relays / circuit breakers

[Pappa-G]

Ok now I know youre thinking this should be in the "electrical" section but thats under restorations and this is a modification question.
I pulled the black out drive light apart and replaced the tiny bulb with a 28volt 250 watt aircraft landing light bulb. It looked good so i made anotherone and mounted it on the other side. But I want to wire the pair up on thier own switch so here are the questions:
1. Should I use a relay? If so can I use a 12 volt relay and what amp should it be rated for?
2. I'm sure I should use a fuse or c/b but I can only find them rated for 12 volts.
3. I know that when you double the voltage you cut the amps in half so does that mean a 20 amp 12 volt cb is good for 40 amps on a 24 volt sys?
4. what gauge wire should I use?
It's only been 30 years since I had an electronics class and I'm sure it's not like figuring fuel loads for the space shuttle but I would rather not burn my truck to the ground! Thanks ~Pappa-G

 

[cranetruck]

If I understand you correctly, you want to switch a lamp load of 500 watts.
Use a relay rated at 50 amps/28VDC. It can handle a lamp load of about 20 amps, which is what you will need.
Use a circuit breaker like the ones you find on the MV's rated at 30 amps.
The switch need only be rated for an amp or so to energize the relay.
Good mil-spec relays pop up all the time on ebay. Keywords: "Leach" & "aircraft relay". The mil-spec ones have much higher margin of safety and are sealed.

Bjorn

 

[rmgill]

What Bjorn said. He reccomends a larger breaker and relay because even though you're pulling 20 amps total, you're going to be a bit more at switch on and that could be right at the edge. Going up to the larger rated size helps protect every thing. Now, make sure the wiring can handle 30 amps. The breaker must be lower than the rating of the wires you're providing power over.

 

[Pappa-G]

So should 12awg be good enough? I want to power lights not make an electric heater.

 

[cranetruck]

14 awg should be about right.

 

[Pappa-G]

A quick update, I was able to install a continuous use relay for a snow plow ( retail $11.90) The parts place had a 24 volt unit for a golf cart for $64.00 HA!
The 12 volt relay works fine and the lights will smoke the eyes out of any on comming traffic!!
Thanks for all the help ~ Pappa-G