Wire Ampacity Calculator
Wire ampacity is the maximum current a conductor can safely carry continuously without exceeding its insulation temperature rating. Choosing the correct wire size is one of the most critical tasks in electrical work—undersized conductors overheat, damaging insulation, starting fires, and violating the National Electrical Code. This calculator looks up ampacity values from NEC Table 310.16, applies ambient temperature correction factors, and recommends the appropriate circuit breaker size.
The ampacity of a conductor depends on three factors: wire gauge (AWG size), insulation temperature rating, and ambient temperature. Larger wire gauges have lower resistance and can carry more current. Higher-rated insulation (such as THHN at 90°C) allows more current than lower-rated insulation. But when the ambient temperature exceeds the standard 86°F (30°C) baseline, the ampacity must be derated because the wire starts closer to its maximum operating temperature.
Electricians use this information every day when sizing branch circuits, feeders, and service entrance conductors. Per NEC 240.4, conductors must be protected by overcurrent devices that do not exceed their ampacity (with certain exceptions for standard breaker sizes listed in NEC 240.6). This calculator helps you quickly determine the right wire and breaker combination for your installation.
NEC Table 310.16 — Ampacity at 75°C
| AWG Gauge | 75°C Ampacity | Standard Breaker |
|---|---|---|
| 14 AWG | 20 A | 15 A |
| 12 AWG | 25 A | 20 A |
| 10 AWG | 35 A | 30 A |
| 8 AWG | 50 A | 40 A |
| 6 AWG | 65 A | 60 A |
| 4 AWG | 85 A | 80 A |
| 3 AWG | 100 A | 100 A |
| 2 AWG | 115 A | 110 A |
| 1 AWG | 130 A | 125 A |
| 1/0 AWG | 150 A | 150 A |
| 2/0 AWG | 175 A | 175 A |
| 3/0 AWG | 200 A | 200 A |
| 4/0 AWG | 230 A | 225 A |
Ambient Temperature Correction Factors (75°C Rated)
| Ambient Temp (°F) | Ambient Temp (°C) | Correction Factor |
|---|---|---|
| 70–77 °F | 21–25 °C | 1.05 |
| 78–86 °F | 26–30 °C | 1.00 |
| 87–95 °F | 31–35 °C | 0.94 |
| 96–104 °F | 36–40 °C | 0.88 |
| 105–113 °F | 41–45 °C | 0.82 |
| 114–122 °F | 46–50 °C | 0.75 |
| 123–131 °F | 51–55 °C | 0.67 |
| 132–140 °F | 56–60 °C | 0.58 |
How to Use This Calculator
- Select the wire gauge — choose the AWG size of the copper conductor (14 AWG through 4/0 AWG).
- Select the insulation type — THHN, THWN, XHHW, or USE-2. The insulation type determines the temperature column used.
- Enter the ambient temperature — defaults to 86°F (30°C). Increase for hot environments like attics, rooftops, or near heat sources.
- Read the results — the derated ampacity and recommended breaker size are displayed.
Frequently Asked Questions
What is wire ampacity?
Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating. It is determined by the wire gauge (size), insulation type, and ambient temperature. NEC Table 310.16 lists the standard ampacity values for copper and aluminum conductors at a 30°C (86°F) ambient temperature.
How does ambient temperature affect wire ampacity?
Higher ambient temperatures reduce the ampacity of a conductor because the wire's insulation is already closer to its maximum operating temperature. NEC Table 310.15(B)(1) provides correction factors. For example, at 40°C (104°F) ambient, a 75°C rated conductor's ampacity is multiplied by 0.88, reducing it by 12%. In hot attics or near heat sources, this derating can be significant.
What size wire do I need for a 60-amp circuit?
For a 60-amp circuit, you typically need 6 AWG copper wire (THHN/THWN rated at 65A at 75°C). However, the NEC allows using the 75°C column for termination temperature matching with most modern breakers and equipment. Always verify the temperature rating of your terminations and consult NEC Table 310.16 for the specific insulation type.