Verify the load on a breaker is within the 80 percent continuous-use limit.
NEC 80% check
Load breakdown
Usage Tip
Lights and receptacles on the same circuit share that limit, so total everything that runs at once.
safe limit = breaker rating × 0.80
Green means the load is within the continuous limit; red means it is over.
Am I overloading this circuit?
A circuit is overloaded when the current drawn by everything plugged in exceeds what the breaker is rated to carry. Current in amps is the load in watts divided by the voltage, so a 1,500W heater on a 120V circuit pulls 12.5 amps. Add that to a 1,200W microwave (10 amps) on the same 20A circuit and you are at 22.5 amps on a 20-amp breaker — over the limit, and the breaker will trip. This planner adds up your appliances, converts to amps, and tells you the capacity used, what is left, and whether it is within code.
The NEC 80% rule
The National Electrical Code limits a continuous load — one running three hours or more — to 80 percent of the breaker rating. That headroom keeps the breaker and wiring from running hot for hours on end. The continuous limit for common breakers:
| Breaker | Continuous limit | At 120V |
|---|---|---|
| 15A | 12A | 1,440W |
| 20A | 16A | 1,920W |
| 30A | 24A | 2,880W |
| 40A | 32A | 3,840W |
| 50A | 40A | 4,800W |
Non-continuous loads can use the full breaker rating; continuous loads count at 125 percent when sizing, which is the same 80 percent rule seen from the other side.
Circuit capacity chart
Maximum recommended load (the 80 percent continuous limit) by breaker and voltage:
| Circuit | At 120V | At 240V |
|---|---|---|
| 15A | 1,440W | 2,880W |
| 20A | 1,920W | 3,840W |
| 30A | 2,880W | 5,760W |
| 40A | 3,840W | 7,680W |
| 50A | 4,800W | 9,600W |
Watts vs amps
Breakers are rated in amps, but appliances are usually labeled in watts, which is where people get stuck. They connect through voltage: watts equal volts times amps, so amps equal watts divided by volts. A 1,200W microwave on 120V draws 10 amps; the same wattage on a 240V circuit draws only 5. That is why higher-voltage circuits carry more power for the same amperage, and why dryers and ranges run on 240V.
Dedicated circuit guide
Some appliances draw so much, or run so often, that code or good practice gives them their own circuit with nothing else on it:
- Microwave — dedicated 20A
- Dishwasher — dedicated 15 or 20A
- Garbage disposal — dedicated 15 or 20A
- Refrigerator — dedicated 15 or 20A recommended
- Electric dryer — dedicated 30A, 240V
- Electric range — dedicated 40 to 50A, 240V
- EV charger — dedicated 40 to 60A, 240V
Why does my breaker keep tripping?
Most trips are simple overload: too many things drawing power on one circuit at once, especially heat-producing appliances like heaters, hair dryers and toasters that each pull 1,500W. Move some loads to another circuit. If a breaker trips instantly even with little plugged in, that points to a short circuit or ground fault, not an overload — and that is a job for an electrician. A breaker that trips at well below its rating may simply be worn out.
Frequently asked questions
How many watts can a 20-amp circuit handle?
2,400W at 120V at the full rating, but only 1,920W for a continuous load under the 80 percent rule.
How many amps is 1,500 watts?
12.5 amps at 120V (1,500 divided by 120), or 6.25 amps at 240V.
What is the 80 percent rule?
A continuous load (3+ hours) must not exceed 80 percent of the breaker rating — 16A on a 20A breaker.
Can I put a microwave and toaster on the same circuit?
Together they can pull over 2,300W (about 19A at 120V), which maxes out a 20A circuit. Better on separate circuits.
What is a continuous load?
A load expected to run for three hours or more, such as a space heater or lighting. It counts at 125 percent for circuit sizing.
Why does my breaker trip with nothing running?
That usually means a short circuit or ground fault rather than overload. Stop using the circuit and call an electrician.
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Estimates are for planning and education, not a substitute for the National Electrical Code or a licensed electrician. Local codes vary, motor and inrush loads need extra margin, and permanent wiring should be designed and inspected by a professional. When in doubt, do not add the load.