The voltage on the ammeter doesn't matter... the gage reads amps not volts. The only time voltage matters on an ammeter if you're on 12 volts and there's a 6 volt backlight on the meter. It'll burn out the backlight.
The issue may be that the alternator puts out more current (amps) than the meter can handle. If you have a 60 amp alternator (many have their output rating stamped in the housing) and you ever need full power out of the alternator such as charging a nearly dead battery, a 20 amp ammeter is going to get hot and possibly fail.
Hope this helps
Ken in AZ
