Battery fuses

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Talmine

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I am currently rewiring our Biscay 36. The current wiring all works but it is such a rats nest of cables, with new bits spliced on in random places, that if anything failed to work it would be virtually impossible to track down the fault. The current set up doesn't include any main battery fuses and although I've sailed without them until now it seems foolish not to include them in a rewire. My set up is 2 x 105 amp hour AGM service batteries and 1 x 110 amp hour wet engine start battery. These are switched through a standard 3 way. My question is how many fuses I should fit, what rating, and where to fit. I have searched through previous posts and found a link to some 425 amp forklift truck fuses and carriers but am not sure if this is the right approach and capacity.
Any ideas?
Jim
 
Ebay, search for say "100A Fuses" or whatever you need (see VicS post above). Personally I go for this because of the plating and likelihood of good QA for Sterling. But others much cheaper abound.
 
I have Megafuses which are fairly cheap and do the job fine. They're much like the Sterling one linked above except the ends are rings rather than slots so you need to take the nuts right off to change them. The Sterling one looks like a better design that only needs them loosened - but I guess you could modify the Megafuses if required.

Pete
 
Thanks for that. Really useful and I'll make my load calculations. Only remaining issue is how many? A single fuse on the output from the 3 way or One on the common for domestic pair and one on the engine start or one on each battery. Also any idea of the current draw for engine start with a newish 35 hp Beta which I guess will be the major load.
Again thanks for your help.
Jim
 
Talmine said:
Only remaining issue is how many? A single fuse on the output from the 3 way or One on the common for domestic pair and one on the engine start or one on each battery.
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Remember that the purpose of a fuse is to prevent a wire overheating. Every wire should have a fuse "upstream" of it that is rated lower than the wire is.

This principle should allow you to work out where fuses are needed. For example, if you have a wire going to the domestic panel whose rating is lower than what your starter motor draws, you can't protect them both with a single fuse - either the wire will not be protected or the fuse will blow whenever you try to start the engine!

Personally, I have a 100A fuse on the top of the service battery (followed by a cable to the service panel which will carry this much) and a 400A (I think, could be 300) on top of the engine battery. But I don't have a 1-2-both switch to complicate matters.

Pete
 
Thanks for that. I've spoken to beta who tell me my engine can take up to 400 CCA to start so I guess I'm looking at 450 - 500 for that. My other big user is the anchor winch which is seperately protected by a 100 amp breaker so I guess maximum load for everything else would be covered by a 100 amp fuse. time to spend some more money.
Jim
 
Talmine said:
who tell me my engine can take up to 400 CCA to start so I guess I'm looking at 450 - 500 for that.
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I seriously question whether there is any point in fusing at that sort of current on a normal yacht. Under that sort of load the batteries will be depleted fast; voltage (and therefore current) will fall very quickly .... and fuse will not fail.

(If you are intent on fusing, remember that a standard fuse ruptures at about double its rated load)
 
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Vic
That makes sense and probably explains why no previous experience of this size fuse on this or previous boats. I'm not trying to do work for the sake of it only to improve on the system. I think I need to do a bit more thinking.
Jim
 
Not sure of the logic there. The role of a main battery fuse is to protect the wiring from catching fire should there be a massive short circuit, for example a positive cable coming adrift and ending up on a negative cable, or the engine if it is a negative earth. If you connected the positive and negative of a battery directly together with heavy duty cable, surely it is still likely to catch fire before the battery runs out of power - or the battery will explode.

Neil
 
VicMallows said:
I seriously question whether there is any point in fusing at that sort of current on a normal yacht. Under that sort of load the batteries will be depleted fast; voltage (and therefore current) will fall very quickly .... and fuse will not fail.
Click to expand...

My reason for putting fuses on top of the batteries was that there are large, live cables (to starter and alternator) in close proximity to a lot of metal (engine and all its accessories) that is firmly connected to battery negative. No doubt a short from the starter cable to the block would be exciting whatever happened, but my hope was that a fuse would prevent the "excitement" from being terminal.

I don't really have any experience with large currents; are you suggesting that such a short wouldn't blow the fuse? What would be likely to happen instead?

Pete
 
Fuses

It is generally the arrangement in cars (older) and boats that the starter wiring (heavy) is not protected by fuses. The wiring should be short and well supported away from potential combustable material. As said it is likely the battery would die before the wire could become incandescentand hopefully fuse through before doing any real damage. (fire) It raises interesting questions if you are using huge AH batteries for starting small engine so smaller cables.
The circuits for charging and lower current loads (domestic) can be made in lighter wire which will get red hot in a short circuit situation. They tend to be run near to combustable material and should be fused.
Hence 15 or 20 amp is typical fuse rating to protect the wire.
Separating the different circuits and individual fuses is the usual way to manage fusing. good luck olewill
 
I wish I could say I'd decided on the way forward but all this thinking is making my head hurt. The main battery cables are heavy and the new cables I have put up to the new bus bars and switch panel are 16 sq. mm. so reasonably heavy. Any other thoughts?
Jim
 
We have a 100amp Mega fuse on the domestic side - simple to fit and vey cheap as it's standard gear for lots of other applications. The starter battery has an isolator switch and a 600amp mega fuse. Nigel Calder recommended them after being on a boat with a starter battery with a huge short on the engine, he said if the fuse hadn't blown they'd have had to cut the cable with an axe as a last resort. The windlass has a mega fuse as does the bow thruster. They're all very simple to fit, though the number of crimpings involved meant it was worth buying a £55 hydraulic crimper.
 
Talmine said:
Thanks for that. I've spoken to beta who tell me my engine can take up to 400 CCA to start so I guess I'm looking at 450 - 500 for that. My other big user is the anchor winch which is seperately protected by a 100 amp breaker so I guess maximum load for everything else would be covered by a 100 amp fuse. time to spend some more money.
Jim
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That is "in-rush" current and the duration is VERY short on the order of 0.1 - 0.2 seconds. The average starting current for a Beta/Kubota is often less than half the in-rush..

For battery fusing where the battery bank may be used for starting, or on any system with a 1/2/BOTH switch, the fuses should be an MRBF, ANL or Class T style fuse. These fuses are specifically designed to handle motor starting loads and the trip delay curve is more than long enough to handle this without nuisance tripping.

Blue Sea System out of Washington, State USA makes the MRBF style up to 300A and ANL from 30A to 750A they also offer Class T fuses.

I wrote a pice a while abck about battery fuisng and it explains it more in-depth than I can here.

Battery Fusing How? (LINK)
 
Lazy Kipper said:
We have a 100amp Mega fuse on the domestic side - simple to fit and very cheap as it's standard gear for lots of other applications. . . . .
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Right, that fuse is almost certainly too high. If you run 100 Amps on your domestic side, you either have a super yacht or a cruise liner. I wager there is no way will you run 100 Amps even with every bit on domestic demand running. I would seriously reduce this fuse probably to somewhere 30 Amps (but I do not know your equipment loadings). A 100 Amp fuse will be too late, you will see acrid smoke before that fuse blows as the fuse amperage will state the continuous rating not the blow rating!

Lazy Kipper said:
. . . . . The starter battery has an isolator switch and a 600amp mega fuse. Nigel Calder recommended them after being on a boat with a starter battery with a huge short on the engine, . . . . .
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OK, again, this 600 Amp fuse is also well over rated, far, far too high!. A 600 Amp fuse will have a blow rating at somewhere up near 1200 Amps.

The cable from Engine battery to Starter motor is probably rated at 400 to 500 Amps maximum continuous and will start to get hot, go soft and melt long before that fuse will blow!

I suspect your starter motor has a cranking current of somewhere near 200 to 300 Amp and this starter motor current is intermittent not continuous so this fuse can also be possibly reduced to 150 Amp but you will need to check the starter motor current.

Nigel Calder recommended a mega fuse BUT he certainly did not mention 600 Amp? :eek:
 
lenseman said:
Right, that fuse is almost certainly too high. If you run 100 Amps on your domestic side, you either have a super yacht or a cruise liner. I wager there is no way will you run 100 Amps even with every bit on domestic demand running. I would seriously reduce this fuse probably to somewhere 30 Amps (but I do not know your equipment loadings). A 100 Amp fuse will be too late, you will see acrid smoke before that fuse blows as the fuse amperage will state the continuous rating not the blow rating!



OK, again, this 600 Amp fuse is also well over rated, far, far too high!. A 600 Amp fuse will have a blow rating at somewhere up near 1200 Amps.

The cable from Engine battery to Starter motor is probably rated at 400 to 500 Amps maximum continuous and will start to get hot, go soft and melt long before that fuse will blow!
Click to expand...

All of this depends upon the wiring connected to the fuses and it's max ampacity. In a dead short situation the fuses blow quickly well before properly sized wire melts.. I have blown a fair number of them myself while working on boats.

One I blew was improperly sized and was using a 4GA alternator wire connected to a 350A ANL fuse. While working on the alt my clamp accidentally slipped and shorted the live B+ to the manifold. The fuse blew and the wire was perfectly fine despite its max ampacity under ABYC standards being 160A at 100% ampacity and 240A using the 150% rule...
 
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