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Old 04-08-2009
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how big should the fuse be ?

I have a trailer sailor and have a Honda 9.9 with a 12 amp alternator on it. I have two group 27 deep discharge batteries with a selector switch for all power on board. I do not have a fuse of any kind on the batteries, they feed a power panel with all marine breakers for each circuit.

It appears from the Honda manual they usually just attach the two wires from the directly to a 12 volt starter battery. I use the deep cycle batteries to start the motor, seems to work fine.

After a survey it was suggested I mount a fuse on the common negative wire very close to the batteries. The surveyor thought 70 amps would be about right. While I do not want it any bigger than required if the starter motor should blow it as the motor starts then it will fry the alternator as it will then be running without a battery. I'm thinking about using a 100 amp fuse. Can anyone provide advice on the max. starter motor draw for a 9.9 4-stroke ?
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A 70 amp fuse is probably more than enough for a 9.9 HP outboard. I think that's what I have on my boat and its a 20 HP outboard. I seriously doubt that the draw for starting a 9.9 HP is more than 50 amps.
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Check your owners manual or ask a local Honda Shop. The BFS115 (for example)outboard requires a 90 amp fuse. But the owner's manual should include requirements for electric start and fuses required.
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Quote:
Originally Posted by Roberthussey View Post
After a survey it was suggested I mount a fuse on the common negative wire very close to the batteries. The surveyor thought 70 amps would be about right. While I do not want it any bigger than required if the starter motor should blow it as the motor starts then it will fry the alternator as it will then be running without a battery. I'm thinking about using a 100 amp fuse. Can anyone provide advice on the max. starter motor draw for a 9.9 4-stroke ?
I'm thinking about this because something about it seems intuitively wrong to me, but I can't put my finger on what it is yet. I guess if you blow that fuse you'd still have your earth ground, so you wouldn't suddenly start using alternate paths to ground (such as through a shore side telephone line), you'd just lose the ability to charge/discharge the battery. Still, trying to use the starter after that fuse has blown is going to fully load your solar panels, alternator, etc, ????
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thanks

Thanks for sharing your ideas and experience. As you suggested I'll stick with the 70 amp fuse as also indicated in the survey.
Bob
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The fuse is there to protect the wires in case of a short. You can look at the different amperage ratings for different gauged wire. Off the top of my head, 10 ga wire is suitable for carrying 30 amps. If your fuse is too big and you have a short, the wire gets hot, the insulation melts off and it can start a fire and all of this can happen before the fuse pops. I learned this the hard way by putting a piece of aluminum foil around a fuse in a car when I was a kid. I still hate the smell of melted insulation.
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