How to Wire a Garage Heater

Make sure your garage heater is safely and correctly installed.

Your garage might be your workshop, but working in it when the temperature is only 20 degrees F is no fun. Adding an electric garage heater might be just what you need to take off the hat and coat when you get down to work. Relatively inexpensive and moderately economical to operate, electric garage heaters work well for up to 500 or more square feet of space if it is well insulated.

Things You'll Need

• 10 AWG 2 wire + ground sheathed cable
• Junction box
• Cable straps
• 30 amp double circuit breaker
• Screwdrivers
• Wire strippers
• Utility knife
• Wall-mounted line voltage thermostat
• Black permanent marker
• Wire nuts
• 1/2-inch flexible metal conduit and connectors
• 10 AWG THHN, THWN-2 or XHHW-2 black wire
• 10 AWG THHN, THWN-2 or XHHW-2 green wire
• Pliers

Instructions

New 240V Circuit

1. Run sheathed cable from your circuit breaker panel to the thermostat location. Run additional cable from the thermostat location to the ceiling where the heater hangs. Install the junction box at that location with two screws. Use cable straps to support the sheathed cable every 4 feet and within 12 inches of the panel, thermostat and junction box.

2. Turn the main circuit breaker off and remove the panel cover. Install the double circuit breaker onto the bus bars according to the style of circuit breaker you have. Most have slots that catch tabs to the right or left of the conducting bus bars, and the breaker clicks down onto the bus bars once you hook the tabs on.

3. Strip away the cable sheathing inside the panel box, and 1 inch of insulation from the two wires. Connect the bare ground wire to the ground bus bar. The two wires go on the breaker terminal lugs. Be sure to tighten the lug screws tightly. Color the white wire black for 2 or 3 inches close to the circuit breaker. Replace the panel cover when you are done.

Thermostat Wiring

1. Strip 6 inches of sheathing from both cable ends at the thermostat. Remove 1 inch of wire from the ends of all the wire. Recode the last inch of white insulation black with the permanent marker on the wire.

2. Attach the two wires that lead back to the circuit breaker panel to the two wires marked Line or L1 and L2. Attach the wires that lead to the heater unit to the wires labeled Load or T1 and T2. Cap the connections with wire nuts. Twist the bare ground wires together and attach them to the green ground wire from the thermostat.

3. Mount the thermostat to the wall with screws while keeping the wire connections inside the thermostat enclosure.

Heater Wiring

1. Use an FMC connector to attach the flexible metal conduit to a knockout hole in the junction box. Use a second FMC connector to attach the other end of the flexible metal conduit to the junction box on the heater.

2. Strip 6 inches of cable sheathing from the end of the cable at the heater. Put the cable into the junction box. Run two pieces of 10 AWG black insulated wire and 1 piece of 10 AWG green insulated wire through the conduit so that at least 6 inches extend into both junction boxes.

3. Strip 1 inch of wire from all five wires in the ceiling junction box. Twist the two black wires together and cap with a wire nut. Recode the white wire black for at least 1 inch of insulation near the end. Twist it together with the remaining black wire and cap with a wire nut.

4. Ground the metal junction box by twisting the two ground wires together and attaching them to the box with a screw. Put the cover on the box.

5. Strip 1 inch of insulation from the three wires in the heater junction box. Connect each black heater wire to a black supply wire. Connect the two green ground wires together and cap all the connections with wire nuts. Replace the heater junction box cover securely.

Tips & Warnings

• The white wire in the sheathed cable is being used as a "HOT" wire and must be recoded to meet 1999 NEC code requirements. A permanent marker is best, but black tape will do as well. Be sure to check your heater's instructions to verify the wiring. This covers most, but not every, garage heater wiring scenario. The sheathed cable with have solid copper wire inside the sheathing and insulation. Buy solid copper wire to run through the conduit as it will make the connections easier to make. There is no neutral wire in standard 240 volt wiring; 240V appliances that have a neutral wire have at least some part of the appliance using only 120 volts.

• This procedure is safe provided the power is off while you are performing it. Installing the new circuit breaker poses the highest risk, as the terminal lugs on the main breaker are still live. Never touch the main breaker terminal lugs with anything; they are always live Working on your home's circuit breaker panel and electrical wiring exposes you to potentially dangerous and deadly electrical currents. If you are not comfortable working with electricity, wiring or circuit breaker panels, hire a professional or get the help of someone who understands what they are doing.

References

• Wiring Simplified, 40th Edition; Richter, Schwan, Hartwell; 2002

Resources

• Choosing a Garage Heater
• Installing Circuit Breakers
• Understanding and Wiring 240V Circuits