Donââ"šÂ¬Ã¢"žÂ¢t get me wrong, I'm not an expert on IC ballasts, but from everything that I've read, the only thing that varies the power supplied between individual bulbs on the same ballast is 1- the type of bulb; 2- the length of the bulb; 3- the bulbââ"šÂ¬Ã¢"žÂ¢s age; and 4- how long its had to warm up. The IC ballasts are said to provide proper power to each bulb regardless of the bulb-type combination, and they donââ"šÂ¬Ã¢"žÂ¢t vary the load depending on the # of feet you have hooked up (unless you exceed the recommended maximum). For example, a specific size (T5) and length (36") bulb will draw the same power (~60 watts) no matter what else is on the circuit (or whether you use a 430 or 660). Additionally, the IC ballasts will only overdrive a bulb up to about 50% above its stated wattage, because much more than that can destroy the bulb.
Quoted from ASH...
"If you use a 54-watt T-5 HO lamps on a Model 430 (one or two lamps) or a Model 660 (2,3 or 4 lamps) they will operate at 85-watts per lamp. The output provided and energy required have been confirmed on countless threads on RC about using T-5 HO lamps."
If you've found a thread that states connecting fewer bulbs on an IC ballast causes them to be overdriven more, please share it because I have a 660 and I thought I understood how it operates.