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Old 12-04-2016, 08:02   #16
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Join Date: Aug 2011
Location: Oakland
Boat: Cross 40
Posts: 1,393
Re: alternator direct to house bank

Originally Posted by dave777 View Post
What size fuse/breaker would you use on a 60A alternator Yanmar 3YM30 engine? Also thinking of upsizing to a 100A alternator.

Don't know if alternators put out surges of current in some conditions, which could complicate things.

I'm thinking that I'd put in a switched breaker so it would serve to dual purpose of depowering the line to the alternator when working on the engine.
The 150% cited above it about right. For a 60A alternator you could go 80A or 100A on the fuse. I'd probably select 100A.

One problem with breakers is their fault clearing capability, that's why fuses are recommended. A dead short on a battery can provide a tremendous fault current (could be thousands of Amps). With a high enough fault current you can actually weld the contacts of the breaker shut, or with DC simply arc between the contacts, and the breaker doesn't do its job. You need to look at the DC interrupting current rating of the breaker and then at the available fault current of your battery system and make sure the interrupting current rating is higher than the available fault current (the same is true for fuses as well, but fuses generally have a much higher IC rating than breakers). For most battery systems you are better off with a switch and a fuse rather than a breaker.

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