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Discussion Starter · #1 ·
Hi All,

This might be hard to believe... but my 35 year old boat doesn't seem to have a main DC breaker. As some of you may remember my engine wiring posts, there was no electrical protection for any engine components (i.e. electric fuel pump, starter, dash lights, or glow plugs). I have since installed circuit breakers for the engine components. Everything else aboard is properly fused or has a switch/circuit breaker. But.... the big giant red cables go directly from the batteries to the battery selector switch and then directly to the bus bars and the starter. What are your thoughts and recomendations? Thanks.

Dave
 

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Learning the HARD way...
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MRBF - Quick and done!

 

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Fuse at 80% of max. amperage for cable.
Jim
For what load, should be the real question??? For some loads this is fine for others it can result in nuisance trips. The ABYC standards for Table VI A Max Ampacity (There are also tables B &C for bundled wire) allow for up to 150% for certain loads.

The 80% figure is generally for average use loads and not exceeding 80% of a fuse or breakers rating.



You can always fuse lower than the wire can handle but you need to be sure you won't have nuisance trips. For older boats wanting to fuse battery banks the wires were pretty grossly undersized, compared to what we use today, so many find they need to push to the 150% range in order to avoid nuisance trips.. As an example Catalina used to use 4GA wire on the Universal M-25 today with basically the same engine they use 1/0 or 2/0.... Most small diesels will require 250 - 300A fuses if the motor could be started on that bank. Some older engines may need 300A+...
 

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One for each battery? Guess on amperage?
One for each bank (not necessarily each battery).

My M-25 with upgraded AWG 2 would start the engine with a 90A fuse, as long as the battery was over 90% charged. When the battery dropped below 80%, the voltage dropped, so the starter motor drew more amperage, and pffft... the lights went out. Fortunately, this was only an experiment, and I had a spare 175A fuse. Since then, (3 years ago) I've been running with 175A fuses, and had no problems.

AWG 2 is rated at 175A outside of engine spaces because of the expected heat of the engine space. I figure that when I start the engine at the beginning of the day, everything is cold, so no problem. When I re-start after a short break, the engine is warm, and starts more easily (and therefore uses less current, for a shorter duration) than when it's cold:).
 

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Discussion Starter · #10 ·
MRBF - Quick and done!

Did a draw measurement, using the glow plugs AND cranking at the same time. Showed around 226 amps. So I installed a 250 amp fuse on each battery. I guess I feel better know that if something shorts, I have a fuse to keep the boat from burning down.
 

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That is a big draw, but not surprising. What was the duration of that load? Realize that any fuse will take brief loads substantially above the fuse's rating.. I've posed the amps vs time plots from BlueSeaSystems in different threads a couple of times before, but you can look it up as easily as I can.

ANY amperage fuse is better than (what you had) no fuse at all.
 
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