Running a branch circuit for a 240-volt baseboard heater with a double pole across the line thermostat is a relatively simple but labor-intensive project. If you will be installing the baseboard heat during new construction, the walls will still be open, and running the wire will be quick and easy. If you are installing baseboard heat to a finished room, you will have to fish the cable through finished walls and floors while causing minimal disruption to the structure.
Calculate the size/rating circuit breaker required by multiplying the heater's FLA, ,or full load amperes, by 1.25. The National Electric Code, or NEC, classifies baseboard heat as a "continuous duty load," which can be expected to pull full amps continuously for three hours or more. Breakers serving continuous duty loads must be rated not less than 125 percent of the full-load current.
Compare the calculated over-current protection device (OCPD) rating with the standard ratings given in NEC Article 240.6(A). If the calculated rating falls between two of these standard values, use the next higher rated CB.
Calculate the branch circuit conductor's required Ampacity. Conductor "Ampacity," like the OCPD rating for a continuous duty load, should not be less than 125 percent of the full-load current.
Compare the computed minimum required branch circuit conductor Ampacity to the standard Ampacity given in NEC Article 310.15 and NEC Table 310.16 using the 140-degree Fahrenheit column. If the required Ampacity falls between two standard ratings, use the wire size for the higher rating.
Locate the device box for the thermostat on the opposite wall from the baseboard heater to ensure even heating of the room. Mount the thermostat's device box 46 inches above the finished floor line.
Route the cable from the thermostat location to the baseboard heat unit and from the thermostat location to the service panel.
At the baseboard heater, secure the cable in place and strip off the outer jacket so that there's 6 inches of free conductors in the splice box.
Strip ¾-inches of insulation from each of the conductors and splice the red and the black conductors exiting the cable to their counterpart on the heater.
Connect the bare copper grounding conductor to the heater's frame. Depending on the unit's design, this may be accomplished with a green grounding screw, a grounding pigtail, a metal clip or some other method.
The white, neutral conductor may or may not be used depending on heater design. Check the wiring diagram that came with the baseboard heat unit.
Install both cables into the thermostat's device box and strip the jacket off them so 6 inches of free conductors are available in the box and secure the box in the wall by pulling the box's wings up against the inside of the finished wall.
Strip ¾-inches from each of the conductors and splice the two white neutral conductors together.
Using a 6-inch length of bare copper wire, make a pigtail splice with the two bare copper grounding conductors in the device box.
Using the needle-nose pliers, form loops in the free ends of all the conductors in the box, in preparation for connecting them to the thermostat.
Install the line thermostat. Connect the red and black wires from the service panel to the "line " side of the line thermostat and the heater cable to the "load" side of the thermostat. Connect the grounding pigtail to the green grounding screw.
Install the 2-Pole CB in the service panel connecting the red and black wires to the breaker's terminals and running the white neutral conductor to the panel's neutral and the bare grounding conductors to the panel's grounding bar.