CRM
Active Member
Various posts suggest a heavy duty fuse in the starter circuit - to be safe !
How does one calculate the size required
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How does one calculate the size required
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Starter Fuse ?
- Thread starter CRM
- Start date
Jean
Well-Known Member
Fuse needs to be suitably sized to protect the cable under short circuit conditions (assuming that you are considering fitting one at the battery terminal). Hence, check cable size and currant rating can then be checked. "Simple" as that, although, in practice, perhaps not that simple. Last week, I measure the current drawn by a started motor starting a cold 36 HP (I think, but yet to check) diesel engine, it peaked at 260 amps. Fuses generally have a time/current relationship, so using a "slow blow" fuse (if one is available at say 200 amps), you can generally get away with one of less than the peak load current (useful, if for example if cable is rated for less that the peak start current). I'm planning fitting one soon so would be interested to hear what size others are using with fuses already fitted.
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chriscallender
Well-Known Member
I'm also trying to figure this out. I found out that for a Bukh 20 the starter motor is 1.3kW, so the current when its turning should be around 100A. However as Jean says there is going to be a surge when you first turn the key. I was planning to try 175A fuses and anyway you definitely need to carry a good few spares. If I ever blow one in normal use then I'll go up a size or two.
Its one of those things on my to-do list, however progress on the to-do list has almost ground to a halt now the boat is afloat!
Chris
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Its one of those things on my to-do list, however progress on the to-do list has almost ground to a halt now the boat is afloat!
Chris
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HaraldS
Well-Known Member
I've made myself a little spreadsheet to show all the important data for a cable based on cable size and for one thing used it to size fuses. Fuse size is based on the idea of how warm (hot) a cable gets under certain load. Then I went and set a comservative limit of dissipating 1W of heat per 6 square centimeters of surface. Guessing a bit on the heat conductivity of the insulation, I figured that would give temperature difference of 50C relative to the surrounding air, given the current would flow for a long time.
That gives the following:
cable size fuse
35mm2 264A
AWG 1 305A
50mm2 345A
AWG 0 363A
AWG 00 432A
70mm2 443A
AWG 000 514A
90mm2 535A
If you want, drop me a PM and I send you the whole spreadsheet.
I put a fuse into my engine circuit a year ago. Found the cabling to be partially 90mm2 and 70mm2. So I took a 355A fuse. The surge wouldn't do much as melting fuses are slow. My engine starting circuit is completely separate and the battery only for starting engine or generator. I have a Yanmar 4JH3 and the starter motor is listed with 1.4kW, yet I have not measured more than 100A while crancking.
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That gives the following:
cable size fuse
35mm2 264A
AWG 1 305A
50mm2 345A
AWG 0 363A
AWG 00 432A
70mm2 443A
AWG 000 514A
90mm2 535A
If you want, drop me a PM and I send you the whole spreadsheet.
I put a fuse into my engine circuit a year ago. Found the cabling to be partially 90mm2 and 70mm2. So I took a 355A fuse. The surge wouldn't do much as melting fuses are slow. My engine starting circuit is completely separate and the battery only for starting engine or generator. I have a Yanmar 4JH3 and the starter motor is listed with 1.4kW, yet I have not measured more than 100A while crancking.
<hr width=100% size=1><A target="_blank" HREF=http://www.taniwani.de>http://www.taniwani.de</A>
snowleopard
Well-Known Member
'official' advice is that it is acceptable to have no fuse on a starting circuit provided the cable runs are kept to a minimim.
the only way to be 100% sure is to have the fuse mounted right at the +ve terminal of the battery so there is no unprotected cable.
with a fuse of 350A you would be protecting the cable itself but if the short were caused by something falling across live terminals (loose wire, control cabel etc.) a current of under 350A could cause a nice blaze.
i think the safest way is to fit a cut-out switch and turn it off except when starting. i have such an arrangement (not for safety reasons). it works but it's a right pain!
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the only way to be 100% sure is to have the fuse mounted right at the +ve terminal of the battery so there is no unprotected cable.
with a fuse of 350A you would be protecting the cable itself but if the short were caused by something falling across live terminals (loose wire, control cabel etc.) a current of under 350A could cause a nice blaze.
i think the safest way is to fit a cut-out switch and turn it off except when starting. i have such an arrangement (not for safety reasons). it works but it's a right pain!
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