The best breaker size depends on the heater’s actual amperage, the wire size, and whether the heater is considered a continuous load. The first thing I would do is look at the nameplate on the heater, because that tells you the real current draw. If the heater is 240 volts and listed in watts, divide the wattage by the voltage to get amps. For example, a 5,000-watt heater at 240 volts draws about 20.8 amps. That kind of load usually pushes you toward a 30-amp circuit, not a 20-amp one, especially if the heater will run for long periods.
For fixed electric heaters, the load is often treated as continuous if it runs for three hours or more. In practice, that means the circuit should not be sized right at the heater’s running amperage. A common rule is to size the circuit at 125% of the heater load for continuous operation. So if the heater draws 20 amps continuously, you would generally want a circuit rated for at least 25 amps, which often means stepping up to a 30-amp breaker if the wiring supports it. The breaker is there to protect the wire, so you cannot choose the breaker first and hope the wire will be fine.
Wire size matters just as much as the breaker. A 20-amp breaker usually goes with 12-gauge copper, while a 30-amp breaker typically requires 10-gauge copper. If the heater is a hardwired unit, the wiring method, insulation rating, and run length also affect what is appropriate. A long run to the workshop can create voltage drop, which may make the heater perform poorly or trip more easily. In that case, upsizing the wire can help, but only within code and the equipment ratings.
If the heater is a plug-in model, don’t assume any outlet and breaker combo will work just because the plug fits. A 1,500-watt portable heater only draws about 12.5 amps at 120 volts, so a 15-amp or 20-amp circuit may be fine. But larger workshop heaters are often 240-volt units and should have their own dedicated circuit. Sharing that circuit with lights, tools, or receptacles usually causes nuisance trips and can create a safety issue.
My practical advice is to check the heater label, confirm whether it is continuous load, match the breaker to the wire size, and give the heater its own circuit if it is anything larger than a small portable unit. If you are unsure, a licensed electrician can verify the load calculation and installation details quickly. For those who have installed one, the most useful tips are usually about heater size, breaker sizing, and whether the circuit was dedicated or shared.
