I do not know much about fluorescent tube ballasts, but I can give some general guesses. 1) Wattage is going to relate to total amount of heat dissipation that the ballast can handle. 2) Tube length (each in parallel or both in series) relates to the voltage needed to first strike and arc, and then maintain the arc that gets converted into light. 3) Tube diameter relates to steady state current needed to change the full width of the tubes. This would be summed (by area, nit diameter) for tubes in parallel. Looking on McMasters-Car, you can pull the same trick of double the input voltage, double the output voltage, two tubes in series on a ballast only rated for one: [1]http://www.mcmaster.com/?orderview=new#fluorescent-ballasts/=8dz6aa NOTE: this is only rates for 120V mains voltage. All the listings at MaMasters for 208 or 240 are for compact fluorescent folded tubes. It seems that the F8T5 tube is only directly supported on 120V ballasts. You might do better to buy a cheap 2 tube, 120V mains F8T5 ballast and a 240 to 120 transformer. Or just buy a brand new two tube lamp at the store and swap for the high UV bulbs. Mike
References 1. http://www.mcmaster.com/?orderview=new#fluorescent-ballasts/=8dz6aa
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