Ampacity Calculator
Calculate the safe current-carrying capacity of a wire.
Last updated 2026-08-21
What Is a Ampacity Calculator?
An ampacity calculator finds the safe current a conductor can carry continuously based on its gauge, insulation rating, and how many conductors share a raceway. It applies the code derating factors when bundled wires trap heat. Use it to confirm a wire size when several circuits run together in one conduit.
How It Works
Adjusted ampacity = Base ampacity × conductor adjustment (4–6: 80%, 7–9: 70%, 10–20: 50%).
Example
12 AWG at 75°C with 5 conductors: 25 A × 0.8 = 20 A.
How to Calculate
- Pick the wire size, for example 12 AWG copper, and the insulation temperature rating, often 75°C for most terminations.
- Enter the number of current carrying conductors in the raceway, say 5.
- The tool starts with the base ampacity of 12 AWG, about 25 A at 75°C, then applies the adjustment factor.
- With 5 conductors the derating factor is 80%, so the adjusted ampacity is 25 A × 0.8, which equals 20 A. Use this reduced value to protect the circuit.
Conductor adjustment factors
| Number of conductors | Adjustment factor | Result |
|---|---|---|
| 1 to 3 | 100% | No derating |
| 4 to 6 | 80% | 20% reduction |
| 7 to 9 | 70% | 30% reduction |
| 10 to 20 | 50% | 50% reduction |
| 21 to 30 | 45% | 55% reduction |
Sources
Frequently Asked Questions
What is ampacity?
The maximum current a conductor can carry continuously without exceeding its temperature rating.
Why does conductor count matter?
Bundled conductors trap heat, so NEC requires derating when more than 3 are in a raceway.
Which insulation rating is right?
Most modern wiring uses 90°C-rated THHN, but terminations often limit you to 60/75°C ampacity.
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