Fusing at Batteries/Switch Help Please

[abraxus]

Having just got some new batteries and a charger I'm now looking at wiring and fuses and what I need.

The 40 amp charger is connected to each battery bank with 10mm cable, which has 50 amp fuses near each battery.

The battery banks are one at 125ah and the other at 500ah. Items connected directly to the batteries (bilge pumps) are fused near the battery.

Each battery bank then has 50mm cable (2.5-3ft long) going to positions 1 and 2 on a 1,2, both, off switch. Connected to the common post on the switch is then the starter cable and what looks like 10mm cable for the house electrics.

My question is what fusing do I need from the batteries and/or switch?

I was thinking of putting a 50 amp fuse after the switch on the house load cable, would that be correct?

Also do I need any fuses on the two 50mm cables from the batteries to the switch? Given that either bank might be used for starting then these would have to be pretty hefty I imagine (5.7L Mercruiser). What about a fuse after the switch on the starter cable, would I need one there as well?

 

[VicS]

Having just got some new batteries and a charger I'm now looking at wiring and fuses and what I need.

The 40 amp charger is connected to each battery bank with 10mm cable, which has 50 amp fuses near each battery.

The battery banks are one at 125ah and the other at 500ah. Items connected directly to the batteries (bilge pumps) are fused near the battery.

Each battery bank then has 50mm cable (2.5-3ft long) going to positions 1 and 2 on a 1,2, both, off switch. Connected to the common post on the switch is then the starter cable and what looks like 10mm cable for the house electrics.

My question is what fusing do I need from the batteries and/or switch?

I was thinking of putting a 50 amp fuse after the switch on the house load cable, would that be correct?

Also do I need any fuses on the two 50mm cables from the batteries to the switch? Given that either bank might be used for starting then these would have to be pretty hefty I imagine (5.7L Mercruiser). What about a fuse after the switch on the starter cable, would I need one there as well?

Ideally mega fuses on the connections from the batteries to the switch and a 50 amp fuse in the connection from the switch common terminal to the house distribution panel.

That may well stilll leave engine wiring that is not protected by appropriate fuses. What you do about that depends on how far you want to pursue the matter and just how,may fuses you are prepared to end up with, but I have not looked a the engine wiring diagram so make no further suggestions

 

[abraxus]

Thanks Vic, that was sort of what I figured, however it does leave me with a question about fuse size.

The mega fuses need to be low enough to blow before the wire from the battery to the switch, and yet high enough not to blow when starting the engine. Does anyone have any idea what size of fuse would be sufficient for a 5.7l engine starter but capable of protecting the 50mm battery cable?

 

[Rigger Mortice]

When I looked into it a small engine starter could use approaching 100 amps so a bigger engine may well be a good bit more than that. An ordinary car battery is designed to be able to push out more than 400amps for cold starting and, whilst it probably doesn't need to deliver all of that, it gives an idea of the kind of currents involved. Heaven knows what a 5.7 litre engine would need.

In the end I didn't bother fitting a fuse for the starter.

 

[abraxus]

Thanks Rigger, my intention is probably not to fit a fuse on the cable from the battery switch to the starter.

However there are also the cables from each battery bank to the switch. Either of these could provide power to the starter, but also to the house loads. If I fuse those (which I assume I should) I need to take into account the starter's power draw. Ideally I suppose I need a fuse that can withstand a high load in short bursts but blow before the battery cables are affected if there's a short, however I'm not sure how to size that.

 

[pvb]

Thanks Rigger, my intention is probably not to fit a fuse on the cable from the battery switch to the starter.

However there are also the cables from each battery bank to the switch. Either of these could provide power to the starter, but also to the house loads. If I fuse those (which I assume I should) I need to take into account the starter's power draw. Ideally I suppose I need a fuse that can withstand a high load in short bursts but blow before the battery cables are affected if there's a short, however I'm not sure how to size that.

I have a 250A MegaFuse next to my battery bank. On the one occasion when I stupidly shorted out the alternator terminals, the fuse blew virtually instantly. If your starter is particularly meaty, you could use a 400A or 500A fuse - if you have 50 sq mm cable the fuse will blow long before any cable damage.

 

[abraxus]

Ok thanks. So it seems that 400A or 500A fuses at the batteries would be appropriate for the 50mm battery cables to the 1,2,both, off switch.

Now for the cables from the switch. It seems after further investigation and a look at a Mercruiser wiring diagram, I got it wrong. The 10mm cable from the common post isn't for the house electrics and instead goes to the power trim pump, which I understand has a 110A fuse at the pump (I'll check). The other cable (looks like 35mm) goes to the engine wiring harness which in turn feeds the starter and also all the other electrics, including the house loads. On the wiring diagram (and on the boat) there's no fuse at the switch end, there is however a circuit breaker on the engine to protect the engine wiring harness. Is this all I need or do I need something at the switch end?

Thanks again with helping me get to grips with this.

 

[pvb]

Fuses protect cables against short-circuits causing fire. So fuses need to be installed at the battery end of each cable.