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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #11  
Old 06-04-2013
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Re: Alternator Output Wire

Quote:
Originally Posted by mitiempo View Post
Lugs for 4 awg are available with holes as small as a #10.
Mitiempo - that is my problem. I will re-check, but I am sure the "post" on the back of the Mitsubishi alternator is a #8.
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Old 06-05-2013
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Re: Alternator Output Wire

I don't recall seeing any smaller than #10. Even if it is #8 using a #10 connector will work.
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Old 06-05-2013
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Re: Alternator Output Wire

Neither do I. Here are 8 Awg to #10, heat shrink:

http://www.mcmaster.com/7036K71
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Re: Alternator Output Wire

I found my answers - go with #10 lug.
I also noticed that my choice of 4awg is not so good....
I visited Maine Sails post on battery fuses where this is discussed and it should be 2awg not 4.

Thank you all once again
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Old 06-05-2013
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Re: Alternator Output Wire

Wait I misread. How did you arrive at 2awg for your alternator? How big is your alternator?

The alternator wire should be sized for the alternator output, not the battery or starting current, and should be fused at the battery. For example I have a 30A alternator and I used 10awg wire from the alt with a 40A fuse at the house battery.

Do you really have a 100A+ alternator?

EDIT: Ok now I see that there is a chance you're talking about a combined starter/alternator wire (going between the solenoid and the batteries/switch)? If your alternator is connected to the batteries via the solenoid then yes, the run between the solenoid and the batteries needs to be large and 2 awg is probably right.

However the small jumper between the alternator and the solenoid, which is what you're asking about still wouldn't need to be 2awg because it's only intended to carry alternator current and is limited to the output of the alt (technically it would be proper to fuse this at the solenoid, but for such a short run that's usually skipped, I know it was on my boat).
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Last edited by asdf38; 06-05-2013 at 02:23 PM.
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Old 06-05-2013
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Re: Alternator Output Wire

He's removed that jumper. That's one of the purposes of the entire changeout.
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Re: Alternator Output Wire

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Quote:
Originally Posted by asdf38 View Post
Wait I misread. How did you arrive at 2awg for your alternator? How big is your alternator?

The alternator wire should be sized for the alternator output, not the battery or starting current, and should be fused at the battery. For example I have a 30A alternator and I used 10awg wire from the alt with a 40A fuse at the house battery.

Do you really have a 100A+ alternator?

EDIT: Ok now I see that there is a chance you're talking about a combined starter/alternator wire (going between the solenoid and the batteries/switch)? If your alternator is connected to the batteries via the solenoid then yes, the run between the solenoid and the batteries needs to be large and 2 awg is probably right.

However the small jumper between the alternator and the solenoid, which is what you're asking about still wouldn't need to be 2awg because it's only intended to carry alternator current and is limited to the output of the alt (technically it would be proper to fuse this at the solenoid, but for such a short run that's usually skipped, I know it was on my boat).
Yes, I removed the alt-starter jumper.
The engine is a Westerbeke 30B3.
Now I am wiring the alternator output directly to the battery - fused at the battery.

So the alternator output @ 24 feet (12X2) 51amp alternator = 2ga cable;
The starter cable to the battery, also fused will be 2/0 ga.
Did I get this right?

Edit: The stater cable will go to the battery switch - not to the battery.

Last edited by SVTatia; 06-05-2013 at 02:45 PM. Reason: Correction:
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Old 06-05-2013
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Re: Alternator Output Wire

With an internally regulated alternator you really want the largest wire you can use that will result in the least amount of voltage drop. This is because they sense voltage at the B+ terminal on the alternator BEFORE any voltage drop issues.. Voltage is pressure and you want to maintain and drive that pressure/voltage up to get the batteries to accept as much current as they can.

A 6% VD at 14.4V will result in the batteries only seeing 13.5V

A 4% VD at 14.4V will result in the batteries only seeing 13.9V

A 2% VD at 14.4V will result in the batteries seeing 14.1V

14.1V will result in awfully SLOW charging for typical flooded or AGM batteries.....

Factory charging circuits are notoriously horrible for deep cycling applications where high current charging is a must..

If we figure the typical round trip to the batteries on many boats is 22-25 feet then we can easily calculate for a 60A alt.

If we figure 45A when hot (I'm not even using the 60A rating) then it is easy to see that you would need 2GA wire on a 45A current to maintain no less than 14.1V at the battery terminals.... 14.1V is still far too low to recharge quickly and your alt will be under performing.

This is one area where an external regulator that physically senses the battery terminal voltage really aids in charging performance. The can compensate for voltage drop in the wire. Instead of sensing BEFORE the wiring it senses AFTER the wiring at the battery terminal...

When we had our internally regulated 50A Mitsubishi alternator on our boat I modified it with 1/0 wire. We had no issues getting 14.3V to 14.4V.... With the factory wiring we could barely get beyond 13.8V and it took a looooong time to charge. Voltage drop in charging circuits is perhaps the worst place for it...

We now have a 140A alt that can run at 130A all day long. The regulator directly senses the battery terminal voltage and still uses 2/0 pos & neg wire.....

The issue of folks undersizing alternator wires got so bad Balmar began shipping alternators with these tags on them despite having a regulator that can somewhat compensate for voltage drop..

This is 3% minimum even with an external regulator. With an internal regulator, that senses the B+ terminal, you ideally want to aim for better than 3%.....

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Last edited by Maine Sail; 06-05-2013 at 03:07 PM.
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Old 06-05-2013
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Re: Alternator Output Wire

Ok I basically retract what I said. Damn I've been bitten by this a few times. I design circuit boards where I have to calculate capacity and heat all the time, but never worry about distance or voltage drop, so I've failed to account for drop properly in a few cases on the boat. I.E. I know enough to get myself into trouble.

Looking at west marine's chart I come out with 4awg for 50A at 24 feet.

The only final comment, is that technically because your return path (negative) is using the 2/0 (right?), the voltage drop calculation isn't as simple as distance multiplied by two. Because the two sides are different. In this case they actually come out in your favor. So with 4awg on one side, and 2/0 on the other, you'll be coming out better than the chart would otherwise suggest.

EDIT:

So for example if you've got 12' of 4awg and 12' of 2/0 awg the number come out like this

2/0: 12'*0.000077 ohm/ft = 0.000924 ohms
4: 12' * 0.00025 ohm/ft = 0.003 ohms

That's 0.000924 + 0.003 ohms or 0.0039 total ohms for a drop at 50A of 0.2V which is only 1.66%. So it would sound like 4 is fine and you could consider moving to smaller wire if you really wanted.

Also, consider whether it might be helpful if you wired the alt to the house battery where the house battery connects to your battery switch. Because the battery is wired to the switch with 2/0 (I assume) this means that another part of the path is heavy gauge 2/0. Depending on locations this may work out in your favor in terms of both voltage drop and wire lengths.
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Last edited by asdf38; 06-05-2013 at 03:42 PM.
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Re: Alternator Output Wire

I think I have it, for now...

I will start the arduous task of ordering - that stuff is dear!
I will not skimp, and have searched everywhere for the best cable prices and no one was able to beat Greg's. I will have to delay the lugs purchase to after I have everything together, as studs, bolts, bus bars, all have different sizes. I will also splurge and buy the FTZ crimper at KL Jack, I figured it will pay for itself.

Thank you all.
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