Wire & Cable calculator

Wire Ampacity Calculator

This ampacity calculator starts with the field checks many users need: 12 AWG copper THHN can begin at a 30A 90°C insulation value for derating, but a 60°C equipment termination caps the final result at 20A. A common derating case, 10 AWG copper THHN at 45°C with nine current-carrying conductors, screens at about 24.4A before the 75°C terminal cap. Use this page for individual insulated conductors checked by table ampacity, ambient-temperature correction, conductor-count adjustment, and 60°C or 75°C termination limits. Use the conduit-fill page separately when the question is how many conductors physically fit in the raceway.

Updated June 14, 2026

12 AWG copper is commonly checked at 20A on a 60°C terminal basis or 25A on a 75°C basis, even though 90°C insulation can start higher for derating.

10 AWG Cu THHN at 45°C with 9 conductors -> 40A × 0.87 × 0.70 = 24.4A before the 75°C terminal cap

Choose conductor size, insulation, temperature, conductor count, and termination rating below to calculate final ampacity

Calculator Inputs

Quick Presets

Calculation Results

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Calculation history

Example Calculations

12 AWG copper branch-circuit check

12 AWG copper THHN at 30°C with three current-carrying conductors and a 60°C termination basis.

Inputs

Wire Gauge: 12
Wire Type: Copper
Insulation Type: THHN/THWN-2
Ambient Temperature: 30
Current-Carrying Conductors: 3
Termination Rating: 60

10 AWG copper with nine current-carrying conductors

10 AWG copper THHN at 45°C with nine current-carrying conductors and a 75°C termination basis.

Inputs

Wire Gauge: 10
Wire Type: Copper
Insulation Type: THHN/THWN-2
Ambient Temperature: 45
Current-Carrying Conductors: 9
Termination Rating: 75

How to Use

How to use the ampacity calculator

Select the wire gauge and conductor material that match the installation.
Choose the insulation family. THW/THWN uses the 75°C column; THHN/THWN-2 and XHHW/XHHW-2 use the 90°C column as the starting point.
Enter the ambient temperature and the number of current-carrying conductors.
Select the governing equipment termination rating. The calculator uses that column as the final cap after derating.
Review the adjusted ampacity, the termination limit, and the final ampacity.

What each result means

Base Ampacity is the table ampacity before any adjustments.
Combined Derating Factor multiplies the temperature factor by the current-carrying-conductor adjustment factor.
Adjusted Ampacity applies the ambient-temperature and conductor-count factors.
Termination Limit is the ampacity permitted by the selected 60°C or 75°C terminal column.
Final Ampacity is the lower of the adjusted ampacity and the selected termination limit.
Limiting Factor shows whether derating or the selected termination column controls the result.
Continuous Load Planning Check shows the current level that corresponds to a 125% conductor-sizing check.

Common reference values

Conductor	60°C	75°C	90°C
12 AWG copper	20A	25A	30A
10 AWG copper	30A	35A	40A
12 AWG aluminum	15A	20A	25A
2/0 aluminum	115A	135A	150A

This calculator is intentionally limited to the table-ampacity workflow for individual insulated conductors. It does not model free-air ampacity, cable-tray rules, rooftop temperature adders, direct burial, or voltage drop. Use the related tools below when your project needs those checks.

For a full conductor-selection workflow, combine this page with the Wire Size Calculator, the Voltage Drop Calculator, and the Conduit Fill Calculator. Move to the Breaker Sizing Calculator only after the load current and final conductor ampacity basis are known, because breaker review also depends on load type and small-conductor limits. For a longer explanation of conductor-count adjustment and temperature correction, see the Ampacity and Derating Guide.

Common Applications

Checking branch-circuit conductors against a 60°C terminal basis

Reviewing feeder conductors with a 75°C terminal basis

Evaluating conductor ampacity in hot mechanical rooms or attics

Comparing copper and aluminum conductor options for the same load

Reviewing multi-circuit raceways before final wire-size selection

Frequently Asked Questions

Why is the final ampacity sometimes lower than the adjusted ampacity?

The adjusted ampacity shows the result after ambient-temperature and conductor-count factors are applied. The final ampacity is capped by the selected equipment termination rating, so a 90°C conductor may still end up limited by a 60°C or 75°C terminal column.

When should I choose 60°C instead of 75°C terminations?

Choose the termination basis that matches the equipment on both ends of the circuit. Smaller branch-circuit equipment commonly relies on a 60°C basis, while many larger lugs and commercial terminations are evaluated on a 75°C basis. The governing basis is the lowest applicable terminal rating.

Does the neutral count as a current-carrying conductor?

Only count conductors that actually carry load current for the installation you are evaluating. Equipment grounding conductors are not counted. Some neutrals are not counted, while others must be counted when they carry load current or significant harmonic current.

Does this calculator cover free-air or cable-tray ampacity?

No. This calculator is limited to the common table-ampacity workflow for individual insulated conductors that are being checked with ambient-temperature correction and conductor-count adjustment. Use a dedicated free-air, cable-tray, or project-specific engineering workflow when those conditions govern.

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