Ampacity and Derating: Temperature and Bundling

Ampacity questions combine three things: the base value from the table, a correction for hot surroundings, and an adjustment for too many current-carrying conductors together. Then the result has to respect the terminal temperature rating and the small-conductor limits. This guide puts those pieces in order and works through three examples.

The tables and sections

• Table 310.16: allowable ampacity of insulated conductors (not more than three current-carrying conductors in a raceway, cable or earth), based on an ambient temperature of 30 °C (86 °F). Columns for 60 °C, 75 °C and 90 °C insulation, copper on the left, aluminum on the right.
• Ambient temperature correction factors: Table 310.15(B)(1) in NEC 2020. Later editions split this table into parts, so check your state's adopted edition for the exact number.
• Adjustment factors for more than three current-carrying conductors: Table 310.15(C)(1).
• 110.14(C): the temperature rating of terminations limits the ampacity you can actually use.
• 240.4(D): maximum overcurrent protection for small conductors (14, 12 and 10 AWG copper, for example).

Values used in the examples

These are the values commonly listed in the tables named above. Confirm them in your own code book.

The order of operations

1. Find the base ampacity in Table 310.16 using the conductor's insulation temperature column. For THHN or THWN-2 that is the 90 °C column.
2. Multiply by the ambient temperature correction factor for that same column.
3. Multiply by the adjustment factor for the number of current-carrying conductors.
4. Compare the result with the ampacity in the column that matches the terminal rating (usually 75 °C, sometimes 60 °C). The conductor's usable ampacity is the lower of the two.
5. Check the overcurrent protection limits, including 240.4(D) for small conductors.

Why start in the 90 °C column if the terminals are 75 °C? Because 110.14(C) and the derating rules allow the higher insulation rating to be used for correction and adjustment, as long as the final result does not exceed the terminal column value. That extra headroom is often the difference between a conductor that works and one that does not.

Example 1: nine conductors in a hot space

Nine current-carrying 10 AWG THHN copper conductors run in one raceway through a space with an ambient temperature of 40 °C (104 °F). The terminals are rated 75 °C.

240.4(D) limits 10 AWG copper to 30 A protection, and the derated ampacity of 25.48 A is the number you protect. A 25 A overcurrent device clearly works. Whether you may round up to the next standard size depends on the conditions in 240.4(B), so check them if the question goes there.

Example 2: four conductors, moderate heat

Four current-carrying 3 AWG THHN copper conductors in a raceway, ambient 35 °C (95 °F), 75 °C terminations.

Example 3: when the terminal column wins

Three current-carrying 6 AWG THHN copper conductors in a raceway, ambient 30 °C, 75 °C terminations.

With no derating, the 90 °C rating gives you nothing extra. The terminals cap the conductor at its 75 °C value.

What counts as a current-carrying conductor

• Every ungrounded (hot) conductor counts.
• Equipment grounding and bonding conductors do not count.
• A neutral that carries only the unbalanced current of a circuit is usually not counted. For example, the neutral of a 120/240 V three-wire circuit, or of a three-phase four-wire circuit serving balanced linear loads.
• A neutral does count in some cases, such as a three-phase four-wire wye circuit where a major part of the load is nonlinear (electronic equipment), because harmonic currents add up on the neutral.

The neutral rules are in 310.15(E) and the grounding conductor rule is in 310.15(F) in NEC 2020. The neutral counting rule was revised in NEC 2026, so if your state has adopted that edition, read the new text carefully.

Exceptions worth knowing

• The adjustment factors do not apply to conductors in raceways not over 24 in. long (nipples). The exceptions under 310.15(C)(1) list this and other cases.
• Raceways and cables exposed to sunlight on or above rooftops have their own ambient temperature rule. Check your state's adopted edition for its current wording, which has changed over recent cycles.
• Different correction factors apply if the table you use is based on 40 °C instead of 30 °C, such as some tables for conductors in free air.

Exam habits that save points

1. Write the column you are using next to every number: 90 °C for derating, 75 °C for the terminal check.
2. Convert the ambient temperature to the scale shown in your table, or use the °F column if it is there.
3. Count current-carrying conductors before you open the adjustment table.
4. Keep the decimals (25.48, not 25) until the final comparison.
5. Remember that derated ampacity and voltage drop are separate checks. A long run may need a larger conductor for voltage drop even after the ampacity is fine. Check it with the voltage drop calculator.