Here is a link to lot's of good info.I am a firm believer in using relays when doing a rewire the first link "Current Topic" spells it out in plain english.http://www.watsons-streetworks.com/tips
One reason the aftermarket stresses relays is the various switches they sell aren't equal to most OEM parts. Relays were fairly rare on vehicles until the mid-70s (with the exception of convertibles or accessory-laden cars) because the switches used could carry the currents involved. So before you throw away the OEM parts, unless you know they're bad, they're probably better than anything you can buy new. Another bee in my personal bonnet is the wire gauges used in aftermarket harnesses. They're nearly always too small, and that leads to problems too. You'll find various charts floating around giving the ampacity of wire sizes and the common error is to run the wire at 100% of ampacity. You should always derate the wire to 80%, that gives you some 'cushion' in case of degradation of any connections or a off-spec part. Also be aware that heat reduces ampacity; whether the heat is from an external source or is caused the connected load, ampacity goes down. One more tidbit; motor loads, whether it's a blower fan, wiper motor, whatever, will try to draw however much current is needed to do the work, regardless of the applied voltage. Where a resistive load like a light will simply get dim if voltage is low with no increase in current, a motor will draw more current until it's using the same amount of power (in watts) at the lower voltage. One last item; don't take a vendors word that a particular bit is rated at X number of amps. I've seen too many who 'cheat' the ratings higher and exceed the OEM suppliers rating. Watson is as guilty as any of them; their '80 amp relay' fed with 10 wire is a fire looking for a place to happen. I wouldn't put more than 30 amps through 10 wire, 25 would be better. For 80 amps continuous load, I wouldn't go smaller than number 4 wire. By the way, I'm a retired electrician and do know what I'm talking about.....
I'm glad you covered that, Steve. I agree with all those points. I wire cars almost every day and am always upgrading the wiring.
Steve, In the intrest or wiring a flame proof car just what is the proper gauge wire to use in it? Fires are expensive, wire is cleap, so what gauge should I buy? Bob
Well here is a chart:http://www.tessco.com/yts/industry/products/itm/automotive/get_wired.html What Steve did not mention is how the length of the wire comes into play,for instance the 10 gauge wire from the 70 amp relay off my ignition switch is about 18" in length and the 12 gauge from the headlight switch to dimmer switch is about 24".
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