Normally I use the ballast data to sized the circuit and I don't bother to read the lamp wattage but useally resvered for ballpark figures plus add about 20% or so so you should land right on the spot.
Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)
I agree with the previously posts - use the Ballast Data.
BTW, A Magnetic Rapid Start Ballast Load for Driving (2) F40T12 Lamps would be:
Line Input - 120 VAC: Line Amperes = 0.69A ANSI Watts = 82W Average Ballast Factor = 0.84 BEF (Eff. Factor) = 1.02
Line Input - 277 VAC: Line Amperes = 0.33A ANSI Watts = 90W Average Ballast Factor = 0.87 BEF (Eff. Factor) = 1.04
BTW, these figures are for F40T12 Lamps driven with the Cathodes being constantly Heated - AKA Rapid Start / Hot Cathode Operation. Values will be lower for Instant Start Ballasting. For Programmed Start Ballasting, the initial or Maximum Values will be similar.
Lamps will almost always draw less Wattage than they are rated at - especially when operated in extremely Cold or Warm environments.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!