[C320-list] Battery Cable fusing question/conffirmation

[Jeff Hare]

Sorry, this got longer than intended.

We use a 250A ANL fuse block on the ground leg placed *very* close to the aft-most 4D battery. My choice to fuse the ground leg is for safety and simplicity, but here goes:

1. The Negative terminal of all 3 batteries (2 house, 1 starting) are all connected directly together with a heavy common wire to form a common ground.

2. Our pair of 4D AGM House batteries are hardwired in parallel with short heavy jumpers.  (+ to + and - to -)

3. The common battery ground leads to the 250A ANL fuse block which leads to a 400a current sensing Shunt, then to a heavy common Ground terminal lug block (where *all* system grounds terminate)  Ie: the starter/engine ground, the DC breaker panel ground, the windlass ground, the shore power ground, etc.. all connect to this ground terminal bus.  Since all the connections in this area are all grounds,  no accidental wrench drop across any of this stuff could cause a dangerous short.

4. Removing/blowing the single 250A fuse disconnects ALL loads from the ALL batteries including removing the batteries from the starter circuit.  There are no paths through any battery that can bypass this fuse.  To disconnect the batteries after winterizing, I simply remove the fuse.

5. All positive leads from the House bank and the Starting battery go straight to a Blue Sea battery/start/house panel directly (no stops along the way).  All of the positive connections out to the breaker panel, windlass, starter, bilge pump, etc. are connected *here* behind a protected panel.  So, this is a dedicated area where there are only Positive connections made (no grounds present).  That makes servicing this area safer.

I believe it results in a cleaner safer circuit overall.  

So:
** Common Bat Neg -> Fuse -> Shunt -> Heavy Common Ground Bus -> To All LOADS(-)

** All Bat Pos -> Blue Sea Panel (like the 8080) -> Starter(+) & House Breaker panel -> Loads(+)

We have an AmpTech 125a alternator and the heavy positive output lead from this does go directly to the house bank positive terminal, but as mentioned before, the Engine ground (and all grounds) connect to the common negative bus terminal block, not the battery. This ensures that all loads go through the shunt and fuse.  Same applies to the shore power charger.  Those two positive charger leads go to the house battery terminals but the negative lead goes to the common bus.

So, in the event of any serious short at the starter or anywhere, that 250A fuse would blow and shut down the whole electrical system.  I keep a pair of spare 250A ANL fuses zip-tied nearby in case I need them.  After the lightning strike I learned that having more than 1 is a good idea.  The first replacement is there to determine if you still have a problem!  The second spare one is to get it all running again after you've fixed the problem... :)  If you're a trial and error kind of guy, you'll want more than 2.  LOL!

300A as danny uses would be fine also.  Fusing on the Positive side is also common and reasonable but you *may* need one or two additional fuses to accomplish the same level of protection for all 3 battery paths depending on your configuration.  Ie: You can easily have a common negative across all 3 batteries leading to a single fuse but you couldn't do this with a single fuse for all 3 batteries on the positive side.

-Jeff Hare
#809 Woodbine II

-----Original Message-----
From: C320-list <c320-list-bounces at lists.catalina320.com> On Behalf Of danny jensen
Sent: Tuesday, February 26, 2019 4:16 AM
To: C320-List at catalina320.com
Subject: Re: [C320-list] Battery Cable fusing question/conffirmation

I'm using a 300A MRBF on my 2x4d 400ah bank of lead acid and a 250ah MRBF on my Group 27 Starter. Marinehowto.com is the source I used to come up with this I think.  Battery MRBF protect against a dead short and the high values offer lower resistance/voltage drop.
Danny

On Mon, Feb 25, 2019 at 4:10 PM Troy Dunn <troutwarrior at gmail.com> wrote:

> My understanding is that the largest draw by far on a stock C320 under 
> normal operations is the starter motor which can pull over 200 amps 
> for a very brief moment due to the inrush of the motor.  The amount of 
> time, small fractions of a second, is generally no where near long 
> enough to blow an ANL or MRBF fuse of 200 amps however.... the key word here is generally.
>   Therefore it is best to size your fuse to the ampacity of your wiring.
> In the case of 1/0 with a 105 temp rating, I would recommend 250 amp fuse.
>    If you size the fuse too small and the starter does blow the fuse 
> (this is generally called a nuisance trip) then you better have spare 
> fuses and quick reflexes because Murphy's law stipulates this will 
> happen at the least convenient time.
>
> To find the ampacity of wiring just google blue seas ampacity chart.
>
> To find additional info on battery fusing.   Check out marinehowto.com
>
> Cheers
>
> Troy Dunn
> Hull#514
>