Originally Posted by rjniles
IMO, return what you bought and buy new 4 foot T-8 fixtures. You will save money and have more efficient fixtures.
Not so fast. He can change to 8' T8. That would be fine if the lights are turned on and left on for long periods. They will chew up the tubes fast with frequent switching, however. If lots of switching is anticipated, get new 4' T8 fixtures or go to a lighting supply shop and get some "belly pans" and socket kits for about 25 bucks a fixture. Ballasts will be extra.
OP, I believe that you can not use those ballasts with your fixtures. Your lamps have one pin on each end, now two on each end, right? If so, you can not use the ballast you bought.
No matter what you do, you should probably switch to T8. You don't have to do anything with the fixtures except change the ballast and lamps. If you turn them on and off a lot, you might consider more extensive changes. The single-pin slimline lamps (six to 8 foot) must be used with instant-start ballasts (T12 or T8). They are very energy efficient, but use lamps fast if switched a lot. If you switch them frequently, get program start T8 ballasts, change to 4' lamps with conversion kits or new fixtures.
You can stick with instant-start T12 if you wish.
Give more detail and let us know what you want to do and I can probably steer you towards less expensive ballasts on Ebay if you are patient. You've got 3, single-lamp, 8' strips or reflectors? At my residence, I have 14, two-lamp, 1 four-lamp and one single lamp. Conversion is in progress. My 4' luminaires are already converted. Ballasts cost me about $10 each in case quantity.