Why does the breaker trip occasionally after I installed a new light fixture?

Question

We bought a house a couple of years ago with a covered light fixture box in the ceiling (a blank cover over a wired box and a blank cover over the switch box). I recently modified a plug-in pendant lamp (no ground) to a hard-wired pendant lamp by doing the following:

1. Cut the plug off the lamp cord.
2. Connect it to the wires in the ceiling box (white to white, black to black).
3. Install a new single-pole switch in the switch box.

The new fixture has no ground, so I'm not sure what to do with the ground wire from the power cable. The fixture is a simple lamp-style socket on the end of a long cord. I have a 12-watt (60-watt equivalent) CFL installed.

The circuit itself, as best as I can tell, controls the overhead lights in the kitchen and the adjacent room. This includes a small (20-watt?) round fluorescent fixture, a ceiling fan that we never use (with a 25-watt incandescent bulb installed), and a track light with three 26-watt (100-watt equivalent) CFLs installed.

The first time I flipped the light on, it immediately tripped the breaker. I reset the breaker and tried again with no problem. Tonight during dinner (I did the mod on Tuesday or Wednesday), the breaker tripped after the lights had been on and stable for quite some time.

I didn't flip the switch off, but I reset the breaker. It immediately tripped, so I reset it again, and it seemed fine. It hasn't tripped since. In the 2.5 years we've lived here, I don't remember that breaker ever tripping before, which is what has me concerned.

I'll be in the attic tomorrow to replace a cable splitter, so I'll try to find the cable and visually inspect it for any damage or kinks.

Is there anything else I should be looking for? Any obvious problems or solutions that I don't know of?

Answer 1

I'm going to take a wild guess that the previous home owner put the blanks up because they had the same problem and didn't want to fix it. You likely have a short circuit and need to track down where the short is occurring. I'd remove the bulb from the fixture, unhook each of the wires that you connected before, and use a multimeter to test for "continuity" between the wires on each run (e.g. hot to neutral, hot to ground, neutral to ground for the line to the switch, for the light fixture without a bulb, and anywhere else that was previously disconnected). If you don't have continuity with a simple low voltage tester, then the problem could be arcing that only occurs when 120V is applied. Either way, get this fixed before turning the breaker back on with this fixture wired.

Answer 2

Your new light is not likely the problem because if it was, the circuit would trip every time you flick the switch. Your problem is more likely caused by the total amount of current being drawn on the breaker at any one time. If you are close to the TRIP point and you turn anything else on, the total current will increase on this circuit and cause the breaker to trip. This is a safety limit that you cannot alter. Run an additional feed from the main panel and start a new branch circuit.

Answer 3

There's a high likelihood of damage in the feed line from the switch to the ceiling box. It could be frayed or rodent-chewed insulation, it could be many things. I once worked on a problem where all the garage and outdoor lights, which were all on 2-way switches, suddenly started working entirely differently, flicking switch A turned on different lights than before, and the companion switch A2 did even more bizarre things. No tripping breakers, but what had happened was a roofer had driven a nail through two separate runs of 3-conductor romex, shorting two of the hots to each other.

Very dangerous and the nail did get quite hot. The nail was not visible from any vantage point, the only way I found it was when I traced the lines and felt the "stop" in wiggle-room where the nail pinned the wires together.

The solution is to run a new line from the switch to the fixture.