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The short answer: a 1,500 watt heater draws about 12.5 amps at 120V, and the National Electrical Code allows only 12 amps of continuous load on a 15 amp circuit — 80 percent of the breaker rating. The heater is over the limit before anything else is plugged in, so a hair dryer, microwave or vacuum on the same circuit trips it immediately. The other causes are a shared circuit you did not know about, an extension cord adding resistance, a GFCI sensing moisture, and a genuinely faulty element.
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How much load is actually on that circuit?
Work in amps. Watts divided by 120 gives the draw, and a circuit is sized for 80 percent continuous: 12 amps on a 15 amp breaker, 16 amps on a 20 amp breaker. A 1,500 watt heater alone exceeds the 15 amp figure, which is why manufacturers print the dedicated-outlet warning on the cord tag. Bedroom and living room receptacles frequently daisy-chain across several rooms in older homes, so the television, a lamp, a phone charger and a heater can all be on one 15 amp run.
| Appliance | Typical watts | Amps at 120V |
|---|---|---|
| Space heater, high | 1,500 | 12.5 |
| Space heater, low | 750 – 900 | 6.3 – 7.5 |
| Hair dryer | 1,500 | 12.5 |
| Microwave | 1,000 – 1,200 | 8.3 – 10 |
| Vacuum cleaner | 800 – 1,400 | 6.7 – 11.7 |
| Window AC, small | 500 – 900 | 4.2 – 7.5 |
| Continuous limit, 15 A circuit | 1,440 | 12.0 |
| Continuous limit, 20 A circuit | 1,920 | 16.0 |
How do I find what else is on the circuit?
Switch the breaker off and walk the house with a lamp or an outlet tester. Anything dead shares the circuit, and in older wiring that regularly includes outlets in an adjacent room, a hallway light, or the garage. Once you have the map, the fix is usually obvious: move the heater to a receptacle on a different breaker, or move the competing appliance. Label the panel while you are there — it takes twenty minutes and saves this exercise every winter.
- Wall outlet only. No extension cords, power strips, surge protectors or non-rated smart plugs.
- One heater per circuit. Two 1,500 watt units on one breaker will always trip it.
- Use the low setting if the room can hold temperature on 750 to 900 watts.
- Check the plug temperature. Warm plug or discolored slots means a worn receptacle — stop and call an electrician.
When is it the heater rather than the circuit?
Three signatures point at the appliance. A trip that happens the instant you switch on, before the element is warm, suggests a short. A trip on a GFCI or AFCI outlet while a standard circuit holds fine indicates current leaking to ground or arcing — moisture inside the housing in a bathroom or garage, or a degraded element. And a trip accompanied by a burning smell, a scorched plug, or visible arcing means unplug it and do not use it again. Electric resistance heaters are cheap and simple; none of those faults is worth repairing.
If the heater is stopping on its own rather than opening the breaker, work through why does my space heater keep shutting off instead. To avoid the problem next time, size the unit to the room using what size space heater for a room, check draw in how many watts does a space heater use, and look at lower-wattage picks in best energy efficient space heater. More in the heating and cooling hub.