Interesting issue alternator-battery fusing. They weren't used in older vehicles (my 1964-1970 vehicles have none).
The last reason I got for their existence was "to protect the wiring", but that was some git that reckoned I didn't know what a mate was talking about when he stated "there is no point breaking the engine-chassis/body ground connection" even though he later repeated the usual problems such as over-voltage frying electrics and currents melting throttle cables, wires and any other engine-chassis paths.
Besides, if it's to protect the, which end for the fuse, or should there be one at each end (using the rule that all power sources should be protected)? (Answer: Battery end.)
My conclusion is that the fuse is there to:
(1) protect the cable from the battery (if the cable shorts);
(2) protect alternator main-diodes in case of reverse connected battery. (Close to useless in practice.)
As to (1), heavy starter cables are not fused. If physical security is adequate, is that sufficient? (Unless alternators can fail in short-circuit mode which won't happen except for direct metal contacts - shorting fragments or loose conductors.)
But as visionsss (the previous replier) indicated, LONG cables are a risk. A long cable - like old Jaguar XJ-6 starter motor cables lol! - should have fusing unless it is physically well protected.
A for (2), nice in theory. But I know of many fused yet failed alternator diodes (probably Bosch!). I have also witnessed MANY repeated reverse polarity connects on an un-fused 1960s vehicle with a 25A alternator. Despite the sparks, the alternator was NOT damaged (Hitachi). (The battery's case-molded polarity was incorrect!!)
As regards the alternator, their stators are generally self protecting.
That doesn't mean the diodes will handle a current surge - but fusing is unlikely to prevent that failure mode anyhow.
[ FYI: I blew 3 Bosch alternators due to low battery voltage alone! The battery was ~7.5V; there was a 40A alternator to battery fuselink; the original system was a 23A generator (a dc generator, not alternator), and the Bosch alternator ratings varied from 75A to 100A. In comparison, my 70A Hitachi alternator happily handles cold starts from a 5.2V battery. And yes - it is a 12V system. ]
Though I have fitted main fuses (a 3 fuselink box mounted off the battery +12V terminal), if I was concerned with voltage drops or added unreliability (fuses/breakers and connections), I would NOT have the alternator to battery fuse. (But I would have fuses/protection to the loads powered directly off the alternator (eg, pre-HID headlights etc).)
As to fuse sizing, many are below the alternator output. I often see older vehicles with a (crappy wire-type) 23A flink (fuselink) from their 45A or larger alternator.
If fitted, it only makes sense to fit a fuse near the battery to protect the cable to the alternator which itself should be rated to carry the max alternator output current.
And I would consider an auto-resetting circuit breaker unless you can tell that the fuse has blown before the battery flattens. (At least it's not critical like headlight fuses!)
[
FYI Only: In my case I have a 40A batt-alt flink with another 30A flink to the headlights from the alternator (in the battery-mounted flink-box).
The total of 70A only matches the present LR-170 70A alternator by coincidence (I also have 90A and later alternators) - the 30A flink is a master for downstream circuit-breaker protected headlights (4 CBs of either 10A or 15A), whilst the alt-batt 40A flink seems to handle even the charge current into a depleted 12V battery of 5 volts.
The 3rd flink is the 30A "main" from the battery to the vehicle. (That doesn't include other +12V terminal taps (fused) to my sound system and auxiliary battery.) ]
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