Can I Use Romex on Furnace Wiring?

Run a length of 8/3 Romex between the furnace and the main breaker panel. The number 3 refers to the number of conductors, but because the manufacturer does not consider the ground wire to be a circuit conductor, the cable actually has four wires.

Locate the terminal enclosure on the furnace, unscrew the cover, and remove it. It may have wires or terminal screws attached to a plate. If it has wires, they will be red, black, white and green. If it has screws, there will be two brass ones, a silver-colored one and a green one.

Cut off about 10 inches of sheathing from the Romex with a utility knife, and separate the wires. Three of them will be insulated, and one, the ground wire, will be bare. Remove a small amount of plastic from the ends of the insulated wires, using the knife.

Connect the Romex to the furnace by twisting the wires together with pliers, if the furnace has wires. Join the conducting wires by twisting together the wires of the same color, and then twist together the ground wires. Screw an 8-gauge wire cap onto the end of each pair of joined wires.

Use ring lugs to connect the wires if the furnace has a terminal plate. Crimp a lug onto the end of each wire in the Romex, using pliers. Remove the screws from the terminal plate, insert them through the lugs, and screw them back to the plate in the following configuration: red and black to the two brass screws, white to the silver one and ground to the green one. Red and black are interchangeable between the brass terminals.

Remove the ground strap joining the ground terminal to the silver one, if there is one, before connecting the ground wire to the ground terminal. Some older 240-volt appliances were designed to be grounded by the neutral, or white, conductor, but current codes require grounding through a separate wire.

Replace the enclosure cover after you have connected the furnace terminals or wires. The cover may have a wire clamp attached to it. If so, pass the Romex through the clamp before you make connections, and tighten the clamp with a screwdriver before you replace the cover.

Connect the other end of the Romex directly to the electrical panel via a double-pole circuit breaker with the same current rating as the furnace. You should be able to find the rating of the furnace specified on a plate affixed to it near the electric terminals.