A great many electrical projects require you to join (splice) circuit cables together. Sometimes this occurs at fixture boxes, but it also can occur wherever you want to branch a circuit into two or more directions. When splicing is done outside a standard fixture box, the project involves making the cable connections inside a junction box with a blank cover that can be accessed whenever you need to work on the wires.
Mastering the splicing technique will make it possible to tackle countless projects that enhance your living space, such as moving an outlet or light fixture, removing a wall, finishing a basement, or taming dangling wires that are improperly connected.
The method demonstrated here is the correct way to splice electrical wires using UL-approved wire connectors joined inside an approved electrical box. These connectors can be the familiar twist-on wire nuts or newer style push-fit connectors. This is the method approved by the National Electrical Code (NEC). The old, informal method of splicing wires together with electrical tape should never be used.
Electrical splices can never left on their own in a wall or ceiling cavity. Instead, all splices must be contained within an approved junction box or fixture electrical box. The box itself must remain accessible and cannot be hidden behind drywall or other building materials that would require removal to get to the box. The junction box provides a safe environment for your splices, protecting them against impact and containing sparks and fire if anything should go wrong. While junction boxes may at first seem unwieldy and unnecessary, you will find that they are easy to work with and will make your work safer.
- This project assumes that the wall surfaces are open to provide access, and that the electrical cables have already been run inside the wall cavities. If not, the process of fishing cables will make the project more complicated and time-consuming.
Making wire splices is not a difficult technique to master, but like any electrical repair that involves handling circuit wires, DIYers should have a good understanding of electrical systems as well as some experience with basic electrical repairs.
Equipment / Tools
- Cable ripper
- Wire stripper
- Work gloves
- Cordless drill with driver bit
- Drill bit extender
- Junction box with cover
- Cable clamps for box
- UL-approved wire connectors
- Wood screws for mounting box
- Grounding pigtail
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, cables 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 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 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.
When the cable sheathing and wire insultion have been stripped, about 6 inches of wire protrudes beyond the edge of the remaining sheathing and individual wires are stripped back and smooth, not nicked.
Remove Knockouts from Junction Box
With a screwdriver and hammer, loosen two opposing knockout disks from the metal 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 metal disks.
Attach Wire Clamps to the Junction Box
To attach wire clamps to the junction box's knockouts, first removing 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. Tighten the screws on the clamp until the cable is securely gripped.
Make Wire Connections
Using approved wire connectors, join the wires with similar insulation colors together. Follow manufacturer's recommendations for this. 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.
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 wires should also be joined together in the box, using an approved connector. For metal electrical boxes, run a third ground wire 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.
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 is flush with the finished wall surface.
After 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.