Thread: Mega fuse

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I am going to run a 140 Amp Alternator. I was wondering what size mega fuse I should run. I have a 60 amp in it. But I don't think this is enough.

Depends on the fuse you're using, I'd say. The Littelfuse brand series 299 slow blow fuse is guaranteed to carry 135% of rating for a minute, and 200% of rating for 4 seconds. Typically it will handle more than the guaranteed values. That works out to your 60A fuse passing 81A for 60 sec, and passing 120A for 4 sec. I'd definitely not go any higher, and in fact I'd probably err on the side of protecting the starter with a smaller fuse to start, like a 50A which would pass 67A for a minute, 100A for 4 seconds. You want your fuse to blow before you start letting the smoke out of anything. The Littlefuse site has a ton of info on fuse ratings.

Depends upon the circuit you are protecting. Typically, there is not a fuse in the main alternator line.

Originally Posted by robot

Depends upon the circuit you are protecting. Typically, there is not a fuse in the main alternator line.

I was thinking alternator to battery, battery to fuse block for distribution. The fuse/fusible link is in the line from the battery to the fuse block, to protect power side wiring ahead of the individual fuses from a direct short, can see the maximum short circuit amp rating of the battery, and needs to be sized for the max draw through the fuse block which is normally the starter.

JMHO, you actually have two power sources on this wire, the battery and the alternator. To protect this wire you need a fuse (or other type of protection device) at both ends.
Some will argue this wire does not require any protection, as the older vehicles didn't have them. However, as current loads increased this wire became protected to prevent electrical fires. Usually OEM used fuseable links at each end as they are compact. This wire can be ordered from OEM with the links installed for the appropriate current, also available in some cases from summit and jegs.