Ampacity calculator.
Calculate adjusted conductor ampacity for common exam setups. Pick the conductor, table column, terminal rating, current-carrying conductor count, and ambient temperature.
This calculator covers the normal Table 310.16 adjustment path. It does not size the overcurrent device or apply every equipment exception.
Direct answer: start with the Table 310.16 ampacity for the conductor and insulation column, multiply by the ambient correction factor, multiply by the current-carrying conductor adjustment factor, then cap the result at the terminal temperature rating.
2 AWG copper THHN, six current-carrying conductors, 35 C ambient
Inputs
- 2 AWG copper
- 90 C insulation column
- 75 C terminals
- Six current-carrying conductors
- 35 C ambient temperature
Result
Base ampacity is 130 A. Ambient correction is 0.96. Six current-carrying conductors use the 0.80 adjustment factor. Final adjusted ampacity is 99.84 A, below the 75 C terminal cap of 115 A.
Here is the trap: the 90 C column can help with derating math, but it does not let you ignore the terminal rating. Always compare the adjusted value to the terminal cap.
Ampacity mistakes that show up on exam questions
- Counting the EGC. Equipment grounding conductors are not current carrying for this adjustment count.
- Skipping the terminal cap. Correct and adjust from the insulation column, then check the terminal temperature limit.
- Treating ampacity as breaker size. Ampacity is the conductor number. Breaker sizing has its own rules.
- Missing the neutral rule. Whether a neutral counts depends on the circuit. Read the words in the question before picking the conductor count.
After the ampacity math, drill the table
Ampacity calculator questions
What does this ampacity calculator do?
It starts with a Table 310.16 conductor ampacity, applies ambient temperature correction, applies the current-carrying conductor adjustment factor, then compares the result to the terminal temperature limit.
Does the equipment grounding conductor count as current carrying?
No. For the common adjustment-factor setup, the equipment grounding conductor is not counted as a current-carrying conductor. Neutrals depend on the circuit, so read the question carefully.
Why can I start in the 90 C column if my terminals are 75 C?
Many exam questions let you use the insulation column for correction and adjustment, then cap the final assignable ampacity at the terminal rating. The terminal cap still has to be checked.
Is the ampacity result the breaker size?
No. The result is assignable conductor ampacity. Breaker sizing depends on load type, standard OCPD sizes, small-conductor rules, motor rules, equipment instructions, and the exact code article in the question.
Which NEC sections are involved?
The common exam path is Table 310.16 for base ampacity, 310.15 for ambient correction and conductor adjustment, and 110.14(C) for terminal temperature limits.
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