What size relay
 

I am trying to wire 3 100W KC Hilights. They come 2 to a box so I ordered a single light so I can mount three on my light bar. The directions say not to wire more than two 130W lights using the supplied wiring harness. I assume this is due to the supplied relay. If this is so how can I determine what type of relay I need and where can I get one? If it is for another reason, what is it?

Any help would be greatly appreciated.

It is likely for the size of the relay, and for the size of the supplied wiring. The key value is the current rating of the relay. A 130 watt light will draw 10.8 amps at 12 volts--you'll need some fudge factor because the bulb isn't always 130 watts and the voltage isn't always 12 volts (more typically 14 volts, but then there is voltage drop across the wiring run and ... and we'll just add fudge factor at the end). So, two 130 watt bulbs draw about 22 amps. Three 100 watt bulbs draw about 25 amps. So you've exceeded the rating of the relay.

You can get a larger relay, and you can upgrade the wiring. I'd recommend a 30 or even a 40 amp relay--price difference is negligible. I caution you that there is a lot of cheap Chinese junk out there that is "optimistically" rated--stick to U.S. made if you can find it, or go higher and pray you don't get some piece of junk that will catch your Jeep on fire--40 amps. You're looking for fan/horn relays, BTW. Well, you don't have to, but they work great in vehicle engine compartments.

As for wire, current rating depends on a variety of factors including size, whether it's bundled (better heat radiation), type of insulation (some melts at higher temperatures), and length (very important--longer wire has more loss which means more resistance which means less voltage to load and more heat dissipated--a winning combination to yet another blaze of glory death for your Jeep). You can find some websites that will help you to judge wire size needed--again, go a bit conservative once you figure all of that out.

Hope that helps!

Here's a good chart assuming that your run is reasonably short (say to the front bumper or the windshield light bar; not to the top of a flag pole, LOL)...

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire breaking strength

It would seem to suggest 14 ga for your load. I used 16 ga for my four 130 watt windshield lights, two lights on each of two separate circuits--not all four on one circuit--and have had no problems.

Your ampacity is dependent upon what type of insulation your wire has. That table ^ doesn't take this in to account. I would go off of table 310.16 of the national electric code and use at least #12 for 25 amps. This is for a wire that has an insulation rating of 75*C or 167*F. This ampacity is based off an ambient temperature of 86*F so you may need to upsize to compensate for this as well.

I guess the OP is using three 130 watt lights. Three 130 watt lights would draw 33 amps. FWIW, I'd use #8 wire here to minimize voltage drop and keep the lights as bright as possible. The relay would probably be rated for some round number like 35, 40, or 50 amps. Two relays having a lower ampacity could also be used in parallel.

X2. I should have actually done the math instead of quickly skimming through the numbers of someones example.

The three lights are 100w each not 130's.

As for relays in parallel, I recommend against that because if one fails, the other is carrying the entire load. You could wire up with two relays and one parallel control circuit and two independent output circuits, one powering one light and the other powering two lights. Or even three relays and output circuits, one for each light. But, that's a lot of complication when high amperage relays are easily and cheaply available (I have a nice 80 amp fan relay to turn off my engine fan for water fording).

http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

And this is my source for wire (local company) and the wire I use (higher temperature rating than what KC provided on your wiring harness) ...

http://www.spectrowireandcable.com/c...L-Primary-Wire

That's what I meant by parallel relays. I did not explain it completely as you did. Tx.

So they are. Got mixed up between your original post and the following posts. So, you are looking at a nominal 25 amp load. 10 Ga wire will work fine, and the relay should be rated for 25 amps or more.