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Old 09-15-2003, 08:22 PM
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Wiring harness from scratch, fuse values

I DO NOT want to use a wiring harness kit like what painless wiring offers, its not what I am looking for. I have a 1971 BMW 2002 that has a rather scerwed up electrical situation so i have planned to make my own harness entirely custom and more reliable with more features actually within the harness itself.

What I am wondering right now is how do I calculate fuse values for particular circuits? Like for example say that the headlights and a couple of other things (like blinkers, etc) are on the same fuse in the original wiring, but through my own schematic it is different. Just theoretical right now, I havent drawn up any routes for this yet, but I know the stuff wont be all the same. This would change the fuse values, right? How would I tell what the new fuse is that I would need?

Also, stock, my 2002 has a 12 fuse fusebox, which does not have a common power bus. This allows the original wiring to have switches, relays and components (lights, blinkers, etc) on separate sides of the fuse. The fusebox that I am looking at is nearly identical in dimension and has 16 fuses, but with a common power bus for all 16. This forces me to have the power coming in one side for EVERYTHING then all the relays, switches and components on the other side of the fuse. Would this change fuse values itself too or is that not important in the electrical world? I am totally new to this and need all the help I can get and be pointed at good resources.

Thanks a lot, I'll clarify some things if you dont understand what I am talking about somewhere.

on edit ---- does this mean (having a common power bus), mean I would need either two busses or two fuse boxes due to ignition powered circuits and straight battery powered ones? BTW, the fusebox I am looking at is the BUSSMANN RTA Fuse Box
It says it can be available with one or two busses but I'm not sure if the number of curcuits per bus can be custom, looks like it would probably be by multiples of 4 fuses which is the size of each module for the box, if you look at the PDF file you'll know what I mean.


Last edited by norcal02; 09-15-2003 at 08:42 PM.
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Old 09-16-2003, 12:54 AM
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I was allways taught in school for commercial/residential wiring to rate a circuit to 80% or less.In other words a 16amp load requires a 20amp circuit and 12gauge wire.A 12amp load requires a 15amp circuit and 14 guage wire.In other words if ya had to go past 80% you need to run a larger curcuit value.But being as 20A in 110V is as big as it goes if you had to load past 16A in a 20A circuit you needed to start another circuit.
I'm not exactly sure how cars are done,Pretty much I'd just see what size fuse was factory for each device and use the same.
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Old 09-16-2003, 01:14 AM
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Cool, that makes sense. Thanks for the tip about the 80% thing, that helps a lot. Just whatever I can put together knowledge-wise will get me just that much closer.
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Old 09-16-2003, 11:03 AM
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Make sure you use good flexible wire not the extremely stiff stuff. Vibration is a wire killer. If you have the room you might want to consider each wires use. The main circuit should be a heavy duty gauge wire, or even a double wire to spread the load across both. Most cars aren't designed for the loads of big stereo's cel phones and other goodies that people are adding to their cars today. Since your building your own you have the option to allow for all of this now. If your original wire is say a 12 gauge go to a 10 or 8 gauge. Good luck, buy a good crimping tool, and use heat shrink instead of electrical tape were possible. Don't use a lighter on heat shrink, go buy a good heat gun for about 15.00 at a hobby shop, it's worth the $$$

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Old 10-09-2003, 06:24 AM
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One of the best books I have found is "How To Do Wiring" by Skip Readio and published by Tex Smith. It's available over the web or in some larger book stores for around $19.95. It explained everything I needed to go from no wires to eveything is wired and working. It gives you all the values for just about all the different accessories and necessities you'll want to run.
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