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post #1 of 7 Old 06-16-2011 Thread Starter
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Fuse Positioning help

OK, so there's been a lot of discussion about fuses. I've been working on the plans for some essential electrical system upgrades on our Sabre 34. We're not rewiring the whole boat, just upgrading in certain areas.

1. Adding a battery charger (don't currently have one)
2. Moving two separate Group 31s into a single house bank, then adding a reserve battery for the other bank.
3. Wiring the alternator and battery charger straight to the house bank, and charging the reserve battery using an Echo charger

In the process of these upgrades, we're doing our best to add appropriate fusing to all wiring going to the batteries. Currently, our boat has very few fuses near the batteries, and none on the primary cables. As Maine Sail has mentioned in other threads, all wires going to the batteries should be fused close to the battery.

As much as I read about fusing locations (and wiring), I'm not sure I have it "right". Below is an image which outlines what I think our plan would be--just the part around the batteries really. Red squares represent MRBF fuses, and the yellow squares would be smaller fuses on non-primary cable (like for a bilge pump or battery monitor). Note that specific fuses sizes are TBD. I'm just trying to get the "layout" together at this point.

The idea here is to have a DC Neg bus on one side of the bank, and a DC Pos bus on the other side, so only one cable actually goes to the battery.

So a couple of questions:
1) Do I only need the MRBF on the cable to the first battery in the house bank, or do I need to put an MRBF on both batteries in the same bank? It seems to me it could be one, if wired so a blown fuse would cut the circuit for both batteries, but I could be wrong.

2) As for the MRBF location for the house bank, is it good where it's shown, or should it be on the POS bus where the cable goes to the Perko Switch? Or both?

3) Does this plan look like a good way to do this, or am I way off base in my understanding of where these fuses should go?

Thanks in advance for the help!
-J


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post #2 of 7 Old 06-16-2011 Thread Starter
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Actually, as a follow-up, should the fuses for the battery charger and the alternator be at the source end of the wire, not the battery end? I believe that's what I read in Calder's book.

My other questions still stand of course, just modifying that one aspect of my diagram...
Thanks!
-J

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post #3 of 7 Old 06-16-2011
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J

To answer your last question first. the fuses should be closest to the batteries.

In a parallel bank only one fuse is required, on the positive feed wire to loads.

On the wire going from the positive bus of the house bank to the Perko switch the fuse should be larger - ANL preferred. Alternator output fuse should be an ANL as well. Or MRBF fuses on the bus for each.

The rest looks pretty good.

Brian
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post #4 of 7 Old 06-16-2011
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Quote:
Originally Posted by mitiempo View Post
J

To answer your last question first. the fuses should be closest to the batteries.

In a parallel bank only one fuse is required, on the positive feed wire to loads.

On the wire going from the positive bus of the house bank to the Perko switch the fuse should be larger - ANL preferred. Alternator output fuse should be an ANL as well. Or MRBF fuses on the bus for each.

The rest looks pretty good.
In 98% of situations yes but there are a number of boats out there where half the bank is on port and half on starboard. In order to "protect the wire" it is best in a situation like that to fuse both halves of the parallel bank.

Jos does not need that on his boat as the location will only fit two group 31's and is too short height wise for 6V batts, unless you were to really modify it. So he really only needs one fuse on the house bank.

In marine electrical terms a "source" would be considered AC shore power, inverter, generator or batteries. An Iota type charger, than can act as a power supply when the batts are removed could also be considered a "source" when acting as one.

Anything connected directly to the batteries should ideally get a fuse as close as possible. ABYC calls for 7" but that is darn near impossible on many boats and on your MRBF's may or may not fit under the lid depending upon brand of battery. I know they fit with Deka's because I recently finished 34 MK I with Deka's in it, but it is darn close to the lid. Most battery chargers are either internally fused or have fuses close to the charger so there should also be a fuse at the batteries.

The only small item I might change is to put the battery monitor sense wire on the positive post of the house bank for the most accuracy. Your shunt will go between the DC negative buss and the battery.

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-Maine Sail / CS-36T


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Last edited by Maine Sail; 06-16-2011 at 11:16 PM.
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post #5 of 7 Old 06-17-2011 Thread Starter
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Thanks to you both!

So quick recap:

- Alternator fuse should also be a larger MRBF or ANL.

- Even with the large MRBF fuse on the house bank, I'll need another similar fuse at the POS bus to Perko battery selector switch.
Question: If the wire is the same gauge, wouldn't the MRBF at the battery protect it?
- Wire for battery monitor should be right on the battery post (makes sense).

And one last question for Maine Sail, what size battery cable did you end up using on that S34-I? I'm trying to decide that next.

Thanks again for your help! This is all new to me.
Cheers,
J

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post #6 of 7 Old 06-17-2011
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Thanks Stu, I suppose I need that same "spec" for the Westerbeke W27. That plus the distance of the run to the starter and back would tell me what I need.

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