Originally posted by Joe Johnston
Most folks think fuses (or breakers) are there to protect people. Ultimately they are, but more specifically they protect wires from causing fires. Alternators have diodes that are always powered. At any time, one could short to ground, causing the battery to unload through the alternator's battery wire (the big one). This doesn't happen often, but it could happen while the car is safely put to bed with nobody around.
Rule #1: THE POWER SUPPLY MUST MEET (OR EXCEED) THE DEMAND.
Rule #2: THE WIRE MUST SAFELY CARRY FULL-LOAD CURRENT.
'Safely' means, the wire needs to be de-rated because of heat rise caused by current flow and/or engine compartment temps.
The fuse must be sized to protect the WIRE, but duty cycle needs to be considered. Most wire can sustain 400% of constant current limit for a second. Fuses blow in far less than a second so we can go up about 125% in fuse rating. OEM battery cables are a good example of duty cycle. Notice, all the considerations when choosing wire. All wire insulation has a temperature rating (which is the wire’s ability to shed heat). If the copper size is large enough, the wire will produce very little heat. 60˚C THHN wire is available at Home Depot. It works just fine and is relatively inexpensive.
Alternators come in rated sizes that continue to increase because automotive systems add more accessories. The big jump came with Electric Fans. Before EF’s 75-amp alternators were large enough. After EF’s, 100-amps were bare minimum but I have seen 400-amp alternators. We never want to drain the battery with any accessory while the engine is running. Battery power must always be there to start the engine.
Littelfuse - MEGA Fuse is a perfect fuse choice and widely available at local and online stores. The price range is wide too:
OReilly’s charges - $6.99 for the fuse (UPC: 79458016826) and $9.99 for the holder (UPC: 79458022100)
Delcity.com charges - $3.97 for the fuse and $8.97 for the holder plus shipping.
This fuse really belongs mounted on the fender apron, connected to the Starter Solenoid, in parallel with the battery cable. This connection offers another stud for connecting existing (yellow) power wires. I suppose the alternator wire could be bolted directly to the fuse, but I would not put any undue stress or vibration on a fuse lug. That’s why a two-post holder with a cover is available.
I invite all discussion, cheaper prices, and concerns regarding the need for this fuse. - Dave
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