By Paul L on Wednesday, April 11, 2001 - 12:35 pm: Edit

I wanted to mount a couple of low power lights (55watts) to my (temporary) Yakima roof rack and didn't want to screw around running wires up thru the firewall, etc etc...The switch will be waterproof and mounted underside the rack, accessible thru the sunroof. Will I need relays and all that stuff? I plan to fuse the lines (obviously) and I want to hook it to the battery. Whaddya think??

By mike on Wednesday, April 11, 2001 - 04:39 pm: Edit

Run Relays. You are talking about a $15.00 part (or less if you shop around). I think a lot of people underestimate the importance of doing this kind of a job correctly. You can easily end up with a real mess on your hands (like starting a fire, frying your alternator, dead battery, etc.).

Run the wires down your front A pilar down to under your hood. I mounted my relays right next to the windshield washer tank. It looks very neat. Use black duct tape to hide the wires if you want something temporary. For a more permanent solution, use heat shrink to wrap the wires and punch three small holes in the black rubber trim and use small zip ties to secure the wire to the A pilar. When you want to remove it, just cut the zip ties. Also, you can wire in waterproof quick disconnects (from Pep Boys) between the lights and the relay to make it easy to remove the lights and wiring harness. This is what I did and it works great.

You will really have a lot of juice flowing through the lights. Many switches are not designed to take that kind of a load. I think is more a function of the number of amps that you are drawing than the wattage of the bulbs. Also, be sure to use the proper guage wire; too small and it will heat up the wire enough to melt it and the lights will not be able to perform to their full potential. Depending on the length of wire run, I think that 12 guage ought to do the trick. You might be able to get away with a little smaller guage though. Most places that sell wire usually have a wiring application chart that will give you the optimum size wire for your application. So be sure to do your homework and I think that you will be VERY happy with your results!

Email me if you want pictures of how I wired my lights.

LoL,
Mike

By Axel Haakonsen (Axel) on Thursday, April 12, 2001 - 03:16 pm: Edit

The wattage of the bulbs do affect the number of amps you draw. If you remember from high scool physics class, the formula is P=UxI, or in english: Wattage = Voltage x Amperage

To figure out how much amperage you will draw, just solve for I, i.e:

Amperage = Wattage/Voltage. Using that formula, each 55 watt bulb will draw approximately 55W/12V, which comes out to about 4.6 amps.

So, for two 55watt lights, your switch will have to be rated for about 10amps, which is a lot.

Most automotive switches is rated between 2-5 amps, which is why we use a relay.

Axel