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How to Connect Old Wiring to a New Light Fixture

Fix old-house wiring problems. Bring old light fixtures wired with knob-and-tube wiring up to code by installing an electrical box in the plaster wall. Here's how to do it without breaking the plaster and lath.

By the DIY experts of The Family Handyman Magazine

Mount a fixture on a plaster wall

Houses built before World War I often have plaster walls and original “knob-and-tube” wiring, which was installed according to old, outdated electrical code that did not require electrical boxes for light fixtures. When you change the fixture, The National Electrical Code requires you to install an electrical box and update the wiring method to the current code. The wires themselves are still acceptable as long as the insulation on them is intact. However, the connections must be made within an approved electrical box.

If the wires emerge alongside a stud or other framing member, you can screw a metal box directly to the stud. However, it's likely that the light fixture was mounted in the middle of a stud cavity, which makes mounting a box that can support the weight of the fixture more difficult.

Several types of remodeling boxes can do the job, but we recommend a 2 x 3-in.metal remodeling box that's 2-1/2-in.-deep (for fixtures up to about 6 lbs.). The trick to mounting it is to position it so that you only cut completely through one lath (Photo 3). The photo series illustrates the process. Work carefully to avoid destroying any of the plaster “keys” on the back side and thereby weaken the wall around the fixture.

After you find the center lath and mark the box outline, cut out the keys along the top and bottom. Then cut about three-quarters of the way down one side of the center lath (Photo 1). This will keep the lath firmly in place while you cut the other side. With both sides cut, pop it out. Then cut out the remaining box profile (Photos 2 and 3). Once you've made the short side cuts, score the plaster horizontally with a utility knife, tap it with the knife handle and it'll crack off cleanly. Split off the small pieces of lath behind. Cutting the hole accurately is critical so that the box ears have solid bearing on the plaster (Photo 4).

Remodeling boxes have internal clamps for the wires. Push the wires through these clamps and work them farther in as you insert the box into the wall. We also pulled in a ground wire (green) because none was used in the original knob and tube system. (A ground wire isn't required in every situation. Ask your local electrical inspector to advise you on this detail.) We're anchoring the box with metal box supports (Photo 4). Insert one and bend one leg around the box to draw one side back tight. Then insert the other support and bend both arms around the box to tighten the other side. Bend the second arm of the first support around the box. The box ears should rest tightly against the plaster. Then tighten the clamps on the wires.

Finally, mount the fixture according to the directions. When you attach the mounting strap to the box, use the same size screws you use to mount receptacles and switches.

Caution!

Turn off the power to the fixture, then check the wires with a non-contact voltage tester before beginning work.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

    • Needle-nose pliers
    • Non-contact voltage tester
    • Utility knife

You'll also need a keyhole saw.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

    • Wire nuts
    • Electrical box
    • Metal support arms

Comments from DIY Community Members

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February 24, 2:05 PM [GMT -5]

I question the necessity of a metal box when the rest of the organically insulated wiring is not enclosed. I suppose that a short is most likely to occur where connections are made, since the wires are in close proximity, but the wiring as a whole will overheat if a short occurs, and if a properly working fusing device is not included in the circuit. A box will do nothing to protect the rest of the wiring, which is still capable of producing fires. I also don't see why people feel that knob-and-tube is much more ill-advised safety-wise than today's Romex. Benefits of Romex are superior insulation longevity (non-organic) and fire RESISTANCE. However, it is just as vulnerable to nails and screws, and if the fuse or breaker fails, it, too, will melt and start a fire. If you are going to install a metal fireproof box, then you should install fireproof BX or conduit sheathed wiring. Otherwise there is no purpose to the box, except perhaps as a convenient mounting means.

December 12, 10:09 AM [GMT -5]

I recently finished updating all the wiring in my home built in 1912. I found having an oscillating power tool, such as the Dremel Multi-Max, to be invaluable. Cutting plaster and lath with a drywall saw can be extremely difficult, and most of the time you will knock off more plaster than intended leaving a messy patch job. I used a grout blade to cut through the plaster and then a wood blade for the lath. These tools will give you a perfect cut out every time. They are especially handy when making circular cut outs for ceiling boxes. If you are going to tackle a wiring project any bigger than just putting in a couple new lights I would highly recommend picking one of these tools up.

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