Dear Mr. Electrician: I want to replace a very old ceiling light fixture in an old house. After removing the existing light fixture ceiling canopy cover that hides all of the wiring, I found that there isn’t a standard ceiling electrical outlet box. Instead, there is a black round disc (about 3 inches in diameter) which has a 1 inch long screw type rod sticking out of it. The wires are around it. The light fixture that I want to install comes with a flat hanger bracket and the instructions tell me to install the hanger bracket onto the ceiling outlet box. Since there is no outlet box, I don’t know what to do. What are my options?
Answer: I suspect that the round disc is actually an old black enameled metal pancake box. It’s approximately the diameter and thickness of a hockey puck and made out of metal. Sometimes an old gas pipe protrudes from the middle of this pancake box instead of a fixture stud. A setscrew tightens the box to the gas pipe. NOTE: Text links below go to applicable products on Amazon.com.
Old Style Ceiling Electrical Boxes
That was how old houses that originally had gas lighting converted over to electric lighting. The pancake box was mounted on to an existing disconnected gas pipe. The cables were attached to the pancake box and splices were made. An additional fitting or two was threaded onto the gas pipe to support the light fixture which was made for this type of installation. The canopy of the old style ceiling light fixture acted as the junction box for the spliced wires. On very rare occasions I have found that the gas pipes were still connected to the main gas line, so proceed with caution.
Other types of old pancake boxes were mounted directly onto ceiling joists using wood screws. Instead of an opening for a gas pipe, they would have their own fixture stud for attaching a light fixture to. The fixture stud had the same size thread as a gas pipe of the same diameter.
Around the pancake box are four holes, each with a setscrew or clamp which engages the cable, preventing it from being pulled out, and provides electrical continuity for the equipment ground for the armored cable.
The wiring is most likely armored cable (BX). However ungrounded non-metallic cable (The Type Before Romex), or knob and tube type wiring is possible as well.
Beware if the gas pipe has a cap on it. Usually the caps are removed when the gas line is taken out of service. However, until you can verify that the gas line is dead you should not remove the cap.
In a situation where the original wiring insulation has degraded, the best thing would be to install new wiring. However that not only could be expensive, but damaging as well. Walls and ceilings may need to be broken open to re-route or remove old dried and brittle cables and install new wiring.
Replacement of Old Style Ceiling Electrical Box
Turn off the power. You will need to remove the old pancake junction box. Do it carefully so as to not damage the existing wiring. Often, wiring from that era of home construction now has dry, brittle insulation on it. Any disturbance to the wires could cause the insulation to break off.
Make sure that you identify each conductor in the box before you take apart the splices. The color coding is almost gone on these wires. The wires are likely to be soldered and taped. The insulation on the wires will break off as you separate the conductors. Get a roll of white and a roll of black electrical tape. You may also need rolls of red and blue electrical tape.
The gas pipe will be a hindrance to installing a new modern octagonal ceiling electrical box. Also it is quite likely that there is a piece of wood supporting the gas pipe or the pipe may be attached to the side of a ceiling joist.
Sometimes a pipe wrench can be put on the gas pipe to unscrew it from the elbow. An internal pipe wrench might be needed. Other options are cutting it out with a multi-function oscillating tool or a reciprocating saw, or relocating the ceiling box. Relocating may not be an option due to the existing lengths of the wires.
Once all obstacles have been removed and the existing cables have been identified and taped, a new electrical box can be installed. A new pancake box can be installed or sometimes an old work ceiling fan brace and box can be fitted in place through the existing hole. A side mount type fan box is also available if the joist is right there. Try to get the deeper boxes.
Use the proper cable connectors and anti-short bushings for the cables and bring them into the back of the box. In this case it may be best to put the cable connectors on the ends of the cables with the locknuts removed. Tape the ends of the wires together. As you push the box into the ceiling, pull the cables through each connector. Tighten the locknuts by hitting them with a screwdriver driven by a hammer after the box is secured to the bracket. The BX cable metal armor is the grounding conductor, so it needs to be a tight connection.
My post on repairing a loose ceiling electrical box may be helpful.
Also my post on installing a ceiling fan electrical box is good to read.