I don't understand alternator wiring sizes

[gozierdt]

OK, I admit I'm a "Mechanical" Engineer, but I don't understand why a 100 amp alternator only needs about a 10 gage wire, instead of a #1 or #2 cable. I know i= e/r, so if you have a low resistance you can run a large current. But I always thought starters only took a 100 amps or so. How big is the starting current? Is that why you need such large cables for the battery?

 

[bruceb58]

Re: I don't understand alternator wiring sizes

If you have a 100A alternator, I would step up a size depending on the wire length run. Most boats don't come with 100A alternators so that is why you have a 10 gauge.

Yes, starters draw way more than 100A.

 

[gozierdt]

Re: I don't understand alternator wiring sizes

Yes, I went up to a 100 amp alternator. I'm going to run it to the batteries through an isolator. Using this ABYC Wiring Standard

http://www.acbsphl.org/Tips_and_hints/ABYC_Wiring.htm

, it looks like I should go to 6 gauge wiring to the isolator, and from it to the 2 batteries. Do I need to keep the original red 10 gauge wire connected to the alternator, or just run to output directly to the isolator? I looked at the generic wiring diagrams, but didn't see enough detail to tell if I still need the original wire.

TIA

 

[bruceb58]

Re: I don't understand alternator wiring sizes

Sure 6 will be fine. You don't need the original wire.

Personally, I don't like using isolators because of the voltage drop. I prefer ACRs

 

[gozierdt]

Re: I don't understand alternator wiring sizes

Thanks Bruce, I've been thinking about an ACR instead of an isolator as well. If I go with an ACR, it looks to me (from the Blue Sea website) that I'll run the alternator output directly to the starting battery, then to the ACR as shown on the BlueSea Website (see attachment).

Then I'll wire in the starter switch to the ACR to create the start isolation, and move all the instrument leads over to the second battery, and leave all the engine connections on the Starting battery, correct?

Sorry if I'm asking questions with obvious answers, but I'm not that well versed in engine wiring.

 

[bruceb58]

Re: I don't understand alternator wiring sizes

Are you doing this on the 20' boat in your signature? I would just leave everything where it is and have a switch that switches between two batterries. The only reason I would do different is if you have heavy hitters that need to be on their own battery. If thats the case you can ajust have them come off the other "deep discharge" battery. For small boats, there really is no reason to move everything over to the second battery. Not worth the effort.

 

[gozierdt]

Re: I don't understand alternator wiring sizes

Yes, it's for the 20' Sea Ray. And I don't have any really heavy usage electrical items on board- only a radio and depth finder. I'm looking for the best reliability in terms of starting, so I'll plan to use the ACR without the automatic battery isolating function. I already have a Perko battery switch. I'll leave that normally on "1", and depend on the ACR to keep the second battery charged. Sound like an OK plan?

And thanks again for all your help.

 

[bruceb58]

Re: I don't understand alternator wiring sizes

That's exactly what I do. Both of my batteries are deep discharge so I really don't worry about the draw down. If one of your is a start battery and other is a deep discharge, you cal always switch to the deep discharge battery when you think you will be discharging for awhile while the motor is off.

 

[JustJason]

Re: I don't understand alternator wiring sizes

The other thing to keep in mind with alternators is that the rating of it is peak, but they typically will only put out 50-75% of their rated peak all of the time without melting down.