I understand, do you understand why they are rated higher than what you should run them at. Marketing? why not say rated at 35A which is what you are doing derating.
I think most any mechanical device will last longer or shorter depending on how heavily you use it. We specified device will tell you what their continuous duty rating is, and also various forms of intermittent duty. We probably see this the most with marine engines where the same engine will have a continuous duty rating of X, and an intermediate duty rating of 2x. Marine gears are the same.
There are also lots of products where no distinction is made because in typical use it's not an issue. Take a shop vacuum. What the hell is "peak HP"? It's some maximum something that someone saw somewhere, sometime, and is the biggest number they could defensibly put on the product. So yes, marketing.
Engine alternators are designed for two things, recharge a start battery which takes very little, and carry ongoing loads for the vehicle like lights, heater fan, HVAC, etc. These loads are max when you first start the engine and battery recharging takes place, then drop down to the background loads. Given this usage pattern for 99% of engines, alternators are rated accordingly, and the nameplate rating is for intermittent, not continuous duty. The engine itself is the same. Your car's engine may be rated at 200hp, but they know it will never be run that was for more than brief moments, and I can assure you it won't last very long if you ran it flat out continuously. Construction and farm equipment is different and is expected to run continuously, so engines are rated accordingly. With boats, it depends on the boat's usage, so marine engines are rated accordingly.
Getting back to alternators, I don't think anyone ever asked about alternator ratings until the advent of large house battery banks, and even then lead batteries get to acceptance pretty quickly and reduce the alternator load. The bottom line is that with the possible exception of large alternators designed for busses and emergency vehicles, all alternators are rated for intermittent duty.
I have NEVER seen an alternator manufacturer specify a duty rating. Nor have I ever seen a spec for max allowed rectifier temp, or max allowed stator temp (the two hottest parts of the alternator). All they specify is max ambient temp, e.g. the temp under the hood of a car, but without any loading specs or assumptions. It's utterly inadequate for designing any sort of power system, but it's all we get. Thanks guys, we really appreciate the help... but empirically it's been shown by people messing with large house banks, especially LFP where full output will be demanded for 100% of the charge cycle, that the continuous duty rating of most alternators is 70-80% of the nameplate rating. Big alternators can typically do more, and small ones less. Mine put out about 85% when regulated to limit the rectifier temp to 100C.
So yes, marketing.
