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Ampacity derating calculator

Start with the conductor's ampacity from Table 310.16. Add the ambient temperature and the number of current-carrying conductors, and you get the derated ampacity.

Built on the 2026 NEC · Free, no sign-up · Updated October 2026

From Table 310.16 for the conductor's temperature column, for example 30 A for 12 AWG copper at 75°C.

Worst-case site ambient at the conductor.

Count those in the same raceway, cable, or earth. A neutral on a 3-phase circuit counts only when it carries harmonics.

The result updates as you type.

Result

35.2 A

How it was calculated

  1. derated = base × ambient × adjustment
  2. derated = 50 × 0.88 × 0.8 = 35.20 A

Assumptions

  • Base ampacity 50 A from the 75°C column of Table 310.16.
  • Ambient 40°C → correction factor 0.88 per Table 310.15(B)(1)(1) (30°C base).
  • 4 current-carrying conductors → adjustment factor 0.8 per Table 310.15(C)(1).

Sections used

310.15Table 310.15(B)(1)(1)Table 310.15(C)(1)

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

How derating works

Table 310.16 assumes an ambient of 30°C (86°F) and no more than three current-carrying conductors together. When either is untrue, you multiply the table ampacity by two factors:

  • Ambient temperature correction, from Table 310.15(B)(1)(1). Hotter air means less ampacity. The factor depends on the conductor's insulation temperature rating.
  • Adjustment when a raceway or cable holds more than three current-carrying conductors, from Table 310.15(C)(1): 80% for 4 to 6, 70% for 7 to 9, 50% for 10 to 20, and lower beyond that.

Derated ampacity = table ampacity × temperature factor × adjustment factor.

Which conductors count as current-carrying

  • Every ungrounded (hot) conductor counts.
  • A neutral counts when it carries real current, for example on a 2-wire circuit, or on a 3-wire circuit fed from a 3-phase wye. It also counts when most of the load on a 4-wire wye is nonlinear. A neutral that carries only the unbalanced current of its own circuit doesn't count. See 310.15(E).
  • Equipment grounding and bonding conductors never count, per 310.15(F).

Use the 90°C column, then check the terminals

With 90°C wire like THHN, you can start derating from the 90°C column of Table 310.16. The result can't be more than the ampacity allowed at the temperature rating of the terminals, which is usually 60°C or 75°C under 110.14(C). Take the smaller of the derated value and the terminal-column value.

Small conductors also have overcurrent limits in 240.4(D), such as 20 A for 12 AWG copper, regardless of the derated ampacity.

2023 to 2026: our list has 4 changes in Article 310. See the changes

Worked example

A conductor with a 50 A base ampacity, 75°C insulation, 40°C ambient, and four current-carrying conductors in the raceway:

  • Base ampacity (amps): 50
  • Insulation rating: 75°C (THWN, RHW, XHHW)
  • Ambient temperature (°C): 40
  • Current-carrying conductors: 4

Result

35.2 A

How it was calculated

  1. derated = base × ambient × adjustment
  2. derated = 50 × 0.88 × 0.8 = 35.20 A

Assumptions

  • Base ampacity 50 A from the 75°C column of Table 310.16.
  • Ambient 40°C → correction factor 0.88 per Table 310.15(B)(1)(1) (30°C base).
  • 4 current-carrying conductors → adjustment factor 0.8 per Table 310.15(C)(1).

Sections used

310.15Table 310.15(B)(1)(1)Table 310.15(C)(1)

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Questions electricians ask

Does the neutral count as a current-carrying conductor?

Sometimes. It counts on 2-wire circuits, on 3-wire circuits from a 3-phase wye, and on 4-wire wye circuits that mostly serve nonlinear loads. Otherwise it doesn't. See 310.15(E).

Does the ground wire count for derating?

No. Grounding and bonding conductors are never counted under 310.15(F).

Can I derate from the 90°C column?

Yes, if the conductor is rated 90°C. The final ampacity still can't exceed the terminal temperature limit under 110.14(C), usually the 75°C or 60°C column.

What about conduit on a rooftop?

Raceways and cables exposed to sunlight on or above rooftops have extra rules in 310.15(B)(2). Check them before you pick the ambient temperature.

Practice it hands-on

Pick the breaker and wire for each load while a gauge shows how heat and crowding cut ampacity.

Try Wire Size

Teach it in one class

Free slides, quiz, and instructor note: Wire sizing (NEC 2026 310.16, 240.4, 110.14(C), 210.19(A)).

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How this calculator was built

The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.

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