Battery Fuse Sizing - How, What...?

[OPossumTX]

I have been curious about the accepted practices for marine electrical installations but haven't searched for it. Now I find most of my questions answered right here.

Thanks!
O'

[josrulz]

OK one more question, but it's still related for the most part. Hope you don't mind!

My alternator is a 90 amp Balmar with ARS-5 external regulator. I don't know what gauge the charging wire is currently. Regardless, what should it be for a 5 foot run to the house bank?

According to the Blue Sea chart that Maine Sail posted, that looks like 4 AWG. But when I use the circuit wizard on the Blue Sea site, it comes out as 6 AWG, even up to 100 amps. Of course it will usually be a lot lower than 90 amps from my alternator.

So how do I size the alternator wire? I know the fuse will be MRBF and I'll size it according to the alternator wire size.

Thanks for the additional help!
-J

[Maine Sail]

Originally Posted by josrulz

OK one more question, but it's still related for the most part. Hope you don't mind!

My alternator is a 90 amp Balmar with ARS-5 external regulator. I don't know what gauge the charging wire is currently. Regardless, what should it be for a 5 foot run to the house bank?

You actually have a 10 foot run, there and back. Remember all DC circuits are a loop. There should be a negative wire that is a minimum of 4ga and a positive wire that is 4ga. This will still result in a 2% voltage drop or 0.3V drop @90A. So at 14.6 you'd be seeing about 14.3v at the battery. 2ga would be better than 4ga.

In either case the alternator should be grounded direct to the house bank, or a closely located buss, for best performance. It should not just be connected to the engine ground then back to the bank. Most Balmar's are isolated ground not case grounded. This will help reduce any more VD than is absolutely necessary. Both of these wires, neg & pos, should be led to the house bank for the best performance.

Above all else your ARD-5 regulator should sense the house batteries not the alternator but this needs to be done with care and wired properly. This will eliminate any VD issues as the regulator will compensate the voltage. You'll still have VD in the wire but the banks won't see it because the regulator will compensate for it.

Originally Posted by josrulz

So how do I size the alternator wire? I know the fuse will be MRBF and I'll size it according to the alternator wire size.

You can size it lower than the wire size and still be fine. The fuse should be as close to the batteries as possible. We are trying to get as little VD as possible and that requires large wires. With battery sensing you can get away with smaller wires but bigger is still almost always better.

[josrulz]

Thanks Maine Sail. I just stopped by the boat, and found some interesting details:

First, the current alternator "out" wire appears to be a #10 AWG, and it wraps around the block, and then connects to the starter solenoid. From the starter solenoid, there's a 1 AWG cable that goes to the battery switch on the center post.

Is there any good reason why the alternator would go through the solenoid, or was this more likely a matter of convenience with the wiring harness?

Second, my alternator is only 75 amp, not 90. Though by Blue Sea chart, that's on the line between 4 AWG and 6 AWG, so might as well go with 4.

Third, I can't find a ground wire attached to the alternator that runs back to the ground. I can find a couple wires that supply ground to other things like gauges and the voltage regulator, but it appears that the Balmar is grounded by its mounts. Is this even possible?

If this is getting too far off the topic of fuses, please let me know and I'll start another thread.
Thanks!
-J

[Maine Sail]

Originally Posted by josrulz

Thanks Maine Sail. I just stopped by the boat, and found some interesting details:

First, the current alternator "out" wire appears to be a #10 AWG, and it wraps around the block, and then connects to the starter solenoid. From the starter solenoid, there's a 1 AWG cable that goes to the battery switch on the center post.

Typical factory installation..

Originally Posted by josrulz

Is there any good reason why the alternator would go through the solenoid, or was this more likely a matter of convenience with the wiring harness?

Only that it is cheap and easy for the manufacturer and when boats came with only two batteries and a 1/2/BOTH/OFF it allowed the user to select where the charge current went. Bottom line it is cheap and easy and builders still do that to this day.

Originally Posted by josrulz

Second, my alternator is only 75 amp, not 90. Though by Blue Sea chart, that's on the line between 4 AWG and 6 AWG, so might as well go with 4.

The only thing that hurts by going bigger is the wallet..

Originally Posted by josrulz

Third, I can't find a ground wire attached to the alternator that runs back to the ground. I can find a couple wires that supply ground to other things like gauges and the voltage regulator, but it appears that the Balmar is grounded by its mounts. Is this even possible?

Some older Balmar's had a case ground option but most still shipped with a ground stud.. If there is a ground stud on the back of the case make good use of it....

[josrulz]

Originally Posted by Maine Sail

Only that it is cheap and easy for the manufacturer and when boats came with only two batteries and a 1/2/BOTH/OFF it allowed the user to select where the charge current went. Bottom line it is cheap and easy and builders still do that to this day.

...The only thing that hurts by going bigger is the wallet..

Thanks again, Maine Sail! OK, so when I rewire, I can skip the route to the solenoid, and run my 4 AWG output from the alternator directly to the POS busbar or battery--where the MRBF fuse will be. I plan on a 125 amp MRBF for the 4 AWG wire.

Originally Posted by Maine Sail

Some older Balmar's had a case ground option but most still shipped with a ground stud.. If there is a ground stud on the back of the case make good use of it....

I did some more research--I think the alternator is a 9075, which did indeed come with the case ground option. I will check if there is a ground stud though. If there is, I'll run 4 AWG cable to the NEG bus bar.

Thanks again!
-J

[josrulz]

Maine Sail, just a couple of quick questions...

First, I have the bus bars and MRBF terminals in hand now, so I've been scoping out where to place them. I've seen that pic of the S34 battery compartment, where you have the NEG bus and shunt along the after bulkhead of the battery compartment. Do you mind me asking--where did you put the POS bus? Is it opposite? That looks like a tight spot on our boat.

I'm thinking of putting the MRBF fuses on the bus bar, only because the clearance above the batteries is tight. They appear to fit, but they are very close.

Second, the POS studs on my batteries are 5/16, not 3/8. Is this going to be an issue with the MRBFs since they're made to mount on 3/8 studs?
Thanks!
-J

[Maine Sail]

Originally Posted by josrulz

Do you mind me asking--where did you put the POS bus? Is it opposite? That looks like a tight spot on our boat.

Water heater compartment works well.. Just drill the wires through the bulkhead.

Originally Posted by josrulz

I'm thinking of putting the MRBF fuses on the bus bar, only because the clearance above the batteries is tight. They appear to fit, but they are very close.

With some batts they will clear but with most they won't. The buss bar is fine joust route the POS into the water heater compartment.

Originally Posted by josrulz

Second, the POS studs on my batteries are 5/16, not 3/8. Is this going to be an issue with the MRBFs since they're made to mount on 3/8 studs?
Thanks!
-J

Many are and Blue Seas does not make a 5/16" MRBF, been round and round with their engineering department on this.

Blue Seas says it is perfectly safe and "acceptable" if your post has a wide enough base to use a 3/8" on a 5/16" stud.. Personally I would rather see a 5/16" MRBF but they simply don't exist..

[josrulz]

Thanks on all counts, Maine Sail! I really appreciate all you do here and in other forums.
Cheers!
-J