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Ampacity derating practice

Read the table, apply the ambient and conductor-count factors, hold the result to the termination column, then the small-conductor device limit.

Ampacity with correction and adjustment

Problem

Problem code AMP-01BJ9F

NEC 2023Worked to the 2023 edition as used by this site's calculators; your exam may use another edition. The 2026 edition renumbered some articles, so check any number against the edition your exam uses.

6 conductors of 10 AWG copper, THHN / THWN-2 insulation (90°C), run together in one raceway through a dry space where the ambient temperature is 40°C (104°F). All 6 are current-carrying. The terminals at both ends are rated 75°C. Find the ampacity of each conductor, step by step, then the largest overcurrent device the small-conductor rule allows.

Conductor
10 AWG copper, THHN / THWN-2 (90°C)
Ambient
40°C (104°F)
Current-carrying conductors
6
Terminals
75°C
  1. A

    Must match

  2. Within ±0.005

  3. Within ±0.005

  4. A

    Within ±0.5 A

  5. A

    Within ±0.5 A

  6. A

    Must match

Every problem has a code. The same code always gives the same problem and key.

Check the numbers in the ampacity lookup, which gives the same answers from the same functions.

What these problems are based on

Every problem is generated by this site's own calculators from the published tables they already use. No question is taken from an exam, a code book or a prep book, and no code text is quoted.

Worked to the 2023 edition as used by this site's calculators; your exam may use another edition. The 2026 edition renumbered some articles, so check any number against the edition your exam uses.

Assumptions

  • Correction and adjustment multiply the table value of the insulation's own column.
  • The ampacity is the lesser of the corrected and adjusted value and the table value at the termination rating.
  • For 14 to 10 AWG the small-conductor rule limits the overcurrent device, not the ampacity, so it is asked as its own step.
  • Exceptions (short runs, cable types, rooftop adders) are not modelled.

Rounding and grading

  • Factors are decimals to two places (0.82). A factor typed as a percent (82%) is read as 0.82.
  • Amperes are rounded to one decimal place, volts and percents to two, areas to four (square inches).
  • Table lookups and choices must match exactly. Calculated lines are accepted within the tolerance shown beside them, the same amount above or below the key.

Exam practice only. AmpSizer is not affiliated with, endorsed by or connected to any code publisher, testing company or licensing board. A real installation is sized by a licensed electrician to the locally adopted code.