What is this single pole switch to empty ceiling box blows breaker when switched on

Question

I would appreciate some help thinking through what are the possible problems in the following situation?

A newly wired circuit from breaker box ( 20 amp 12 AWG wire) to front of the house (longest run in house) has a set of outlets, and at the end of the run a three gang switch box with 3 single pole switches for 20amp. One switch works an outside light, the second switch a switched outlet, the third switch a ceiling fan yet to be installed. The ceiling box is empty of a fixture and is covered. Inside that ceiling box is a green ground screw with the ground wire attached, and the capped off white and black wire. All looks properly sound. The wires in the 3 gang switch box also look properly and neatly wired. All the switches and wiring are new.

Problem: When all the outlets on the circuit are not in use, and all three single pole switches in the 3 gang box at the end of the run are switched off, if someone turns on the single pole switch to the empty ceiling fan box the circuit blows. If that switch is off and anything else on the run is used, whether it be one of the outlets, outside light, switched outlet, or some combination of them, the circuit works fine. Even when the switch to the empty fan box had been switched on in the past, the circuit continued to work fine. Then one day the switch to the empty fan box was switched on and blew the circuit. It is only when that switch is turned on that the circuit breaker switches off. This happens even when nothing else is running on the circuit and the only thing done is to switch on that switch to the empty ceiling fan box. Since the room has not been finished yet, one can see that no screw, drywall or otherwise, has damaged the wire. Further upstream cabinets were installed, but the wires in the wall at the location were protected from damage with plates at every stud and any damage would have theoretically affected all the down stream outlets, not just this one switch run.

I suppose it possible that the switch, although new, could need to be replaced, and the new wire between the fan to the switch may also have to be replaced if no other explanation can be found.

Answer 1

Something is wrong with the wiring to the ceiling fan or with the connections to the switch.

Possibly in between the last succesful test and the first unsuccessful use of that switch something has damaged the wiring (e.g. a nail into the wall? strain on the wires?)

I would isolate the circuit at the breaker box, test for voltage at the switch and then test the wires attached to the fan switch for unexpected continuity between live & neutral, live & earth. I'd then detach the switch from all wiring and test for continuity across it's contacts with the switch in each state.

Answer 2

It's common practice (sort of) for the ungrounded (hot) switched conductor, to be terminated together with the grounded (neutral) conductor inside a "for future use" box like you've described. It's not the best practice, since it causes the breaker to trip when the switch is turned on. However, it does prevent the switch from being left on, with nothing connected to the circuit.

If the black and white wires in the ceiling box are connected together, the circuit is short-circuited when the switch is turned on. This should cause the circuit breaker to trip. To remedy the problem, separate the white and black wires, and/or install a fixture in the box.

If the white and black wires are not currently connected together. You have a short somewhere else in the circuit, and you'll have to investigate to locate it.