How big should my starter motor fuse be?

[demonboy]

I'll be ordering some fuses from West Marine shortly but I'm unsure what size fuse for my starter motor to starter battery and I'm not sure how to measure this. The engine is a Perkins Prima M60 (59 hp). I have a Perkins service manual but not the specification of the starter motor itself. I know there was some talk of applying the corrrect fuse to the WIRE rating as opposed to the rating of the amps from the source. Does that apply here?

 

[NickRobinson]

While chunky fusible links are available, do battery to starter cables have fuses?

I may be behind modern practice (old cars, older boats!) but I've not seen any-

Fused energising circuit (via ignition switch) another matter-

Nick

 

[prv]

While chunky fusible links are available, do battery to starter cables have fuses?

Mine does, but I think it's relatively uncommon in this country.

Can't remember the size of mine offhand; 250, 300, and 500 amps all ring a bell.

Pete

 

[PCUK]

No fuse required.

 

[demonboy]

OK, thanks. Perhaps then you could tell me what fuses you have where on the main circuit? Obviously I have my circuit breakers but do you have anything between charging units, isolating switches and batteries?

 

[pvb]

My 43hp Volvo's starter circuit has a 250A MegaFuse next to the starter battery. For your bigger engine, you might need a 300A fuse.

 

[Sandyman]

OK, thanks. Perhaps then you could tell me what fuses you have where on the main circuit? Obviously I have my circuit breakers but do you have anything between charging units, isolating switches and batteries?

Hi Jamie

You say you have cct breakers but do you not have a main breaker that these are fed from ?
A main breaker that may typically be in the range of 50 amp ?

Cant see the need for a fuse for your starter motor. What purpose would it serve ?

 

[demonboy]

Cant see the need for a fuse for your starter motor. What purpose would it serve ?

Dunno, I misunderstand the need for fuses, obviously!

I have a main circuit breaker for all my 12v electrics via the control panel. What I am trying to do is establish what fuses I need based upon SJF's now infamous wiring diragram he put together to show how he installed his Sterling alternator charger.

PVB is saying he has a fuse on his starter circuit, whereas others are saying this is not required. So my next question is what other fuses should I be considering on my charging circuit (anything inbetween my alternator/sterling/batteries/isolator switch).

 

[CreakyDecks]

I suppose it depends how things are wired up. If your starter is wired to a huge battery bank it might be a good idea to have a fuse. On a car, or if you have a separate starter battery, I don't suppose it really matters because if there was a short at the starter the battery would probably be dead before the wire burned out. Car batteries today seem smaller than I remember!

 

[pvb]

PVB is saying he has a fuse on his starter circuit, whereas others are saying this is not required. So my next question is what other fuses should I be considering on my charging circuit (anything inbetween my alternator/sterling/batteries/isolator switch).

Fuses protect wires. Specifically, they prevent the wire overheating and causing a fire in the event of a short-circuit. In years gone by, they were rarely fitted to boats. Today, especially as owners upgrade their electrics, fuses are being fitted. People who say they're not needed may be correct - in the same way as fire extinguishers aren't needed. People who say fuses aren't fitted to car starter circuits overlook the fact that you can walk away from a car fire.

The real danger on a boat is a short-circuit allowing the battery or batteries to overheat the wire. For this reason, fuses should be fitted as close as possible to the battery end of each wire which is being protected.

 

[demonboy]

People who say fuses aren't fitted to car starter circuits overlook the fact that you can walk away from a car fire.

I like your thinking. I will be ordering some fuses. Didn't realise that the fuse blocks cost as much as the fuses though!

So, how about fuses for the domestic battery? Is that required too? If so, what rating? As big as the biggest pull (i.e. windlass)?

 

[pvb]

So, how about fuses for the domestic battery? Is that required too? If so, what rating? As big as the biggest pull (i.e. windlass)?

The fuse protects the wire, so has to be matched to the wire's current carrying capacity. Imagine you have a big cable going to your domestic distribution board; then you'd have a suitably big fuse as close to the battery as possible. From the distribution board, smaller wires usually go to individual lights, radio, pump, etc, and each of these smaller wires needs protecting with a suitably smaller fuse. Most distribution boards have banks of switches with associated fuses or breakers.

 

[Ruffles]

People who say fuses aren't fitted to car starter circuits overlook the fact that you can walk away from a car fire.

My understanding is that the connection block fitted to the battery on modern cars forms the fuse in some way. I suspect that it's not designed to be replaceable!

 

[prv]

Cant see the need for a fuse for your starter motor. What purpose would it serve ?

Mine is there to prevent serious damage if the cable to the starter motor either comes off its terminal or the rubber cover comes off the terminal and something metal touches it. In either case, it's then more or less inevitable that contact will be made with the engine block, which has a good connection to battery negative.

With a well-charged starting battery and 25mm^2 cables, that's a lot of energy pouring in, so while I've never seen it happen I assume the result would be a fire, a minor explosion, and/or welding-style damage to the engine block.

Seemed a good argument for a fuse to me.

Pete

 

[demonboy]

My 43hp Volvo's starter circuit has a 250A MegaFuse next to the starter battery. For your bigger engine, you might need a 300A fuse.

Should I go for an 'ignition-protected' fuse on my starter battery circuit?

suitably big fuse

So to reiterate, a 'suitably big fuse' is chosen purely for the size of my cable coming from my domestic bank before it gets to the distribution panel? My cable is 35mm2 (I'm sure someone, probably Vic, put up a formula on one of my other threads to work this out).

 

[pvb]

Should I go for an 'ignition-protected' fuse on my starter battery circuit?

No need - they're for petrol engined boats.

So to reiterate, a 'suitably big fuse' is chosen purely for the size of my cable coming from my domestic bank before it gets to the distribution panel? My cable is 35mm2 (I'm sure someone, probably Vic, put up a formula on one of my other threads to work this out).

Yes, the fuse protects the cable. You want the fuse to blow before the cable glows red hot and sets the boat on fire.

Incidentally, I once very stupidly connected the wires wrongly on my alternator. When I turned the power back on, there was a little "phut" as the fuse blew. Better than the alternative!

 

[savageseadog]

The real danger on a boat is a short-circuit allowing the battery or batteries to overheat the wire. For this reason, fuses should be fitted as close as possible to the battery end of each wire which is being protected.

Doesn't just protect the cables, batteries can melt and explode as well. Fit a fuse.

 

[CreakyDecks]

I think the argument would be that the cable is securely mounted to the starter and so very unlikely to come off. Adding a fuse simply moves the potential for a fault along the cable a few feet, to where the battery cable is connected to the fuse. So actually it probably doesn't result in a system that is any safer.

 

[lenseman]

Hiya Jamie - The very fact that you are considering a fuse in the starter circuit means that you are conscientious enough to plan for the worse case situation.

I would sooner sail with you that those who never bother to check that their wiring is safe. As an emergency, you could get away with a H/D switch which would be able to handle 600 Amps or so. It would be better than nothing.

Just as an aside, I has a main bilge pump wire, a 16 Amp, short out due to the pump going short circuit. As the pump wire glowed cherry red, it effected the adjacent starter battery wire (to which it was tie-wrapped) and took that down to deck too, luckily the boat was in the marina at the time.

Apart from sinking, fire onboard is probably the most worrying disaster that can effect a boat.

 

[demonboy]

Cheers, David. Just before returning to this post I was reading up on CBs and fuses here, which sets the scene with some statistics on boat fires.

A couple of weeks ago we were getting a short and I burnt myself on a white-hot guard-rail. I have no idea how long it had been like this but it was a real wake-up call, hence the new caution as we recommission Esper for some more sailing.