That's a nice industrial timer and should work for a long time without any trouble. The rating is for a resistive load. That means there is no surge in current on start-up. Coil and core type ballasts are an inductive load, which does have a current spike on startup. None the less, that timer will easily handle 20-25 amps continuous HID lighting load. A 20-24 amp load will need # 10 awg wire and a 25 or 30 amp breaker, a 25 amp load would be just a little over the limit for that and code requires the next size wire (#8) and breaker (35 or 40 amp).

FYI, a 30 amp rated wire can only feed a 24 amp load (80% max). If you put 30 amps on a 30 amp breaker you are on the trip current and that's not what you want to happen.