Join Date: Aug 2006
Location: Arlington, VA
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Re: Electrical Upgrades
I agree with MaineSail 100% on this: if you're going to put a fuse in the starter circuit -- and I, too, believe this is a very good idea -- don't fool around with 'barely adequate' size fuses. I use a 400A ANL in my start circuit with 2/0 cable on my 4-108, after having blown lesser fuses in the 200A range.
Electric motors, like starter motors, are generally rated in at least two ways:
LRA = Lock Rotor Amps
RLA = Run Load Amps
When a starter is fed current from a dead stop condition, there is very considerable in-rush current, comparable to the LRA rating. This may be 2 times or 3 times the RLA run load amps, or even more. On cold mornings, or if there is sludge in the engine, or if it is hard starting for some reason, these elevated current draws can be much longer than the "normal" in-rush current times.
Example: a starter on a small to medium size diesel may, indeed, only draw 150A or so while it's turning, but the in-rush current can reach 1,000A for a few milliseconds, dropping to 300A to 400A or so for a few more milliseconds before leveling out at 150A when the starter really gets going.
What you most certainly don't want to happen is to have a fuse blow while you're trying to start the engine. Murphy's Law dictates that this might well be at the worst possible time.
The fuse is there to prevent a catastrophic meltdown and fire.