If your to-do list includes an ambitious DIY electrical project, whether it’s installing a light fixture, replacing a switch, or extending electrical cords to add another outlet in the garage, you’ll need to know the fundamental skill of splicing wires. Learning how to splice wires correctly will not only ensure your electrical repairs and upgrades work properly, but just as importantly, it will also keep you and your property safe. If you’re not sure exactly how to splice wires, read on for more information.
before you start splicing wires
The following instructions assume that you are splicing two romex cables of the same type. (in this example we are connecting a 12/2nmc with ground to the same type and size of wire). Romex is a brand of cable preferred by many electricians that is commonly used in residential applications. stamped markings on outer insulation, “12/2nmc grounded”, indicate wire size and type; in other words, a 12-gauge wire with two insulated inner conductors (one black “common” and one white “neutral”) as well as an uninsulated ground wire.
note: “nmc” is an acronym for non-metallic cable, the most common type of cable in residential applications.
Other types of romex wire used in residential construction are:
- 12/3: 12 gauge wire with three insulated inner conductors and a ground connection, commonly used for switches and light fixtures
- 10 /2: 10 gauge wire with two insulated inner conductors and a ground connection, commonly used for water heaters
- 10/3: 10 gauge wire with three insulated inner conductors and ground, commonly used for electric clothes dryers
- 6/3—6 gauge cable with three insulated inner conductors and ground, commonly used for electric stoves and ovens
It should be noted that while it is possible to splice different types of romex cable (12/2 to 12/3, for example), you should never splice different gauge wire. Wire gauge is determined by the amount of amperage the wire is expected to carry. for example, a 12-gauge wire is capable of handling approximately 20 amps, while a 10-gauge wire is capable of handling 30 amps.
Overloading a cord with more than its rated amperage could cause it to overheat, melt, and possibly catch fire.
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safety precautions for splicing electrical cables
Before beginning any work to connect electrical wires, turn off the circuit breaker that supplies power to the wire you want to splice. Use extreme caution when working with electricity as it can cause serious injury or death if not handled correctly.
verify that the power is off with a voltmeter, a device that measures electrical current in wires. If you’re still not sure there’s no power, turn off the main circuit breaker for the whole house.
additional precautions to take before starting your project:
- find a partner. never work alone on electrical wiring. you want someone close by in case an unfortunate circumstance occurs.
- change your shoes. wear rubber-soled shoes to insulate your body.
- make sure the space is dry. never work on electrical wiring in wet or damp conditions.
how to splice wires
When a home project requires splicing wires, be sure to follow these steps carefully and precisely. a solid electrical cable connection will bring the desired result, for example, a new lamp or a fan. however, a single misstep can result in, at best, a disaster that needs to be corrected, or at worst, a disaster. Please read these instructions carefully and gather all necessary equipment before you begin. Most importantly, if you’re ever in doubt, call a professional.
Step 1: Prepare and install the junction box.
Use electrician’s or lineman’s pliers to remove two of the new junction box’s knockouts, which will house and protect the spliced wires and contain sparks that could cause a fire if something goes wrong. knockouts are pressed into the box at predetermined locations during manufacturing for easy removal.
Most junction boxes are universal and include various sized knockouts to accommodate different applications and a variety of wire gauges. This setting allows you to choose the locations on the box where you want to install the cable and wire connectors during installation.
insert a romex connector into each junction box knockout. make sure you buy a connector that fits the knockouts you are using in the junction box and is suitable for the diameter of the cable you are splicing.
attach the connector to the junction box using its threaded locknut and tightening with needle nose pliers and/or a screwdriver. the connectors act as protective guides that also secure the cables to the junction box. without them, the sharp edges of the knockouts could damage the cables.
Install the junction box correctly (many types attach directly to the wall stud or surface with mounting screws or anchors) and in an area within reach of the existing cable.
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Step 2: Prepare the wire splice connectors.
feed the end of each 12/2 romex cable (the existing cable and the cable you are splicing to it) through one of the romex cable connectors attached to the box. tighten the screws on the sides of the cable connector designed to hold it in place, using the appropriate type of screwdriver.
Step 3: Ground the junction box.
drive a ground screw through the threaded hole in the back of the junction box. the grounding screw grounds (returns excess electrical current safely to ground) the junction box in the event of a short circuit.
Step 4: Strip and ground the wires.
strip approximately 6 inches of the outer plastic sheathing from the end of the wires you are splicing. A utility knife is great for slicing and cutting the outer insulation, or you can use a wire cutter. remove the protective paper wrapping around the insulated wires and the ground wire.
Step 5: Use a twist wire cap.
wrap one of the bare copper ground wires once around the ground screw that is attached to the junction box; you should leave about 6 inches of exposed wire hanging past the screw. Tighten the ground screw with the screwdriver to secure the ground wire to the box.
Now, twist the second ground wire tightly together with the attached ground wire using the electrician’s pliers and secure the joint with a twist cap/wire nut. Bend the attached wires neatly into the back of the junction box.
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Step 6: Twist the wires together.
Using a pair of wire strippers, strip about ½ inch of insulation from the ends of both 12/2 wires, both the white and black wires. Wire strippers are a convenient tool for this task, as they are designed to strip a wide range of wire sizes and are available at most home improvement centers. Similar to a pair of pliers, the tool incorporates sharp edges and predetermined cutting points that allow you to remove the protective insulation from each wire without damaging the wire itself.
Using electrical pliers, twist the bare ends of the corresponding wires of each strand of Romex 12/2, white wire to white and black to black. twist until tight and secure each joint with a threaded wire nut/cap. Fold both sets of wires neatly into the junction box.
step 7: secure the junction box.
align the protective cover with the mounting screws on the junction box and tighten securely with a screwdriver.
Knowing how to splice wires can save you time and money on numerous electrical and lighting projects around the house. however, if you are concerned about working with electricity or don’t have basic electrical skills, don’t hesitate to hire a certified electrician for your project.
While hiring an electrician can easily cost you at least $50 an hour, it’s a small price to pay to protect your family and property from the serious consequences of shoddy electrical work.
frequently asked questions about connecting electrical cables
p. can you splice wires without a junction box?
technically, yes, but splicing cables without a junction box is highly unlikely to be acceptable under building codes and also not safe, so functionally, it shouldn’t be done.
p. how many wires can you connect together?
Depends on the size of the cables you are using, the size of the cable glands/nuts, and local building codes. the packaging of the nuts you are using will provide information on the number of cables of various sizes they will accommodate. or you can find push-in connectors that support a variable number of wires.
p. How many times can you splice a wire?
There is no limit, assuming you make the splices correctly and contain them inside an accessible, and preferably metal, junction box. One word of caution though: every splice is a possible failure, so the fewer splices, the less potential for problems.