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TDLR conduit fill practice

Conduit fill checks that the wires don't take up more of the pipe than code allows, so they can shed heat. This is a TDLR Calculations-part question type, now scored on its own. Here is the pattern, one worked example, and an original question to try.

Last reviewed June 2026

One worked example

Five 12 AWG THHN run in 1-inch EMT. Are you under the fill limit?

  1. 1
    Name the problem

    This is a conduit fill. Bundle too many wires and they cannot shed heat, so code caps how much of the pipe the conductors can take up.

  2. 2
    Add up the conductor area

    Each conductor takes a fixed area from Chapter 9 Table 5. Add them all together.

    5 × 12 AWG THHN = 0.0665 sq in

    Subtotal: 0.0665 sq in (5 conductors)

  3. 3
    Find the allowed fill

    Table 1 sets the cap by how many conductors are in the pipe. One conductor 53%, two 31%, three or more 40%.

    5 conductors → 40% allowed

    Raceway area (Table 4): 0.864 sq in

    Allowed = 0.864 × 0.40 = 0.3456 sq in

  4. 4
    Compare

    Your conductors fill 7.70% of the pipe. The raceway has room. You are under the limit.

    0.0665 sq in vs 0.3456 sq in allowed

Try an original question

Sample question · original

You are pulling 12 AWG THHN through 1/2-inch EMT. The EMT has 0.304 square inches inside, and each 12 THHN is 0.0133 square inches.

What is the maximum number of 12 AWG THHN allowed?

  • A7
  • 9
  • C12
  • D13

Answer B. Three or more conductors cap at 40% (Chapter 9 Table 1). 0.40 x 0.304 = 0.1216 square inches, divided by 0.0133 per conductor, is 9.14, so 9 fit.

  • NEC 2023 Chapter 9, Table 1
  • NEC 2023 Chapter 9, Table 5

Why the other answers tempt you

  • A: 7 uses the two-conductor 31% limit. With three or more conductors you use 40%, not 31%.
  • C: 12 uses the single-conductor 53% limit. That only applies when there is exactly one conductor.
  • D: 13 uses the 60% nipple exception, which only applies to a sleeve 24 inches or shorter, not a normal run.

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