Originally Posted by 1joeyj
What would happen if you installed T8 lamps onto a T12 ballast? They are 2 lamp 3' and 4' luminaires.
T12 is 0.43A (or thereabout) and T8 is 0.265A.
The operating voltage is much higher on T8.
On magnetic T12, the lamps either won't start/flash obviously or if it manages to remain lit, either the ballast or lamp will fail soon.
On electronic T12, the ballast will have enough voltage to strike a T8. The ballast will operate at higher wattage than intended, because the voltage is higher while providing the same current, its likely that ballast will fry.
on the other hand, running T12s on T8 ballast will cause them to operate as if a 25W 2' lamp was fitted in place. They will simply run at reduced power. On an instant start without cathode heater, the cathodes may not stay hot enough and ends may blacken rather early.