Watts, volts and amps
A circuit breaker protects the wiring from overheating by cutting the power when the current exceeds its rating. To know whether a circuit can handle your appliances, convert their total power into current:
- Current (A) = watts ÷ volts
- Breaker capacity (W) = volts × breaker rating
- Continuous-load limit = 80% of the breaker capacity
The 80% rule
The National Electrical Code limits continuous loads, those running for 3 hours or more, to 80% of the breaker rating. Space heaters, EV chargers, water heaters and window AC units often fall into this category. Short loads such as a microwave or a hair dryer can use the full rating briefly, but running near 100% for long periods heats the wiring and trips the breaker.
Worked example
A 1,500 W space heater and 300 W of lamps and electronics share a 120 V circuit on a 20 A breaker.
- Current: 1,800 ÷ 120 = 15 A.
- Share of the breaker rating: 15 ÷ 20 = 75%.
- Breaker capacity: 120 × 20 = 2,400 W; continuous limit: 1,920 W.
- Room left under 80%: 120 W. Plugging in a hair dryer would trip the breaker.
- A 20 A circuit needs at least 12 AWG copper wire.
Circuit capacity
| Breaker | Wire (copper, typical) | Capacity at 120 V | 80% limit at 120 V | Capacity at 240 V |
|---|---|---|---|---|
| 15 A | 14 AWG | 1,800 W | 1,440 W | 3,600 W |
| 20 A | 12 AWG | 2,400 W | 1,920 W | 4,800 W |
| 30 A | 10 AWG | 3,600 W | 2,880 W | 7,200 W |
| 40 A | 8 AWG | 4,800 W | 3,840 W | 9,600 W |
| 50 A | 6 AWG | 6,000 W | 4,800 W | 12,000 W |
Long runs may need a larger wire to limit voltage drop.
Typical appliance loads
| Appliance | Watts |
|---|---|
| Space heater | 1,500 |
| Hair dryer | 1,200 to 1,875 |
| Microwave | 1,000 to 1,500 |
| Toaster oven | 1,200 to 1,500 |
| Window AC | 500 to 1,500 |
| Level 2 EV charger (32 A at 240 V) | 7,680 |
Signs of an overloaded circuit
Breakers that trip repeatedly, lights that dim when an appliance starts, warm outlets or switch plates, buzzing and a burning smell all call for action. Move high-wattage appliances to different circuits, and never replace a breaker with a larger one to stop it from tripping: the wire is sized for the breaker. Have a licensed electrician add circuits or inspect the panel.