How this calculator works
A circuit's maximum power is its breaker rating times the voltage. For a load that runs three hours or more the usable share is 80%. Dividing by the size of each device gives how many a circuit will run.
Worked example: a 15 A, 120 V circuit has a maximum of 1,800 W and a continuous limit of 1,440 W, so it can run 0 space heaters of 1,500 W continuously, and 1 for a short time. A 20 A circuit has 2,400 W (1,920 W continuous, 1 heater). A 50 A, 240 V circuit has 12,000 W (9,600 W continuous). A 15 A circuit runs up to 144 LED bulbs of 10 W.
Watts by breaker size
Maximum and 80% continuous load.
| Breaker | 120 V maximum | 120 V continuous | 240 V maximum | 240 V continuous |
|---|---|---|---|---|
| 15 A | 1,800 W | 1,440 W | 3,600 W | 2,880 W |
| 20 A | 2,400 W | 1,920 W | 4,800 W | 3,840 W |
| 30 A | 3,600 W | 2,880 W | 7,200 W | 5,760 W |
| 40 A | 4,800 W | 3,840 W | 9,600 W | 7,680 W |
| 50 A | 6,000 W | 4,800 W | 12,000 W | 9,600 W |
| 60 A | 7,200 W | 5,760 W | 14,400 W | 11,520 W |
Tips before you buy
- A space heater, hair dryer, toaster or microwave each draws about 1,500 watts, so a 15 amp circuit can run only one at a time.
- An outlet is not a circuit: many outlets and lights usually share one breaker. Add up the whole circuit.
- Kitchens need two 20 amp small-appliance circuits in most codes, and a bathroom a 20 amp circuit.
- If a breaker trips, it is telling you the load is too high or there is a fault. Do not fit a larger breaker.
- Tripping once in a while with a heater running is a sign the circuit is overloaded, and it should be fixed.
Tools & materials for this project
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Frequently asked questions
How many watts can a 15 amp circuit handle?
1,800 watts at 120 volts as a maximum, and 1,440 watts for a load that runs continuously (the 80% rule).
How many watts can a 20 amp circuit handle?
2,400 watts at 120 volts, and 1,920 watts continuous. At 240 volts it is 4,800 watts.
How many watts can a 30 amp circuit handle?
3,600 watts at 120 volts or 7,200 watts at 240 volts, and 80% of that continuously.
How many watts can a 50 amp circuit handle?
12,000 watts at 240 volts, and 9,600 watts continuously.
Can I run a space heater on a 15 amp circuit?
One 1,500 W heater alone is about 83% of the maximum and is over the 1,440 W continuous limit, so it should be on a circuit of its own, with nothing else plugged in.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.