There are a number of different ways that a baseboard heater can be wired, depending on the specific type and model of the heater. How a baseboard heater is wired also depends on whether or not it is connected to a thermostat.
For most standard baseboard heaters that are not connected to a thermostat, the two heater wires are connected directly to two supply wires coming from the building's electrical system. Most baseboard heaters do not have a ground wire, meaning both wires are hot. The majority of baseboard heaters are wired using a 12 AWG loomex cable, which is designed specifically for heating circuits and comes with one black and one red hot wire. Baseboard heaters are typically wired to a double 20 amp breaker in the building's electrical panel.
To wire the unit, one needs to first turn off the building's electricity, connect one heater wire to one supply wire and then connect the other wires. Many baseboard heaters can be wired from either side, which provides more options for where the unit can be installed. When installing a baseboard heater, it must be located directly next to a stud where there are easily accessible supply wires that can be connected to the unit.