Alternator amp rating.
I’d like to see the oem ammeter on a test bench pushed to failure.
They vary from vehicle to vehicle. They're not all built the same. The ones on trucks should have been more robust, but plenty were not. Why trucks different? Because plows and winches are wired off the battery and when that's done, the battery demands for recharging are high and frequent.
I guess the real danger though is the bulkhead connector,
Its one of the weakest links (although there are exceptions as pointed out above by 273). Packard 56 terminals really not great to be carrying more 20 amps for long periods of time. On most A-bodies, another weak area is the minimized wiring to the headlights. Too many connections, long route, wires at near max capacity for load (eg 18 AWG for the final runs from the hi/lo switch).
Factory offered at least 3 alternators for much of the 60s. Getting back to the original question, the shop manual test is not the same as the "rating". The test was at 1250 rpm and 15 V - that gives you a better idea of what it can supply while doodling along at 25 mph. The higher rated alternators have more windings, which means more copper so cost more. The factory wasn't putting them in unless there was a real need or the customer was willing to buy it as an option. That's what was going there. "30 amp" was the lowest in '66.
http://www.hamtramck-historical.com/images/TSBs/1966/66-78_page2.jpg
FSM vs. rating and a graph of a 15Si alternator's power vs. rpm
here.
