Wire/Fuse Size & Relay explanations - JeepForum.com
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post #1 of 1 Old 02-21-2007, 10:43 AM Thread Starter
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Wire/Fuse Size & Relay explanations

As for wires/fuses:

I keep getting asked the same thing over and over so here's a reply I did to a post long ago.

Well, you should always go by the manufacturers specs. Since I normally use AWG sized wiring, I use their charts. But basically, to make it simple, here is a simple wiring chart.

Size -- AWG Rating -- My Limit (Normal run) -- My Very Long Run Limit (more than ~12')
10AWG ----- 50A ----- 40A ----- 30A
12AWG ----- 40A ----- 30A ----- 15A
14AWG ----- 30A ----- 20A ----- 8A
16AWG ----- 20A ----- 10A ----- 5A

These are just my conservative limits to the wires based on the AWG rating (rounded down) and then 10A removed again for more added safety because using too large of a wire is never a bad thing for safety electrically.

To that note, you can figure out what you have to go on. So for running two 55W lights, you have 110W of potential draw. Using a worst case vehicle scenerio of 11V = 10A. So you could do it with 16 AWG but I would probably use 14 AWG myself. (I = P / E or Current = Power / Voltage) Click here for my simple Ohm's Law Calculator for current. So normally with the alternator running around 14V will be a 7.8A draw. We use low voltages for a worst case estimate of draw potential. NOTE: This formula does not translate directly for audio amplifiers since they have many other circuits that can use power besides the amplification alone so do not use this formula alone to figure out what fuse an amp might need. Always consult the manufacturer's notes...

So, for our example, you know a 30A relay and a 10A fuse is plenty for that installation. 30A relay cause that's about as low as they come and a 10A fuse because any more current draw would mean a fault in the circuit. And you fuse it by the weakest link in the system, in this case, with 14 AWG wire and a 30A fuse, 20A would be the wire limit, but the circuit only needs 10A so we use 10A.

Now to flip that over to 130W pair of lights, you end up with a 24A potential draw. So definatly 12 AWG wire and a 30A relay and 25A fuse. Some people would even say 10 AWG wire, and that is perfectly safe and fine also. My limits are safe and reasonable. But going even stricter is still just as safe, just a bit more expensive due to heavier wire. For example, I believe NnF pretty much sticks with limits about 10A lower than even mine or roughly half the AWG standard. This is a perfectly safe practice but just understand that it does not HAVE to be that strict. As long as the circuit is in the rating according to the American Wire Guage standards, it should be ok. But take some extra safety like I and others have to assure that you never have a wire-fire. Trust me, they will really ruin your day.

Pretty simple once you get the understanding of it. Also note, that these formulas do not correlate directly with audio equipment. That's slightly different.

Also here's a standard method to wire a pair of aftermarket lights: Quick how-to writeup.

As for relays:

A relay is simple a high-current capable, electronic switch. Nothing more. How it works is extremely simple.

Power goes into pin 30. Pin 87 would be the output when the switch is turned on, 87A when it is turned off. When power is applied to pin 85 and ground to pin 86 (this can be refersed) this switches the output from the off state of pin 87A to the on state of pin 87. The above image shows a good example of the system. If you think if the wire between pin 30 and 87A as a hinge, and the 85-86 as an electro-magnet, turning on the magnet pulls the pin over to output 87. That's actually how some do work actually.

Now the power to switch the relay (pins 85-86) only takes < 1A. So this is a very light circuit and can be tapped into many different combinations of items. The relay itself (pins 30-87/A) can handle 30A (some 40A or even more). Plus you can wire the power the shortest path instead of back to the dash to a switch and out again which is bad. Beside the switch itself wouldn't have this type of current load.

That's all there is to it. With different combinations and uses and wiring methods from the above, relays can be made to nearly dance right off your vehicle. A vital tool for electrical wiring.

Relay Wiring Summary:
Pin 30 - high-current input
Pin 87A - off state high-current output
Pin 87 - on state high-current output
Pins 85-86 - turn-on signal/ground

Other Public Help:

Wiring Schematic Examples:

Wire up a set of aftermarket lights:

Wire up a set of conventional headlights on their own circuit.

Wire up a full custom fuse panel and relay bank for a custom setup

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