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before World War I often have plaster walls and original “knob-and-tube” wiring, which was installed according to old, outdated electrical code that did not require electrical boxes for light fixtures.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.
Turn off the power to the fixture, then check the wires with a non-contact voltage tester before beginning work.