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Conductor Derating Explained

A chart ampacity is not a field ampacity. Ambient temperature and the number of current-carrying conductors multiply the table value, then the termination rating caps what is left. This guide separates those steps so the upsized conductor makes sense.
Code basis
310.15(B)(1), 310.15(C)(1)
Multipliers
Ambient × bundling
Final limit
110.14(C) cap
Common trap
Capping before derating

The Procedure

5 steps, in the order the code applies them.

  1. Step 1

    Read the insulation column

    Start from the ampacity column that matches the conductor insulation, not the termination. THHN usually starts in the 90 C column even when the lugs are 75 C.

  2. Step 2

    Multiply by ambient correction

    Apply the factor for the air temperature around the raceway. Attics and exterior walls often sit well above the 30 C table basis.

  3. Step 3

    Multiply by conductor-count adjustment

    Count current-carrying conductors in the raceway. Four to six use 80 percent; ten to twenty use 50 percent. The equipment grounding conductor does not count.

  4. Step 4

    Cap at the termination rating

    After both multipliers, the result may not exceed the table value in the lowest terminal-temperature column at either end.

  5. Step 5

    Compare usable ampacity to design current

    If the derated and capped value is below the design current, upsize the conductor or reduce the factors by splitting the pull.

Two multipliers, then a cap

Derating starts with the ampacity column for the conductor insulation. Ambient temperature and the conductor-count adjustment multiply that value.

The terminal temperature rule is different. It caps the final result after the multipliers, so applying it first can undersize the conductor.

The count is not the fill count

Derating counts current-carrying conductors. Conduit fill counts all conductors in the raceway, including the EGC.

A pull can pass fill and still require upsizing from derating, or fail fill while the ampacity calculation is still comfortable.

Decision Guide

The choices that change the answer, and how to make them.

  • Do not apply the terminal cap first

    Capping before ambient and bundling factors produces a smaller answer than the procedure requires and fails a plan review that checks the arithmetic.

  • Derating count is not fill count

    Derating counts current-carrying conductors. Fill counts every conductor, including the EGC. A pull can pass fill and still need an upsize from derating.

  • When to split the raceway instead

    When two factors compound below about 0.7, splitting conductors into a second raceway often restores ampacity cheaper than jumping a conductor size.

FAQs

Why did derating make my wire larger than the ampacity chart?
Charts show tabulated ampacity at 30 C with three or fewer current-carrying conductors. Hot ambient and shared raceways multiply that value down, then the termination rating caps what remains.
Does the equipment grounding conductor count for derating?
No. Derating counts current-carrying conductors. The EGC counts for conduit fill but not for the conductor-count adjustment.
Should I derate from the 75 C or 90 C column?
Derate from the column that matches the insulation. Then cap the corrected result at the termination column. Using 75 C for both steps is a common undersizing error when the insulation is 90 C.

Calculated against the NEC 2023 edition using 310.15(B)(1), 310.15(C)(1), 110.14(C). Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Motor, air-conditioning and welder circuits, which are sized under their own articles.
  • Conductors in free air and photovoltaic circuits.
  • Parallel conductor sets, taps and busbars.
  • Local amendments, which can be stricter than the published code.

Data last verified