Amps, volts, watts — the math that tells you what a circuit can handle
One equation explains most of what people find confusing about breakers, extension cords, and space heaters: watts = volts × amps.
The three numbers
- Volts (V) — the electrical pressure. Standard US household outlets run at 120V. Big appliances (electric dryers, ranges, EV chargers) use 240V circuits.
- Amps (A) — the rate of current flow. This is what your breaker is rated for — 15A, 20A, 30A.
- Watts (W) — the actual power being used. This is what's printed on your appliances and light bulbs.
Multiply volts by amps to get watts. Divide watts by volts to get amps. That's the whole formula, in both directions.
Why this matters at your breaker box
A standard 15-amp circuit at 120V can theoretically handle 15 × 120 = 1,800 watts. But electrical code says continuous loads (running more than 3 hours) shouldn't exceed 80% of a circuit's rating — so the practical safe continuous limit is closer to 1,440 watts.
A 20-amp circuit at 120V maxes out at 2,400W theoretical, 1,920W at the 80% continuous rule.
Where this bites people
A single 1,500-watt space heater is already close to the safe limit of a standard 15A circuit — before you add anything else on that circuit. Plug a space heater into an outlet that's sharing a circuit with a hair dryer (1,500W) or a window AC unit, and you'll trip the breaker, or worse, overheat wiring that isn't protected fast enough.
| Device | Typical wattage | On a 15A/120V circuit (1,800W max) |
|---|---|---|
| LED light bulb | 8–15W | Negligible |
| Phone charger | 5–20W | Negligible |
| Microwave | 1,000–1,200W | Over half the circuit |
| Hair dryer | 1,200–1,800W | Can max out the circuit alone |
| Space heater | 1,500W | Leaves almost nothing for anything else |
| Window AC unit | 500–1,500W | Depends on size |
The breaker isn't being dramatic
A breaker tripping isn't a malfunction — it's the safety device doing exactly its job, cutting power before the wire heats up enough to be a fire risk. If a breaker trips repeatedly on the same circuit, that's a signal to redistribute what's plugged in, not to find a bigger breaker. A bigger breaker than the wire gauge is rated for is how house fires start.
Quick reference
- Watts = volts × amps
- 15A/120V circuit ≈ 1,440W safe continuous load
- 20A/120V circuit ≈ 1,920W safe continuous load
- Space heaters, hair dryers, and microwaves each eat most of a circuit alone
- A tripping breaker is a warning, not a defect — don't upsize it to make the tripping stop