pmagowan
Well-Known Member
the starter motor power is rated at 1.4kw according to the workshop manual. Does that equal 116.6 amps in 12 volts?
Electrics
- Thread starter pmagowan
- Start date
pmagowan
Well-Known Member
I am wondering if this means I can get away with a 150A fuse and a 25mm2 cable at 1.5m one-way cable length? Bluesea seem to do fuses that attach to the battery terminals which might be handy.
Hoolie
Well-Known Member
I have a functionally equivalent arrangement with a Smartguage/Smartbank system. But I have the alternator directly coupled to the starter battery and the charger directly coupled to the house bank. I also have provision for a couple of solar panels that I'll connect directly to the house bank.Give or take some circuit breakers and isolators.
I arbitrarily decided to use 50mm2 for the main battery runs associated with the starter and windlass and 25mm2 for the service connections to the house bank. Our boat is similar size to yours and it's surprising how cable routing constraints use up cable length.
I arbitrarily decided to use 50mm2 for the main battery runs associated with the starter and windlass and 25mm2 for the service connections to the house bank. Our boat is similar size to yours and it's surprising how cable routing constraints use up cable length.
pmagowan
Well-Known Member
I presumed that I would wire any Solar to the same place as the charger as they are essentially the same thing.
VicS
Well-Known Member
Yes if you want it it take advantage of the VSR and charge both batteries.
OTOH it will normally be the house bank that will need charging, rather than the start battery which will be maintained by the engine alternator, so you could simply connect it to the house battery.
lw395
Well-Known Member
No.
Probably a peak of something over 200A on whatever your '12V' sags to.
But the cables don't have to survive peak current for very long.
pmagowan
Well-Known Member
This is quite confusing. I can easily get charts for cable gauge for all normal cables but the starter cable is generally treated differently! It seems to be accepted that the starter cable will accept more amps than is allowed for its gauge in any other use. This seems to be because it will only be used for a very short time. I can find no table that gives you the recommended gauge for starter cable (perhaps nobody will commit to this given they are working outside normal parameters). The next problem comes when you want to fuse this cable. The fuse will also have more amps drawn through it than it is rated for otherwise the fuse will be so big as to not protect the cable. From my research it seems that, plucking numbers out of the air, a 35mm2 cable would be fine (25mm probably fine) and a 200-250amp fuse should be fine and still protect the cable.
I am quite surprised that this is not clearer. It seems it is a special case where even the regulations bodge it! Or am I wrong
I am quite surprised that this is not clearer. It seems it is a special case where even the regulations bodge it! Or am I wrong
lw395
Well-Known Member
A lot of starter cables are not fused.
The starter cable should be sized for resistance giving an acceptable voltage drop.
You could then work out how long it would carry this before the insulation melts, based on the heat capacity of copper.
You can fit a big fuse on the theory that you've worked out the shorted starter will take 'n' seconds to fry the cables and the fuse will blow first.
But this does not always work. If the battery is low and is not quite capable of blowing the fuse, the cables may still get very hot.
It is sometimes better to concentrate on insulating the terminals etc properly so shorts are very unlikely to happen.
The starter cable should be sized for resistance giving an acceptable voltage drop.
You could then work out how long it would carry this before the insulation melts, based on the heat capacity of copper.
You can fit a big fuse on the theory that you've worked out the shorted starter will take 'n' seconds to fry the cables and the fuse will blow first.
But this does not always work. If the battery is low and is not quite capable of blowing the fuse, the cables may still get very hot.
It is sometimes better to concentrate on insulating the terminals etc properly so shorts are very unlikely to happen.
pmagowan
Well-Known Member
On my previous boat, before I rewired, one day the engine wouldn’t start. I traced the fault to a starter cable that had fractured due to fatigue right at the starter motor. It was very close to making contact with the engine and therefore creating a short and probably a fire/explosion. So I would like to do it right. I always shrink wrap connections anyway.
VicS
Well-Known Member
Suppliers often quote a current rating for starter / battery cables. For example Furneaux Riddall quote 170 amps for 25mm² cable.
The standard which covers "Extra low voltage DC installations in small craft", ISO 10133, requires all circuits EXCEPT cranking motor conductors to be protected by a fuse or circuit breaker ( there are some other exceptions as well)
pmagowan
Well-Known Member
Is that not a rather unsatisfactory situation though. It seems you are allowed to get away with this wire not being fused even though this wire is very easily shorted with disastrous consequences. The wire gauge being used is almost always less than that required for the current (in continuous use) and so is in the process of heating up and failing every time you use it only being saved by the short duration of normal use. If there is a failure the wire will glow red hot and start melting things with no fuse protection, likewise with a short. I would have thought there would be a standard size of wire for this job depending on both current and distance and that there would be a standard size of fuse that would protect the wire while allowing it to function in this ‘overcurrent’ normal operation. Perhaps I am too pernikity and I should just suck a finger and hold it to the wind.
lw395
Well-Known Member
Doing it right would include proper support and looming of cables so they don't fatigue and fracture.
And maybe periodic maintenance inspections.
Tens of millions of cars don't have fuses in the starter circuit.
pmagowan
Well-Known Member
Yes, but a systems approach to risk means you don’t rely on the user to maintain or inspect. That is a nice extra. It just seems that it must be possible to specify both cable gauge and fuse size for this function. I am not particularly concerned with how other people have ignored the issue in other less important applications.
pmagowan
Well-Known Member
Can anyone tell me if 35mm2 cable would be ok to work with for all the battery connections? I am thinking if I buy 10m of this in each colour I can make my battery interconnects and main wiring all in the same size. Then I can get smaller cable to work with as per required rating for normal wiring after the distribution panel.
pvb
Well-Known Member
Your diagram doesn't show the negative connection for the VSR. It won't work without the negative connection.
You don't need a fuse in the alternator output - the battery fuse will cope if there's a fault.
I'd suggest a 250A or 300A MegaFuse next to the starter battery, and a 150A fuse next to the house battery (unless you're going to be connecting any high-current users such as an inverter, windlass, etc).
pvb
Well-Known Member
You may find that your rudder post is electrically connected to battery negative, in which case a galvanic isolator would be a good idea.
pvb
Well-Known Member
Yes, I reckon you'd be fine with 35mm2 cable.
pvb
Well-Known Member
The correct approach. Some years ago, when replacing an alternator I foolishly got the connections wrong. When I turned the start battery switch on, there was a gentle pffft as the MegaFuse I'd installed next to the battery blew. Without it, there'd have been red-hot cables before I knew it!
pvb
Well-Known Member
That's a silly comment. The shunt will drop about 10mV max at the sort of currents involved. Infinitesimal in the overall scheme of things.
pmagowan
Well-Known Member
The wiring of the VSR is from the BEP instructions (in fact it is pre wired).
There is a windlass to contend with so I will investigate this.
I like the look of the bluesea fuses mounted to battery terminals so that the fuse is exactly where it needs to be and easy to identify. It also reduces the number of connections.
Looks like I will go with 35mm2 and around 200 amp fuse. I might try a 150amp and see if it works as I think they need significant overcurrent for a while to blow and a very short blast like a starter may be fine and leave even better protection for the cable.
Thanks
There is a windlass to contend with so I will investigate this.
I like the look of the bluesea fuses mounted to battery terminals so that the fuse is exactly where it needs to be and easy to identify. It also reduces the number of connections.
Looks like I will go with 35mm2 and around 200 amp fuse. I might try a 150amp and see if it works as I think they need significant overcurrent for a while to blow and a very short blast like a starter may be fine and leave even better protection for the cable.
Thanks
