Calculating the wattage per square foot in a room for baseboard heating will give you a heads up as to how much electrical power is required for heating that space. As a general rule, the watts per square foot will vary by how efficient the home or room is for retaining heat. A well-insulated room will require 10 watts per square foot of room. A less insulated space will need 12 watts and an older home with no insulation may need up to 15 watts of electrical power per square foot.

Use the tape measure to find the inside dimensions of a typical room. In this instance the width of the room is 12x15 feet. Multiply the two measurements together, and the total square footage of the room is equal to 180 square feet.

Multiply the total square footage by the step 1 wattage requirements for the space. In the case of a well-insulated home, the heating wattage is 1800 watts. For a home less insulated, the wattage needed is 2160 watts. The home with no insulation will need baseboard heating that can provide up to 2700 watts of power.

Find the required amperage for the electrical circuit if the voltage feeding the power is 240 VAC. A basic electrical formula for power is wattage is equal to voltage times amperage (w = v X a). By algebraically manipulating the formula, amperage is equal to wattage divided by voltage (a = w/v). Divide 2700 watts by 240 VAC to find the amperage of the non-insulated space. The answer is 11.25 amperes. Perform the same steps to find the values of the other two wattages. The 2160-watt heater will need 9 amperes and the 1800-watt unit will require 7.5 amperes.

Understand that finding the required amperage for the baseboard heater unit is needed, so it can be wired with the proper size wire and circuit breaker. In most cases a 12-gauge wire can handle up to 20 amperes of electrical power. A larger 10-gauge wire can accommodate a 30-ampere load. Circuit breakers attached to these power carrying wires can never exceed the wires capacity. In other words, a 30-ampere circuit breaker can only be connected to a 10-gauge wire and never to a lower capacity wire such as a 20 ampere rated 12-gauge wire.