Short in Roof A/C

[dale farley]

While at Tom Johnson's, my bedroom A/C ran for a while then tripped a breaker. Actually, it tripped my 20 amp, my main 50 amp inside the bus, and the main 50 amp on the post outside, so I assumed a locked up compressor or a dead short. I then noticed that it tripped the breaker even with the A/C turned off, so I assumed a short.

When I started troubleshooting it, the pictures below show what I found. I repaired the wires and turned the A/C on again. It appeared that the burned wires were not making good connection inside the wire nut. Only a few strands of the wire feeding the unit were wrapped around the wire coming from the breaker box. I think this eventually caused the wires to get hot and start the process of the short. Either that or there was a break in the insulation where it was touching the metal box.

After monitoring the amperage for over 2 hours and watching the temperature of the fan and compressor, I saw nothing out of the ordinary, so I am assuming the problem was in the wires, not the unit. The amps stayed between 12.5 and 13 amps. I made sure the wires are no longer in contact with the metal junction box.

The foam pads in the picture are from Sam's ($18 for 8). They help protect the roof when on it. I still try to walk on the ribs when I can, but the pads help distribute the weight and protect the skin. I use 2 or 3 if I am working in one area for a while. It is nice to be able to lay tools on them without worrying about scratching the paint.

[Jerry Winchester]

Good post and nice pics. UR the man.

[flyu2there]

Dale,

Wires that reside inside of a wire nut should never get hot unless they have been exposed to a very big load. There is something else going on there........ Those are the black ones (+) so I would cetainly look into this further.

Any visible corrosion?


John

[dale farley]

There was no corrosion on any of the wires that I could see. Of course, the one set was burned so badly that I couldn't tell. The ground side was perfectly clean and no sign of any heat at all. I checked the breaker, and it was fine with no corrosion.

As I said before, I kept the amp meter on it with the compressor and fan running continually for over two hours. I then cycled the compressor on/off several times and watched the current draw. It only shot up to a little over 15 amps when it started, then within 20 seconds was back down to 12.5-13. I also inspected all the wires on the unit itself, and everything was clean and corrosion free.

I used my infrared thermometer to check the temps on the fan and compressor, and neither of them ever got overly hot during the 2 hours of monitoring. The hottest spot on the casings were each running 130-150 degrees. I have never heard the compressor struggling when running or starting.

It wouldn't be difficult for a bad compressor or fan to blow the 20 amp breaker if they start overloading, so I intend to just keep an eye on it the next time I use it.

[garyde]

Hi Dale. When twisting wires together, normally two solid or two stranded, I twist them together prior to twisting the wire nut onto the wires. When there is a solid wire connected to a stranded wire, you have to be careful the stranded wire does not slip down.

[dale farley]

Gary, I usually do the same, and I made sure that I got a good wrap before I attached the wire nuts. It appeared to me that the original connection only had a few of the strands touchings the solid wire, and I think that may have been the culprit. I hope it was!

[Jon Wehrenberg]

Loose or poor connections create resistance which in turn generates a lot of heat. This could be true at any connection anywhere, not only at wire nuts.

A loose connection at the back of the circuit breaker on the power pedestal will do the same thing.

[mike kerley]

Jon brings up a good point.

Many of the power pedestals in use in older parks were wired with aluminum wire, which does a great deal of expansion and contraction with heat. This helps them loosen up over time, which causes high resistance, high heat and intermittent power burps and power spikes which new electrical equipment hates. Fires are never much fun either.

Its a good idea to have an electrician check and tighten the connections on older pedestals and on the bus, kill all power and tighten everything from the power cord through the transfer switch down every couple of years. Utility companies will normally check and tighten (when requested) only to the meter. From that point its your problem.

Could save some serious dollars$$.

[dale farley]

All the wires in the bus are copper, but I do think the loose wires were the problem. I checked the connection to every breaker while I had the front panel off the box, and they were all very tight with no signs of heat.

Another thing that makes me think it was a loose connection was the fact that the burned wires were worse at the connection and only extended out about 2"-3". It didn't appear that there was a load getting all the wires hot. If either the fan or compressor was the problem, it should have shown up on the amp meter within two hours of continuous running.

[Jon Wehrenberg]

The heat is generated by the resistance caused when only a few strands or small contact points are trying to carry a heavy load. If the proper sized wire is used it will not ever get warm, but it the wire size is dramatically reduced instead of easily carrying the current, it acts like an electric heating element.

Bad or minimal contact connections exhibit the same characteristics.

On older coaches, especially areas subject to vibrations or corrosion all connections are at risk and should be checked from time to time. It does not matter if the coach has all copper wiring. BTW, these problems are not shorts, but actually a rise in amperage due to the drop in voltage and they impact not only AC current (alternating current), but DC current.