Conductor Ampacity Calculator
Look up baseline conductor ampacity and apply ambient-temperature correction, conductor-count adjustment, and terminal-temperature limits.
Conductor and conditions
Reference table coverage: 14 AWG through 2000 kcmil where listed for the selected material.
Use equipment markings and applicable terminal rules.
Enter the ambient temperature around the conductors.
Count them using the applicable NEC rules.
Final estimate is the lower of adjusted ampacity and the selected terminal ampacity.
For copper 12 AWG, NEC small-conductor overcurrent rules include a 20 A limit, subject to listed exceptions. This is not a breaker recommendation.
About this calculator
Choose a conductor size, material, insulation temperature rating, terminal rating, ambient temperature, and current-carrying conductor count. The calculator shows the baseline ampacity, each correction step, and the lower result after applying the selected terminal limit.
The 2026 NEC is the calculation baseline. Local adoption, amendments, equipment rules, and installation conditions vary. Treat the result as one input to conductor selection, not a final installation decision.
Method and assumptions
Baseline values come from NEC Table 310.16 for the conductor sizes shown in the calculator. Ambient-temperature correction uses the entered ambient temperature relative to the table’s 30°C reference. The conductor-count adjustment uses the number entered. The result is the lower of the corrected/adjusted value and the ampacity listed for the selected terminal temperature column.
The tool does not evaluate load calculations, continuous loads, correction rules for every wiring method or location, parallel-conductor requirements, special-use tables, or exceptions. Verify the full installation against the locally adopted NEC and equipment instructions.
FAQs
Which insulation temperature rating should I use?
Select the rating printed on the conductor insulation. A higher insulation rating does not automatically permit using that ampacity at equipment terminals.
What counts as a current-carrying conductor?
The NEC defines which conductors count for adjustment calculations and includes exceptions. Count them under the rule that applies to the circuit; this calculator does not infer the count from circuit details.
Does the result tell me what breaker to install?
No. This calculator estimates ampacity after selected corrections and terminal limits. It does not select overcurrent protection or determine final conductor suitability.
Why can the terminal limit be lower than the insulation rating?
Equipment terminals can limit the usable ampacity even when the conductor insulation has a higher temperature rating. Use equipment markings and applicable terminal rules.
Sources
- 2026 NFPA 70 (NEC), Table 310.16 and the applicable conductor ampacity correction and adjustment provisions in 310.15.
- Equipment terminal ratings and conductor markings: listed product markings and manufacturer instructions.