short circuit

I have a short circuit somewhere in a string of 12 volt lights and need help with how to isolate the problem. I have a circuit of 10 lights and two switches the switches are 3 pole with 2 traveler wires. When I turn on the lights they trip a circuit breaker within a few minutes of being on. So far I have replaced both switches and the circuit breaker and the problem continues. The wires to the lights are behind a ceiling covering and would require removal of the headliner which I would like to avoid.

Any thoughts on how to analyse this problem without ripping the ceiling.

welcome aboard charleston.

im sure there are plenty of members ready to tackle your problem, but you will need to supply us with more info.

please tell us exactly what bulbs you are using and the the specs on your switches and breakers. it might help to know wire sizes too if you can.

yuk

quiet rural missouri, near KC.

If your system moves you physically and not emotionally, you have wasted your money.

If it runs for several minutes then trips the breaker, I would have to assume that you are just running the breaker too close to it's rated current.  Do you have any idea how much current the lamps draw?  What size wire did you use and what is the current rating of the breaker?

A little more background the lights are in the front of a motorhome and  the switches are at the exterior door and the bath area. The light fixtures all  look  factory installed along with the switches. I pruchased the motohome used and don't know it's history.

The circuit has 8 20 watt halogen bulbs at least i think they are halogen they are very small, about the size of  a finger nail, with 2 prongs that are pushed into the fixtures. The breaker is 15 amps. The wires appear to be 12 gauge and barely fit on to light blue connector crimps. The switches are standard RV 3 pole. Before I changed the 15 amp breaker, the on switch would immediately trip the breaker, since I changed the breaker the lights stay on for a couple of minutes.

One thing I haven't tried to do is see if the lights trip the breaker with the 2 switches in different positions that may help to determine if the short is in one of the traveler wires. I think there are 4 switch combinations.

Good info. 8x20W = 160W = 13.3A@12V or 11.5A @ 13.8V. 13A is a bit high for a 15A breaker. The usual rule is no more than 70-80% of fuse/breaker rating (ie, 13.3/.8 = 16A, /.7= 19A) but 15A should be ok if the weather isn't too hot, and the lamp power isn't underrated. And that closeness should not result in an immediate trip - it should take minutes if not hours (if at all). But with a new breaker they stay on for a few minutes. Maybe the old breaker "wore out" due to its high loading and the new is back to spec, but the lamps draw more than their ~13A. Else yes, there could be some other load, but that too should be warm (ie, if 2A@12, then 24W - think of a stop lamp).     IMO 12G sounds a bit small for that current. Does the 12G get warm after a wile? (Though I'd estimate only 10W-20W is dissipated over the entire cable run.) But if it is too small and gets warm, that reduces the total current... And you know to de-grease halogen bulbs if touched?

Thanks for the information oldspark, can I assume that a hot wire is not in direct contact to a ground?

I have noticed that the switches are warm to the touch but not hot!

Also should I replace the 20 watt halogen bulbs with LEDs to reduce the current?

Do you think this may be poor engineering by the motor coach manufacturer?

Oh dear - big ambiguities.... A "hot" wire is one that has the non-ground power (eg, +12V, or "Active" in ac systems). It's "hot" as in "dangerous". But hot as in heat means excess current ie, exceeding the cable rating. Short circuits (ie between hot and cold, er... ground = +12V to GND) are an extreme case of "excess current", but a warm or hot cable doesn't necessarily mean a short. LEDs draw negligible power compared to halogens etc. That is one way around the problem - and worthwhile if you are using battery power. Equivalent LEDs are probably under 1W compared to 20W halogens. Poor design? It may not be, but it would not surprise me. Certainly if that is original wiring for the same number of 20W bulbs (not 10W etc), then I'd argue yes - it's a poor design. At least from our point of view - it may be great for the trade or supplier that replaces the breakers every so often, or rewires it later... But if your installation and bulbs are standard, I think YOU have shown what a bad design it is. The old breaker failed, and the new one still trips after a while. If you are in a desert at 45°C (110°F) I might understand it, though I'd still argue an "not good" design.