Primary 12v fuses - what rating recommended?

West Coast said:
Fitting "mega" fuses to a 12V primary circuit setup.

2 battery set up, with 1/2/both switch. Going to fit the fuses on the +ve wires, between the battery terminals and the switch. Engine is a VP 2003, so circa 1.4kW starter.

Thinking of buying these http://www.iem-services.co.uk/ShowDetails.asp?id=452

Question is - what fuse rating is recommended?
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The fuses in the link should be fine. Obviously you will need 150 amps or more of fuse. So each battery can start engine on its own.
It is often felt not necessary to fuse the high current starter side of the 12v system. This decision might be based on the wiring being short and direct well supported so it can not short and on the concept that the battery may not provide enough current or long enough to produce a fire in the heavy leads. A car starter wiring is not normally fused.
It is however nromal and good practice to fuse at a lower current the wiring to services engine systems etc.
Of course some standards require all wiring to be protected by fuses in which you will need to do it to pass inspection. good luck olewill
 
If you are thinking of fitting a NASA BM1 or similar, the 100 amp shunt will also act as a fuse in the case of a short on the heavy duty cable to the selector switch. Everything from there should be fused/circuit breakers anyway. I haven't fused the engine battery.
 
This is all part of a a rewire project for SIL boat. The house circuits are all fused with small spade type, but thinking would take the opportunity to fuse the main circuits

Thinking of a 250 amp rating ...... seem sensible?
 
West Coast said:
This is all part of a a rewire project for SIL boat. The house circuits are all fused with small spade type, but thinking would take the opportunity to fuse the main circuits

Thinking of a 250 amp rating ...... seem sensible?
Click to expand...

Yes, 250A megafuses would be fine. Be sure to mount them in proper holders, with covers.
 
pvb said:
Yes, 250A megafuses would be fine. Be sure to mount them in proper holders, with covers.
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.you need to work out just why you are using fuses and what they are protecting. Cables that carry the full starter motor current should be rated to cover that load, just as the cables should be at least rated at that load to. Equally the feed to the house circuits would need a fuse related to the capacity of the supply cable and the likely max load. The load should be the lower of the two and there is really no need for a fuse larger than that.
 
GrahamM376 said:
Oh yes it will, ask me how I know :redface-new: If there's a dead short on the battery cables, the shunt will blow before the battery.
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It might blow before the battery but you wont know at what current it will blow. You may well have a fault that does not blow the shunt but allows sufficient current to burn out the wiring or attached circuts and cause a fire.
Afuse should be rated to protect the circuits and wiring attached to it

It is as pvb says very bad advice to suggest using a battery monitor shunt in lieu of a known, and correctly, rated fuse
 
GrahamM376 said:
If you are thinking of fitting a NASA BM1 or similar, the 100 amp shunt will also act as a fuse in the case of a short on the heavy duty cable to the selector switch. Everything from there should be fused/circuit breakers anyway. I haven't fused the engine battery.
Click to expand...

I fitted one, so put a 100 amp fuse on the positive side (shunt on negative).
 
The starter motor on my boat is 1.7kW rated and I use 250A mega fuses (which have never blown in normal use) located as close to the battery terminal as possible. 25 mm2 battery cable (the stuff about 1/4" diameter) will easily carry 250 amps so this ensures the fuse will blow before the insulation melts out or catches fire in a short circuit situation; which I think is all that needs to be considered as all other DC circuits are protected by fuses and circuit breakers anyway.
 
West Coast said:
Fitting "mega" fuses to a 12V primary circuit setup.

2 battery set up, with 1/2/both switch. Going to fit the fuses on the +ve wires, between the battery terminals and the switch. Engine is a VP 2003, so circa 1.4kW starter.

Thinking of buying these http://www.iem-services.co.uk/ShowDetails.asp?id=452

Question is - what fuse rating is recommended?
Click to expand...
Both my Benes have/had 100 amp fuses in the supply to the domestics. The starter is unfused.
S
 
The most recent boat fire I saw was due to an electric short - in one of the domestic circuits it's believed.

I use 50 amp thermal cutouts on the two battery feeds to the bus bar, no fuses there or in the starter and hydraulic motors - the starter pulls considerably less then the hydraulics (85 amps compared to 140.)
Probably the most likely cause of overheating is not covered by fitting fuses, that of runaway battery overheating due to a smart controller and a duff cell in the battery. You can get perilously close to that situation with a shorepower smart charger, especially over 20 amps. (I know having seen it happen).
Those that think that fitting fuses will make everything safe and dandy need, perhaps, to think again and start with a proper risk analysis of their electrical systems, rather than a piecemeal approach to the matter which might lead to a sense of false security.
 
West Coast said:
Fitting "mega" fuses to a 12V primary circuit setup.

2 battery set up, with 1/2/both switch. Going to fit the fuses on the +ve wires, between the battery terminals and the switch. Engine is a VP 2003, so circa 1.4kW starter.
Thinking of buying these http://www.iem-services.co.uk/ShowDetails.asp?id=452
Question is - what fuse rating is recommended?
Click to expand...

If your fuses are 150 amp, they will let through at least 1.8 Kw at a nominal 12v - that's plenty of power to set fire to something if a fault occurs - without blowing the fuse. Put lower rated fuses in and they may blow the day you NEED to start the engine on a cold morning. AFAIK it is not normal to fuse the main battery supplies - just have an effective circuit breaker in place, and fuses on individual circuits off the main supply.
 
jwilson said:
If your fuses are 150 amp, they will let through at least 1.8 Kw at a nominal 12v - that's plenty of power to set fire to something if a fault occurs - without blowing the fuse. Put lower rated fuses in and they may blow the day you NEED to start the engine on a cold morning. AFAIK it is not normal to fuse the main battery supplies - just have an effective circuit breaker in place, and fuses on individual circuits off the main supply.
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That's not the way short circuits work! If there's a short (which is the main danger the fuse will protect against) the current flowing in big cables will be many hundreds of amps, resulting in the fuse blowing instantaneously. I've done it, there's just a soft pfft sound and the power goes off.

Using an under-rated fuse is a bad idea, and could be dangerous.

The reason it isn't normal is that for years boats were built on the cheap, with little regard for system safety. Today, we know better.
 
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pvb said:
That's not the way short circuits work! If there's a short (which is the main danger the fuse will protect against) the current flowing in big cables will be many hundreds of amps, resulting in the fuse blowing instantaneously. I've done it, there's just a soft pfft sound and the power goes off.

Using an under-rated fuse is a bad idea, and could be dangerous.

The reason it isn't normal is that for years boats were built on the cheap, with little regard for system safety. Today, we know better.
Click to expand...

My thoughts exactly. The biggest danger is a "hot wire" effect where the conductor becomes so hot it will melt and drop out of it's insulation and at about that stage, all hell break's loose. That's why I selected a fuse rating to suit the cable I use, as that's what it's protecting. Now I don't have to worry (too much!) about the positive lead chafing against an engine mount, the alternator lead falling off or the starter solenoid falling to bits and shorting out in the process and creating a potentially dangerous short circuit condition.

I favour using a fuse instead of a circuit breaker for a couple of reasons. Firstly, circuit breakers are notorious for nuisance tripping due to sometimes the briefest of current surges (and they tend to progressively get worse the more they trip); And, secondly, the breaker will become more susceptible to failure as it ages just by nature of it's construction. It's probably also arguable that they have more of a voltage drop across the contacts than a comparatively rated fuse at high loads, too. For an application where the circuit protection is never likely to be required, the fuse makes perfect sense to me as the best choice.
 
charles_reed said:
The most recent boat fire I saw was due to an electric short - in one of the domestic circuits it's believed.

I use 50 amp thermal cutouts on the two battery feeds to the bus bar, no fuses there or in the starter and hydraulic motors - the starter pulls considerably less then the hydraulics (85 amps compared to 140.)
Probably the most likely cause of overheating is not covered by fitting fuses, that of runaway battery overheating due to a smart controller and a duff cell in the battery. You can get perilously close to that situation with a shorepower smart charger, especially over 20 amps. (I know having seen it happen).
Those that think that fitting fuses will make everything safe and dandy need, perhaps, to think again and start with a proper risk analysis of their electrical systems, rather than a piecemeal approach to the matter which might lead to a sense of false security.
Click to expand...

I dont consider my decision to fit fuses to be particularly piecemeal, but a sensible added defence IMHO. I have directly heard of at least 3 boat fires started by an electrical short on the main battery cables, hence fitting fuses to protect the primary wiring
 
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