Finding Cabin Lights Electrical Short

[Parsons]

I have a short somewhere in my cabin lights that is blowing a 20 amp thermal breaker, and I want advice in finding it. There have been no recent changes to the cabin electrical, so it's not a simple matter of "Undo what you just put in". When the breaker started blowing, my first try was to simply replace the 20-year old breaker - no luck.

My next try was to break out the amp meter (this is getting to be actual work!), and validated that with all cabin lights/fans/12v plugs off, it starts with 5 amps and ramps up to 37 amps within a few seconds, before the circuit breaker trips. Now my problem - I know there is a short somewhere between the single line to the panel and the switch to some device, but how do I find it quickly? The single power line disappears into a cable wrap and then behind a panel. I suppose there is a terminal block somewhere to divide it up. Any ideas for the quickest, highest-probability-first search?

I know that this has probably been covered before, so could someone just point me to the right thread?

 

[Whatfiero1]

Most manufacturers daisy chain fixture to fixture from panel thru out lights and sometimes 12v sockets and other devices
I'd start by removing all light bulbs from sockets if possible and unplug any fans or other devices and turn on see what amps then. I'd also shine flashlight and get a good look at all sockets for corrosion or maybe something wedged in there.

If that doesn't fix you could start pulling down lights and cutting splice made behind light and figure where issues are figure the run leaves breaker goes to closest fixture and in most boats makes a clockwise or counter clock around boat spliced at each light go to middle light separate terminals and ohm wires +to- and figure which has lowest then follow that down rabbit hole
A tone tracer good route to follow wires around boat

 

[JoeWhite]

Insert in series a 12v high wattage auto light bulb in the circuit just after the circuit braker. This bulb will limit the current and avoid tripping the braker. It's brightness will indicate if you have located the short. All devices connected to this circuit will have less than 12v to operate, but will help you to isolate where the short is. Since there is a delay before tripping the breaker, it appears that there may be accumulated rust and salt.

 

[rgranger]

Sounds like a mouse made a home someplace behind a panel. Running down a short can be a real PITA. I don't know your layout but at somepoint you may decide it is just easier to make a new run and orphan the existing wires behind the headliner.

 

[DArcy]

Coming from the guy that is chasing down "orphaned wires" - please don't do that. Multiple POs of my boat added and then abandoned miles of wire, it is a real headache trying to sort it all out and could lead to potential problems down the road.
Did you add any new hardware anywhere on the boat lately, say in the last year or two? It is possible a screw that was installed some time ago could have worn through wire insulation under vibration which makes electrical contact with something it shouldn't.
Sometimes it's just corrosion or carbon arc tracking building a bridge between +12V and return. Look at all the fixtures and points of interconnect. I like the idea mentioned above of a 12V light bulb in series with your circuit which would be illuminated until you open the short circuit.

 

[shemandr]

I'm no Maine Sail but 20 amps seems like a lot. Are you sure there isn't something bigger on that circuit? If in fact lighting is causing a 20 amp breaker to trip, I would think you'd find melted wiring or worse. The last wiring job I did used a 5 amp fuse and maybe less for the navigation lights. The aggregate load of everything on the boat being on at the same time maybe could reach 20 amps, which would be unlikely, so I would test the wiring close to the panel first. I guess I'm asking why is there a 20 amp circuit in the first place. My 2 cents and worth about that.

 

[DArcy]

I guess I'm asking why is there a 20 amp circuit in the first place. My 2 cents and worth about that.

I have not converted my lights to LED yet, still the old incandescent bulbs. When I turn them all on at once they draw about 12 Amps. A 20 A breaker sounds about right.

 

[RoyS]

Does not sound like a typical wiring short. Wiring shorts usually cause instantaneous breaker trips. When tracking down a normal short the standard procedure is to open the circuit in about the middle and apply power. Your wiring will come from your circuit breaker and should proceed along the lights in your boat in a logical pattern. Open the circuit inside the center light fixture. After applying power you will know which side of the circuit the short is on. Then divide the shorted side in half again and continue. Time consuming but really the only way to find your short. Previous owners probably messed with your wiring.

 

[Parsons]

I'm no Maine Sail but 20 amps seems like a lot. Are you sure there isn't something bigger on that circuit?

What @DArcy said -- this circuit has the nav/com, cabin lights, and nav lights branching off of if. Why someone designed critical systems on the same circuit breaker as the cabin lights is a question I'll leave for the good folks at Catalina. In any case, the nav/com system pulls about 1.5 a, and 5.5a when VHF is transmitting. The nav and cabin lights are now all LED, but when the boat was built they were those 5 or 10 watt, third-degree burns bulbs. So if that was the draw, then if you were navigating at night with some cabin lights on, and talked on the VHF, you might blow that breaker. Good question though.

 

[Parsons]

Previous owners probably messed with your wiring.

Thanks for the search idea, and I'll start on that next weekend. I've had the boat about five years, so I can't blame those pesky PO's. Boat just recently moved down to salt water, so perhaps some growing corrosion in the past few months.

 

[uncledom]

I does sound, to me, like a dead short through a bad wire/connection. Do you have a wiring diagram for your boat? Can you split off a section of the house lights at a time? Can you isolate the critical systems like the VHF and nav lights to ensure it's not those circuits?

 

[Justin_NSA]

My 30T has cabin lighting and fans on the same circuit so I could see the need for Hunter to install a 20amp breaker. I have converted everything to LED and replaced fans with more efficient Caframo units.

 

[DougM]

My 30T has cabin lighting and fans on the same circuit so I could see the need for Hunter to install a 20amp breaker. I have converted everything to LED and replaced fans with more efficient Caframo units.

Parsons,
If you have fans on the same circuit as the lights, which I personally would not recommend, I would suggest checking out the fans. If they happen to be hard starting or stalled they are going to draw maximum amperage.
What you are describing seems more like a motor than a light. Corrosion could be the cause or just plain old age. Just my two cents.

 

[Parsons]

Corrosion could be the cause or just plain old age.

My doctor said this to me at my last appointment
Thanks Doug, but no fans installed on the lighting circuit. I did branch in one of those SCAD tank readers as it only takes a small amount of power, but that was the first fuse I pulled when this all started. It's only a 1A fuse, so it would have blown well before the breaker, but I was trying to eliminate all the things that I added.

 

[Parsons]

Insert in series a 12v high wattage auto light bulb in the circuit just after the circuit braker. This bulb will limit the current and avoid tripping the breaker.

Could I do the same thing with a rheostat, and just turn it down to limit the current to avoid tripping the breaker?

 

[Captain Larry-DH]

it starts with 5 amps and ramps up to 37 amps within a few seconds,

This sounds to me like a device, not a wire. I'd start with disconnecting/removing all bulbs and other loads (if any) then add them in, one at a time, while taking ammeter measurements.
Edit: If the breaker trips with all loads disconnected, it would prove it's a wire or socket/switch.

 

[DArcy]

Could I do the same thing with a rheostat, and just turn it down to limit the current to avoid tripping the breaker?

You could but then you lose the advantage of the light bulb as an indicator.

 

[JoeWhite]

Parons, the idea of using a light bulb is to provide a visual feed back of the current in the circuit. Initially it will be half the brightness then become much brighter as the current increases indicating a short. A rheostat will not provide this feedback
To protect your circuit braker, you can disconnect the output side and use a 12vdc wall adaptor instead.
DArcy has the right idea.
Haro.

 

[JamesG161]

I just talked with Joe Kerr, who wired many Hunter sailboats. [my talk with him was about other problems]

I mentioned this thread issue to Joe, about tracking down a problem circuit, since my boat got "dunked" and needed a similar check out method.

Me: Joe, do you have any tips or tricks to find at bad circuit?
Joe: The best method is a clamp on DC amp meter.
Me: Faster than an in-series light bulb?
Joe: Yes, plus you can see the amp flow and direction of amp flow.
Me: But what if the breaker trips?
Joe: Good, But the Amp meter is faster reading.

For what its worth
Jim...

 

[Project_Mayhem]

I agree with splitting the circuit in half to narrow down the location of the short. I know with 120v a poor ground can cause breakers to trip slowly. Perhaps corrosion is the cause for a slow tripping breaker in your case?