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Breaker Selection Guide
Determine recommended circuit breaker amp ratings and minimum copper wire gauges for electrical circuits using the NEC 80% continuous load rule.
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Recommended Breaker Size
Minimum Wire Gauge
Actual Running Current
How it works
- Select the circuit supply voltage (120V, 240V, 208V, or 230V).
- Specify whether the load is continuous (running > 3 hours, such as EV charging or space heaters) or non-continuous.
- Enter the total power in Watts or measured current in Amps.
- The calculator applies the 125% continuous safety factor, selects the standard breaker rating, and maps the required copper wire gauge.
The formula
Running Current (Amps) = Total Power (Watts) / Circuit Voltage (Volts)
Continuous Minimum Breaker Rating = Running Current × 1.25 (or Running Current / 0.8)
FAQ
What is the NEC 80% continuous load rule?
According to the National Electrical Code (NEC Article 210.20), an electrical branch circuit breaker must not be loaded continuously (for 3 hours or more) to more than 80% of its rated capacity. Alternatively, the breaker must be sized to at least 125% of the continuous load.
What wire gauge is required for 15A, 20A, 30A, and 50A breakers?
Standard copper wire sizes are: 14 AWG for 15 Amp breakers, 12 AWG for 20 Amp breakers, 10 AWG for 30 Amp breakers, 8 AWG for 40 Amp breakers, and 6 AWG for 50 Amp breakers.
Can I install a larger breaker if my current one frequently trips?
NO! Circuit breakers are designed to protect the wiring inside walls from overheating and catching fire. You cannot increase the breaker size without upgrading the entire wire gauge for that circuit run.
What is the difference between single-pole and double-pole breakers?
Single-pole breakers connect to one hot bus bar supplying 120V circuits (standard lighting and outlets). Double-pole breakers connect across two hot bus bars to supply 240V circuits (EV chargers, central air conditioners, water heaters, and electric ranges).
Does the calculator apply the 125% rule to every load, or only continuous ones?
Only continuous loads get the 125% (80% rule) sizing factor — a non-continuous load is sized directly from its running current, since it isn't expected to run for 3 hours or more at a stretch.
How we compare
| Feature | Online Tool Store | Hardware store charts | Printed electrical codebooks |
|---|---|---|---|
| Automated NEC 80% / 125% continuous rule | ✓ | ✗ (Manual) | ✓ |
| Simultaneous AWG and mm² metric wire gauges | ✓ | Partial | ✗ |
| Supports both Watts and direct Amp inputs | ✓ | ✗ | ✗ |