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wire size

Posted: Wed Jul 12, 2017 10:12 am
by ddependo
Replacing old alternator with one wire 100 amp. I'm thinking I should use # 8 gauge wire & won't need the voltage regulator & need to update to 100 amp breaker.
Should I run # 8 to the switch or just to battery or hot post on solenoid?
Thanks, Wayne

Re: wire size

Posted: Wed Jul 12, 2017 4:27 pm
by honestcharlie56
I would get a new breaker and run it directly to the positive post, but thats just me. Someone with more knowledge will hopefully chime in.

Re: wire size

Posted: Wed Jul 12, 2017 4:36 pm
by barkleydave
#8 should be just fine if run is 4ft or less. Single wire go straight to positive on battery bank. Use a 100 AMP blade fuse which is preferred to a breaker. Locate as close as possible to the positive terminal.

Here is a great guide: ... ctions.pdf Only a guide for wire size. the Alternator in the article is NOT approved for marine use!

safe boating,

Re: wire size

Posted: Thu Jul 13, 2017 11:37 am
by ddependo
Thanks for the good info.

Re: wire size

Posted: Thu Jul 13, 2017 2:49 pm
by bcassedy
(Not to muddy the waters, but....). One wire alternators are dependent on a good ground between the mounting tabs and the engine block. Given the high moisture content of the air in most engine spaces, I'd be concerned about eventual corrosion between the mating surfaces, reducing the effective ground contact (and thus, the alternator's end output). Would it not be a good idea to run a ground cable <like gauge> (copper or brass "eye" terminal'd) from the back of the alternator tabs (coated with dielectric grease to minimize corrosion) to the grounding lug in the engine compartment?


Re: wire size

Posted: Thu Jul 13, 2017 5:29 pm
by barkleydave
No muddy waters here! Good advice. Running a grounding wire to the engine block is more than sufficient. Your engine is grounded to the battery directly or to the stringer. No need to run to grounding lug on bulkhead. (That poor lug has enough to do as it is :)

My one wire alternators are both have a ground wire added. One note: Using dielectric grease should only be used on the outside of the wire connection. Dielectric grease is non-conductive. Now there is a Carbon-electric grease which is black and messy which is loaded with carbon graphite which is conductive. My alternators are going strong after 12 years. I do check for clean connections. Keeping a dry bilge will also greatly reduce corrosion of electrical connections.

save boating,

Re: wire size

Posted: Thu Jul 13, 2017 10:09 pm
by BlueSkye
I went to a higher amp rated alternator to pump up my house bank while I was cruising. It didn't go so well at first. Tech support told me that rating isn't for continuous duty. I ended up putting a resistor on the output to tone things down a bit. I now have an inline fuse (and the resistor) that is much less than the advertised rating of the alternator.

Re: wire size

Posted: Fri Jul 14, 2017 9:41 pm
by EWRice
Alternator/generator ratings and controls are a mystery to many, even good mechanics. I had the good fortune of working in a shop with a starter/alternator rebuild department. This was extremely beneficial in my line of work. Here are a few valuable points I learned on alternator upgrades.

1- Alternator output ratings can be deceiving for most applications. All output ratings are max output. Until very recently, most alternators needed to turn high RPM to generate this amperage. Most have a lower output at idle than a stock alternator. The higher the max output, the lower the output at idle and just off idle. I have found the 85amp 1 wire 10si to be the best of both worlds. We saw this alot with boats and plow trucks. People wanted more power to recharge batteries. We found that in most cases, going to HO alternators made the situation worse because most boats spend most of there time at idle or just above, and plow trucks stay at lower rpm with short bursts of throttle.

2-Going along with above, you are generally better with more reserve capacity in your batteries than higher alternator output. A 27 or 31 series battery, in most cases, will not take more than a 30 amp recharge. Even that would be a very drained battery. Most average around 10 amps. You would have to have a lot of electrical equipment running to need more than 50 amps.

3-With only a few exceptions, all modern one wire alternators are AC turn on regulators. These are far superior to the earlier. AC regs mean that it uses the residual magnetism in the stator (ac side) to excite and turn on the alternator once it begins to spin. These turn on at very low rpm unlike the earlier ones that had to be revd up to excite.

4-An alternator, internal or external regulated, or generator will only work as good as it is wired. Paint is your enemy. You have to have good grounds and power connections. Batteries are the worst place for connections because of corrosion. The only connection at the battery should be the battery cables, period. Everything else should be connected at terminal blocks, disconnect switches, or starter solenoid.

Blue sky, there is no reason you should have had to install a resistor in your alternator wiring. Something is not working right. I would suspect you have a bad ground to the alternator or a shorted regulator or rotor. Was it putting out too high a voltage? If this is the case, it will eventually burn up your alternator, even with the resistor.

Hopefully this info helps others.