Building a headlamp harness

[jaymzx]

I'm going to be building a high/low/fog 3-relay harness to completely remove the power load from the headlamps from the stock multifunction switch/harness. It will retain stock functionality and improve the voltage drop which will hopefully pick up a few extra lumens. At some time I plan to replace the stock fogs with a 55W set, as well.

Couple questions for those who know:

I plan to put a diode in there to allow the mains (& fogs) to stay on with the highbeams to fill in the 'black hole'. Any heat-related issues with the stock headlamp housing?

Where are you mounting your relays & fuses under the hood?

Also, mains fuse rating: Continuous current, all lights burning, would be (12V^2/55W=2.6 ohms/filament. 13.6V^2/2.6ohms=71W X 2 = 142W total low beams or 10.4A) plus high beams (65W, 2.2ohms, 79W x 2=158W @ 13.2V, 12A) plus 55W fogs=(10.4A) 32.8 amps at 13.6V (assuming a .2V voltage drop in warm weather). Should I choose a fuse rating that is 2X continuous current? At that point, should I just fuse each branch circuit individually, since a 60A fuse would be too beefy for 10A wiring?

To improve the voltage drop, I plan to 'home run' the grounds back to the battery. Any perceived benefit? I can calculate the voltage drop with copper wire, but the chassis has an unknown resistivity.

[jaymzx]

After further research it looks like there's some interesting information regarding fusing. ATO fuses don't blow immediately, they will carry their rated current or above for 100+ hours. Even 30% over can take 10 mins to blow.
Reference:http://www.powerletproducts.com/docu...usesizing.html

Also, the free-air current carrying capacity for 10ga stranded copper wire is 55 amps, however, in a bundled scenario where there isn't air passing around it, a conservative rating is 15 amps. Reference:http://www.powerstream.com/Wire_Size.htm

With that said, I think I will choose to wire it as three circuits from the battery, with dedicated grounds and 15 amp fuses per branch. Why dedicated grounds? Steel has 6.8 times the resistivity of copper. Really puts things in perspective for those who run 4ga or thicker hot cables to their trunk for their amps but use bonded chassis grounding. Ground loops for audio are a completely different subject but wow steel sucks Reference:http://www.stealth316.com/2-wire-resistance.htm

[jaymzx]

After thinking about it some more, the resistivity of steel really isn't a huge factor when passing current though such a large conductor as a car chassis. In a wire format, yea, steel would be bad.