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daywilmOffline
Post subject: BMW V12 Alternator Wire Size  PostPosted: Nov 13, 2016 - 10:10 PM
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What size (AWG) wire should I use to run from B+ to the alternator on my v12? The wire that came off the BMW 750 is a 22 square mm which equates to a #4 AWG but I've seen others call for an 8 gauge wire (including most of the wiring harnesses for sale out there). Do I really need a wire as big as #4? Not sure why BMW would use it if it weren't necessary. What have you other v12 builders used?
 
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AdrianBurtonOffline
Post subject: RE: BMW V12 Alternator Wire Size  PostPosted: Nov 14, 2016 - 01:37 PM
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I am using 4

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daywilmOffline
Post subject:   PostPosted: Nov 14, 2016 - 06:15 PM
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Thanks Adrian. I think I'm going to go with #4 as well.
 
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AdrianBurtonOffline
Post subject:   PostPosted: Nov 15, 2016 - 10:24 PM
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how close are you putting the battery?
what all are you running?

If you are keeping it close you will be just fine

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76mxOffline
Post subject:   PostPosted: Nov 16, 2016 - 03:03 AM
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How many amps does a BMW alternator put out? I am thinking about 120 at the most. 8GA is good to 150 or so, 4GA will bring it to about 800 amps. 4GA should also be the battery cable size. Does the circuit continue from the alternator to the starter? If it continues to the fusebox or loops between the fusebox and the alternator and battery, 8GA will power the whole fusebox as well as a 400 watt stereo amp. Use 8GA and save yourself $100 worth of copper.
 
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daywilmOffline
Post subject:   PostPosted: Nov 16, 2016 - 06:34 PM
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Thanks for the replies. I tend to lean towards the side of overkill where wiring is concerned. I'm putting the battery behind the driver's seat and running a #2 cable from the battery to a "thru firewall" connector. This connector (on the engine side of the firewall) will have a cable going to the alternator and 8 gauge wire going to the BMW power distribution box that powers the computers and fuel pump. BMW has two sizes of alternators on the 750il, a 105 amp and a 140 amp. I believe I have the 140 amp. I am not connecting the alternator to the starter. The starter is powered by a separate cable directly from the battery positive post. So, it sounds as if an 8 gauge wire would probably do just fine, however, I happen to have some 4 gauge lying around. I assume it won't hurt to use it.
 
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AdrianBurtonOffline
Post subject:   PostPosted: Nov 17, 2016 - 04:36 PM
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140A will exceed the safety margin of the 8A cable, depending on what you have in the car and when it comes on the inrush current could be 600A or more particularly when the temps go up....

Heck I have built stereo's that I have used 2ga for the amps and 8ga on my subs alone

Just my 2 cents

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76mxOffline
Post subject:   PostPosted: Nov 18, 2016 - 01:22 AM
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If you already have 4GA, then take the advice of my bud AB and use it. Free is a good price. While we are talking about cables, the ground cable is more important. This is off of the top of my head so it may not be perfectly accurate, but the concept is there. Copper wire has a conductivity of about 99%, while a cast engine block is about 40%. Unless the ground goes directly to your starter, 60% of the cranking capacity is lost. This may not be an issue here, but it is a big one for high compression high horsepower applications. I am a dealer for Painless Performance Wiring, but I will still say that there is an excellent several pages of wiring do's and do'nts the Ron Francis Wiring catalog.
 
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AdrianBurtonOffline
Post subject:   PostPosted: Nov 18, 2016 - 04:55 PM
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absolutely correct Charley

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Richard454Offline
Post subject:   PostPosted: Dec 03, 2016 - 07:11 PM
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Why did BMW go with a larger wire when they could have saved some money? Heat. Wire is rated at 70ºF- and the rule of thumb is go up one wire size for under the hood due to the increased resistance of the wire at the higher temperatures.

And as far as the ground? Yes- the other metals in the car are less conductive- but the amount far exceeds a small copper wire...

Richard
 
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