O.K. , some of the bow tie buddies (non mechanics) on my Mercedes group are having a discussion about what gauge wire one should use between a 115 ampere alternator and the battery..... One guy says 4 gauge ! (that's 12 volt battery cables in case you didn't know) What say you all ? .
Nate, here's my $.02 on this. All through the 70's and 80's both GM and Ford had a physically bigger alternator that was a 105 amp IIRC for cars with rear window defoggers. These cars have a 10 gauge wire from alt to battery. I wouldn't think 10 more amps would require much more. I think the error in these non mechanics thinking is that they think the regulator is bypassed with the alternator locked into a maximum output mode, very unlikely and would cause the belt to slip and squeal under such a demand. I know when we put an old external regulator alt on the test machine and hit the field terminal with 12v the alt howls and the belt slips. We both know an alternator doesn't make electricity like a generator, it just receives a signal and amplifies it to meet the demand. Used to walk through the junkyards looking for wires in the back window to locate one of these units which we used to make a couple portable battery chargers. An air cooled engine, a short fan belt, a battery, and the alternator all mounted on a pneumatic tired hand truck made an all terrain battery charger/jump starter unit. We used 10g wire and never heated it that I know of.
This is kind of a trick question. The answer all depends on what the normal alternator output will run at. If it is a bone stock setup, leave it the way GM designed it and it works fine. However, if you are running a high end stereo, with muliple amps, airbag suspension with a compressor, high wattage lamps, etc, then I would recommend dual batteries, if possible, and minimum 4 guage wire. I'm not sure on the cars, but I know a lot of the trucks run the alternator wire to a battery jumper location, then to the battery. GM uses a very undersized wire from the alternator on this setup, and it is indeed a problem if any aftermarket accessories are added. It should be upgraded to a bigger wire as needed. Just my thoughts and experiences. P.S. I am running 4 guage from my alternator on my 1950 3100.............Mike
You need to wire it for alternator max current, even if you don't expect to produce that (very often). From my Dad's "Westinghouse Convenient Tables and Formulas" (no date, but it is so old that the address in the front doesn't have a zip code) Allowable Current-Carrying Capacities of Insulated Copper Conductors in Amperes, Single conductor in Free Air based on room temperature of 30C (86F) #6 wire should do the job. #8 wire is rated a max of 100 amps depending on insulation type. At least as important is to have good tight (and preferrably soldered) connectors on both ends of the wire.
larger loads require larger wires. don't forget shorter wires are helpfull as well. the type of wire, corse ves fine should be debated as well since the corse will fail under hi vibration and (i could be wrong) carry a lower amp load vs the fine wire version... i would try to use a larger cable to the battery (or starter cable) in all situations. if your fear of shorting out your large wire is in the way you can allways drop in a fussable link... GT
Do take into account that modern PVC coated automotive wire has a much higher temperature rating for the insulation. Meaning a smaller wire can handle more. Bill is right on the money tho using that book or at least that kind of chart. Over the years industry has gotten even more conservative on wire sizing, tho I don't have my copy of the National Electric Code handy and it doesn't exactly apply to automotive use, but is a good guideline. If you look into the Painless wiring website (or similar) they might have an amperage chart, and what you will need based on their fairly high-quality wire. All wire that is UL listed (or whatever) has to meet the same standard for the insulation, so as long as its rated for such you can use that amperage chart. The course wire will handle less and should be avoided due to the vibration, just like mylow53 pointed out. Fine wire is best...tell them to get some DLO333...ever seen it on a locomotive? Good stuff, but doubt they make a connector for a 1" diameter wire to a 3/8" stud... There is something to be said for duty cycle of the alternator. 99% of the time it won't be loaded that much (and in fact if it was fully loaded I'd expect it to burn out quick, they are only rated for a peak output...the steady state load is much lower...just look at an output chart if the alternator came with one), but in case it ever is its best to be on the safe side. Course look at the rest of the harness...all the wires are pretty small...so there's some 'engineering' that goes into saving every penny on the amount of copper. I don't apply those principles when designing an electrical distribution at work...always want the safe side just in case. Course I deal with the flying public...if something goes wrong with a facility there's more at stake than just a building burning down... Wait a minute...I'm off today. I shouldn't be answering engineering questions!! Back to fight with a 2-cycle lawnmower that won't start...grrr.
