Do I need a main 'master' fuse ?

[lw395]

Unfortunately a 250 amp fuse will carry 250amps indefinitely. That's 3KW at 12v, probably a good bit more than was cooking your boat, or there'd have been plenty of flames

I'm pretty sure my main battery cables don't go anywhere near each other, but that's more by good luck than good judgement, and I'll be making sure as part of my planned electrical upgrade over the next few months

To be pedantic, not actually indefinitely, as many people have found.

But seriously,
I think you can go too far in worrying about ensuring that the fat cables are fused, it can be better to make sure they are beyond reproach and never going to short out.

 

[VicS]

Look at the engine's book of words to find the power. Amps = Watts/Volts, so 840w/12v = 70amps plus a margin. For that one, I'd guess that 100 or 120a would be somewhere near. The starting load of a motor is a quite a bit higher than the rated load, but a fuse will carry more than its rated current for a while and if the motor doesn't spin up pretty quickly, you'd probably want the fuse to blow.

A quick google finds lots of pages that say you shouldn't fuse the starter circuit because the current's too high. I'm sure there's one somewhere with more useful advice, but I've got work to do!

The current drawn by a starter motor will be more than you calculate from the rated power because thats the (mechanical) power output, not the electrical rating

As an example a 810 W (1.1 hp ) motor will draw 185 - 220 amps ( at 9 volts) when loaded. More than double that when stalled

 

[nigelmercier]

This is what I am going to use. Seems reasonable quality so hopefully no voltage drop...

Looks pretty small and flimsy to me, and those crimp connectors are rubbish.

 

[Storyline]

Looks pretty small and flimsy to me, and those crimp connectors are rubbish.

It is BEP marine and was recommended on here a few years ago. They are just studs, you can use any cable terminals I presume.
Maybe the picture is not clear; it is about 6" long.

This one http://www.power-store.com/view-item.asp?itemid=1199&id=202&

 

[West Coast]

It is BEP marine and was recommended on here a few years ago. They are just studs, you can use any cable terminals I presume.
Maybe the picture is not clear; it is about 6" long.

This one http://www.power-store.com/view-item.asp?itemid=1199&id=202&

I fitted two of these last year, one on engine circuit, one on house circuit, great bits of kit, not flimsy at all.

 

[davidwf]

I like terminal fuses these are directly connected to the battery terminal, easy to fit and they protect the whole cable. See http://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A

 

[West Coast]

Several years ago I had a Hurley 24. One morning after a pleasant night at anchor I went to start the engine and was horrified to see smoke filling the cockpit locker. Everything switched off pdq and then connections knocked off the battery with a large hammer. It turned out the first "dark space" the battery cables went into from the battery involved passing through a bulkhead. Over the years they had rubbed together and slowly worn through the insulation so there was a short. Smoke was from smouldering outer insulation sleeve and wooden bulkhead. Fortunately it didn't get any further than that so no flames or major damage. The following day I had a conversation with Marine Power and ordered fuses for the main cables. Fitted close to the batteries they were much larger capacity than total consumption as they were to protect the system from a short. If I remember correctly about 250A. On subsequent boats I have fitted the same system immediately after taking ownership.

I heard of a very similar story of a Starlight 35 from Strangford, NI, where the cables from the batteries to the engine were taken through a constricted duct along with the seawater cooling pipe. Slight movement over the years wore away the cable insulation, a short occurred starting a fire. I believe a fair amount of damage was done, but repairable.

 

[jac]

I like terminal fuses these are directly connected to the battery terminal, easy to fit and they protect the whole cable. See http://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A

Like the look of those but whilst there seem to be several retailers who stock the holder I couldn't track down a retailer of the fuses themselves. Would be nervous of getting something that you can't easily replace the fuse in. Where do you get yours?

 

[davidwf]

Foxs sell them http://www.foxsonline.com/acatalog/Blue_Seas.html or try searching for littelfuse terminal fuse they are quite often on ebay.