How to Splice Electrical Circuit Wires

Remove Outer Sheathing From Cable

Make sure that you are joining two similar cables. The cables must match in terms of wire gauge and the number of individual conductors in the cable. Modern wiring (up to 50 years old or so) will have the gauge and number of wires printed on the outer sheathing of the cable. For example, a cable labeled "12/2 w ground" contains two 12-gauge insulated conductors plus a bare copper grounding wire.

First, expose the individual conducting wires within the tough outer plastic jacket by using a cable ripper to slice through the sheathing. Do not use a utility knife, as you risk cutting into the individual wires. Insert the cable into the hole on the cable ripper until it is about 6 inches from the end of the cable. Lightly press the sides of the ripper together and draw the tool off the end of the cable to slice through the sheathing.

Trim away the severed portion of the sheathing and paper filler with the cutting jaws on a wire stripper or with a utility knife.

Strip Insulation From Conductors

Every conducting wire in the cable, except for the ground wire, will be coated with color-coded plastic insulation. Use a wire stripper to remove about 1/2 inch of this insulation from each wire. The stripper tool has slots to match various wire gauges; use the slot that matches the wires in your cable.

Inspect the Cables

Check the wiring on both cables for any sign of damage, such as cut, chewed, or burned insulation or nicked wiring. When the cable sheathing and wire insulation are properly prepared, about 6 inches of wire should protrude beyond the edge of the remaining sheathing, and individual wires should be stripped back and smooth, not nicked.

Remove Knockouts From Junction Box

With a screwdriver and hammer, loosen two opposing knockout disks from the junction box. Use pliers to pry off and completely remove the discs. Often, you have to rock the disk back and forth several times before it will come free. Dispose of the disks.

Attach Cable Clamps to the Junction Box

Plastic cable clamps usually just snap into the knockout openings in the box. For metal cable clamps, first remove the threaded tightening ring, then insert the clamp through the knockout opening. Screw the tightening ring back onto the clamp from inside of the box, and use pliers to tighten the ring securely. Do not turn too hard or you may break the clamp.

Insert the Cables

Insert one cable into each junction box knockout, through the clamp. Make sure that the cable is positioned flat on the clamp—if you accidentally position it sideways, you risk damage to the cable. The cable sheathing should extend past the clamp into the box by 1/4 to 1/2 inch. Plastic cable clamps usually have a tab that you force closed in order to grip the cable. With metal clamps, tighten the screws on the clamp until the cable is securely gripped.

Attach the Box and Cover Plate

The junction box has holes in the back and sides to allow you to screw it to a wood framing member. If necessary, you can attach a bit extender to a drill to help you drive screws without disturbing the wires. Make sure the box is securely attached to the framing member at a depth that will allow the cover plate to fit securely over the box once the finished wall or ceiling surface is in place. For example, if the wall will be finished with 1/2-inch drywall, then the front edge of the box should extend 1/2 inch beyond the front face of the stud. This will ensure that the box will be flush with the finished wall surface.

Make the Wire Connections

Using approved wire connectors, join together the conducting wires with similar insulation colors.

With standard wire nuts, some electricians prefer to twist the wires together first with pliers, then screw the wire nuts over the ends of the wires. However, some wire nut manufacturers instruct to simply hold the two parallel wires together, then twist the wire nut over the bare ends of the wires in a clockwise direction. However you do it, the wires should be connected securely enough that they don't come free from the wire nut when you tug on them. There should be no bare wire exposed at the bottom of the wire nut. Some electricians like to wrap a loop or two of electrician's tape around the base of the wire nut and wires to help reinforce the wire connection.

Another type of approved wire connector is the push-fit connector. With these connectors, you simply push the bare end of the wire into a grip-fit socket on the connector.

The bare copper circuit grounding wires should also be joined together in the box, using an approved connector. For metal electrical boxes, run a third grounding pigtail (bare copper or green insulated) to the two bare copper circuit grounding wires, using a wire connector. The free end of the pigtail is then connected to a threaded screw opening on the metal box, using a green grounding screw. This technique grounds the electrical box and improves the safety of the circuit.

After completing the connections and finishing the wall or ceiling surface around the box, attach the cover plate securely. Some cover plates have a matte surface that allows them to be painted.