Electric baseboard heaters are resistive heaters, which means that electrical resistance causes their heating elements to glow red hot when current passes through. Residential current can be supplied at 120 or 240 volts (which are nominal values that may vary slightly), and manufacturers make heaters that operate at one or the other. If you're replacing your baseboard heater because it broke or because you prefer a different style, it's important to know the voltage at which it operates so you can search for a heater that operates at the same voltage.
What Happens If the Voltage Is Wrong?
The main advantage of a 240-volt heater over a 120-volt one is that the one with the higher voltage draws less current, which makes the heater run more efficiently and consume less power overall. A heater rated for 1,500 watts, which is typical, draws 6.25 amps at 240 volts, and at 120 volts, it draws twice that, or 12.5 amps. Moreover, because a 240-volt appliance has two hot wires, each connected to a different leg on the main electrical panel, it draws less from each leg than a 120-volt appliance, which uses one leg only. This greatly reduces the likelihood of overburdening the panel.
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You can replace a 240-volt heater with a 120-volt one by simply connecting one circuit leg and capping the other. The heater will operate normally, but you'll lose the advantage of drawing power from both legs of the panel. If you replace a 120-volt heater with a 240-volt one, you'll only be able to connect one hot wire on the heater, and it will run at half the power for which it was intended.
How to Identify a 240-Volt Circuit
The quickest way to identify the voltage of your heater is to look for a label on the heater that displays it. If it hasn't fallen off over the years, it's most likely on the inside of the access cover to the electric terminals on one end of the heater. If you can't find the label, there are two other places to look, but first, you need to know how to distinguish a 240-volt circuit from a 120-volt one.
A 240-volt circuit has two hot wires, one of which is black and the other red. The wires pass through a line-voltage thermostat on their way to the panel, and in the panel, each one is connected to a separate circuit breaker in the panel. The breakers are coupled and their switches connected to make sure that both will trip in an event that overloads one leg of the circuit and not the other. A 120-volt circuit, on the other hand, has a single black wire that also passes through a thermostat, and in the panel, it's controlled by a single breaker.
Identifying the Voltage of Your Heater
If you can't find a label on the heater, you can identify the voltage at the panel by noting whether the circuit breaker controlling the heater circuit is a single one or a double one. If the panel isn't properly labeled and you don't know which breaker controls the heater, there's another way to identify the voltage, but it's potentially hazardous, so it's best to get an electrician to do it.
Remove the cover from the thermostat so you can see the wires. This is similar to uncovering the terminals of an electrical outlet, so don't touch anything inside, or you'll get a dangerous shock. Just look and if you see one black wire coming in and one going out, it's a 120-volt heater. If you see two wires coming in — one black and one red — and two wires of the same colors going out, it's a 240-volt heater.