Things You'll Need
Black electrical tape
20-amp circuit breakers
Insulated wire connectors
Baseboard heaters are a fast and cheap way to supplement the heat in your house. The number you need depends on how cold your climate is and how well your house is insulated. The usual location to install baseboard heat is under your windows, making sure that the heaters run at least the length of the window--just be careful not to install them behind any furniture.
Buy 240-volt heaters with internal thermostats--they use less current and are much easier to install.
Divide the total wattage of your heaters by 240 to determine how many amps of current you will need. For example, if the heaters add up to 4,000 watts and you divide that by 240, you get just over 16 amps. In this case, you will need to install two separate 20-amp circuits for the heaters, because you can't safely draw more than 16 amps over 12-gauge cable. You can use 10-gauge cable on a 30-amp circuit instead, but you will find it much more to difficult to run the wire due to its thickness.
Run the cable from the service panel to the heaters. You cannot have any receptacles installed above the heaters--you have to either remove the receptacle, put the heater in a different place or buy heaters that come with inserts for power receptacles. If you buy the heaters with receptacle inserts, you cannot connect those outlets to the same circuit that the heater is on--they need to be on the 120-volt receptacle circuit that runs through the room.
Take off the cover plate on your heater and mount the thermostat. Pull about six inches of cable through the NM connector installed in the back of the heater, and screw the connector securely to the cable.
Wrap black electrical tape around the white wire from the service panel to indicate that it is hot. Splice it to one half of the thermostat wire, and splice the black wire from the service panel to the other half. Splice the other ends of the thermostat wires to the heating elements. Consult the manufacturer's directions to verify the connections are correct.
Connect the bare ground wire from the service panel to the green grounding screw on the heater. Screw wire nuts over all splices and ensure that no bare wires are exposed. Tuck the wires neatly into the heater junction box.
Open the windows before you test the heaters, because they will smell and possibly smoke until the factory coatings burn off.
Verify the connections against the manufacturer's instructions before you turn the power on, because if you wire across the thermostat you will short it out.