Trailer wiring

[Zephod]

At some point I will want a trailer. Before I get too advanced with construction, I want to do the trailer wiring. I figure a single power wire with relays tripped by the power at the rear light clusters. Given that the cable needs to carry enough power to light two reds, two ambers and possibly side lights too, what gauge of wire should I use?

[gumpy]

12ga or 14ga to the trailer plug. 10ga from the batteries to supply the relays.


You should also consider that many buses have 4 circuits (tail, brake, left turn, right turn), whereas most trailers only have 3 (tail, left brake/turn, right brake/turn).
You probably want to build a system that combines your brake and turn signals.

http://www.gumpydog.com/Bus/MC9_WIP/Electrical/Tail_Light_Converter/tail_light_converter.htm

[oltrunt]

Gumpy, great post--well explained.  Although I didn't have to deal with the 24/12 volt issue, I did have the same problem with the 4 circuit to 3 circuit conversion device weakly flashing the opposite turn signal particularly with several signal lamps in use.  The actual cause of the problem is insufficient amperage being provided by the 4/3 converter to operate the several bright signal bulbs.  I solved the problem by using the output of the conversion device to activate a pair of 4 terminal 12 volt relays, one for left and one for right.  The output of the relay came from a fused link directly from the toad's battery which thereby lit the lamps.  The tail lamp circuit seemed to provide enough amperage to work directly from the 4/3 converter so I left it alone.  Since the stop and turn signals are in only intermittent use, the draw on the toad's battery is negligible.  Jack

[bevans6]

Commercial trailers use 14 gauge for the lights and 10 gauge for the brakes, 12 volt systems.

Brian

[TomC]

My bus is straight 12v. To pull my car or any other trailer, I made a 4 light magnetic lights with a long cable. As mentioned, I have running, brakes, left turn, right turn, ground-I use a 5 prong plug. I just run the wire down the side of the tow car and plastic tie wrap to the door handles. I tried wiring the car's lights, but wasn't bright enough. With running LED lights on the tail lights of the bus and LED's on the 4 magnetic lights, no relays were necessary. Just another thing to keep it simple. Course, if you have a 24v bus, that's a different story. Good Luck, TomC

[Lee Bradley]

With a 24 volt bus, you run 24 volt lights in the magnetic lights and lighter wiring.

[bevans6]

I have a "Gumpy-made" 24 volt to 12 volt converter, it works flawlessly.  I am very proud of it, I show it off to everyone who gets to see the passenger side of my engine compartment...   

Brian

[akroyaleagle]

I don't know what kind of bus you have. I have an Eagle. 12V.

I burned out so many converters I finally just installed another pair of lights on the trailer.
That works perfectly. I've done all my trailers that way.

[Zephod]

I have a schoolbus and it all seems to be 12v. Would it perhaps be better to have my own lighting board to tie over any pre existing board on a hired trailer?

[gumpy]

Not sure I understand your question.

If you have split brake and blinker circuits, then I would recommend you install a relay system in the bus. That way, you can pull any trailer
that your bus can handle and won't have issues with the lighting, other than maybe needing a plug converter (6 prong, 4 prong, etc).

The relay system on my website will work for 12v, also. You just need to use 12v relays.

[Zephod]

So... Relays coming off the rear lights with power to the trailer lights supplied by a single 10 gauge wire?

[Oonrahnjay]

So... Relays coming off the rear lights with power to the trailer lights supplied by a single 10 gauge wire?

    That's what I'm planning.  I'm also going to use LED trailer lights to lessen the load. 

[Oonrahnjay]

    Accidental double post ...

[gumpy]

So... Relays coming off the rear lights with power to the trailer lights supplied by a single 10 gauge wire?

Yes, you can use the rear light signals to drive the relay coils. Switch the larger wire supply through the contacts and the outputs drive the trailer plug.

The schematic on my website does the combining of the separate brake and turn signals into a single brake/turn signal for each side.
There are some other schematics on the site for doing that combination, but they are basically the same. Most use less expensive relays
than I do. Mine uses the same relays already found in the MCI buses. Any decent DPDT relay will work.

Search for those other threads. This has been discussed a lot here.

[Zephod]

Thanks. I'm praying 10g will be enough for sidelights indicators and brake lights. I like your idea of having two cables. From my reading, car lights use unusually high amps for low watts output.