[rsk289]

Well, if there are no components drawing more than 30A, it doesn't have to be able to handle anything over 30A. Unless there is a huge stereo system, extra lights everywhere etc. the only thing that might draw high current is the cooling fan. A generator/alternator won't produce 30A unless a component is asking for it.
Having said that, I had my 30A BM4 ammeter upgraded by Speedycables internally to handle 50A just in case, as I have a pretty big aftermarket electric fan to try to keep the thing cool in traffic. There's no way of knowing visually that the ammeter has been uprated - it just doesn't deflect quite as far as it used to. Frankly I think they're only useful for showing charge/discharge.