Alternator wire

[wired1236]

Hey guys. Has to replaced alternator in my MP 5.7. It had a 70amp stock in it and I got a new one from MP that's a 95amp that's a direct swap, sort of. My question is that the bat wire going to the starter is only a 10awg so only rated at 55 amps? Should I replace with a 6 awg to handle the 95amps?

[chpthril]

I would certainly upgrade it if it is only 10ga.

[IDBoating]

x2 on ChpThril's advice. In fact, I regularly upgrade cabling in charging and starting circuits when replacing them. As an example, jetskis are often a bit undersized on the cables running between the battery, starting solenoid, and starter, making them more sensitive to batteries that have slightly discharged from lack of recent use. I usually bump those up at least one standard gauge (common example: OEM was 6 gauge, replace with 4) on both negative and positive to reduce the IR drop across their length. In your specific case, 10 gauge is way smaller than I'd use for that application. Definitely upgrade.

[wired1236]

K. I figured so it will be much easier to just run another 10awg in parallel than replace with a larger asg. That should giver plenty of spare capacity. It just doesn't make much sense that they would undersize it right out of the gate. Maybe they figure no one would drain their battery that far to run the alternator at full capacity.

[chpthril]

I would actually lay in a new wire, taking the original out of the circuit, as opposed to laying in a 2nd. That would be 2 ring terminals on the alt stud, which is probably short to start with. You be surprised how little amperage an alt actually puts out, compared to its rated capacity. Takes one hell of a heavy load to get one rev'd up to putting out a high amperage. A 3ft length is 10ga is probably sufficient, for a 70A alt.

[IDBoating]

x2 again on ChpThril's comments. Don't run two wires, for a lot of reasons. The "weakest link" in any cable connection is the terminals at the end, and running two in parallel doubles their number.

Hey guys. Has to replaced alternator in my MP 5.7. It had a 70amp stock in it and I got a new one from MP that's a 95amp that's a direct swap, sort of. My question is that the bat wire going to the starter is only a 10awg so only rated at 55 amps? Should I replace with a 6 awg to handle the 95amps?

I read this earlier and didn't comment on it, but it's been bothering me so.... Your words here say "the battery wire going to the starter". If you really meant that, then that cable shouldn't have anything to do with your alternator's current rating. That wire is only used to start the engine, and all of its current comes from the battery because the alternator isn't generating current until it's spinning. Again, if your words were exactly accurate, the cable you're talking about is a separate topic from the alternator. If it's what you said, then a 10 gauge wire for your starter's current seems WAY undersized to me. On the other hand, if what you really meant was that the cable coming from the alternator is 10 gauge, that's less of an issue because as ChpThril said the alternator won't actually generate 90+ amps except in the most extreme conditions. If you do, in fact, have those conditions then there shouldn't be any 10 gauge cable anywhere in your battery/alternator wiring at all.

Hope this helps and doesn't confuse.

[wired1236]

Sorry. I confused you. I meant the alternator wire going to the starter. It's a 10 awg. My bad.

[IDBoating]

Since the alternator doesn't power the starter, they must be using the alternator or starter as a convenient connection point. So... how does the battery connect to the starter? Does it have its own (hopefully heavier) cable going to the starter? Trying to understand the current flow here.

Whatever the answer to the above question, you should still follow the advice here and beef up those cables. But I would like to know how the battery connects to the starter/solenoid.

[chpthril]

WAB,

Its very common on many boat engines with single battery, for the main B+ to connect to the starter B+ post, as well as the alternator's output cable and helm B+ feed. His main B+ and B- are likely 2/0ga. No need to upgrade these.

[IDBoating]

That's what I expected - that the starter is being used as a convenient connection point. Hope you're right about the battery cable gauge (that's why I asked the question). Interested to hear what he reports.

Thanks!

[wired1236]

Yes. Looks like a convenient spot to get to the battery. It's a factory wire harness so I have to believe most are that way. The cable going to the battery switch is a 2/0 as predicted. Then I have a 1/0 going from the switch to battery 1. (Dual battery setup with ACR)

[IDBoating]

OK, then you have reasonably sized cables actually handling the starter current. Beef up the cable coming from the alternator and you should be good.

[wired1236]

Will do. Thanks guys.