does not apply to longer raceways.

When do you round up the number of conductors in conduit fill?

Only when all the conductors are the same size and the decimal part of your answer is 0.8 or more. Chapter 9 Note 7 then lets you use the next whole number; otherwise round down.

Sources

About the tables: The exam is open book. These problems cite the 2026 NEC section and table so you practice finding them in your own book. We never print NEC text or tables.