Dale,

You don't need to think about overdriving fixtures until you have run out of room over your plants to add more fluorescent fixtures. But frequently you still won't have as much light on the plants as they would like, and then you should start overdriving your fixtures, because at that point you can't add more fixtures.

MM

Maineman, So it is indeed a space issue. because if I can put in more fixtures, do that until i have no more room? Then overdrive?

Dale

Yup, its a very simple point that is rarely made. Overdriving is just for people that can't fit enough tubes in the space to get the light they need.

Also look at T5 tubes which are thinner so you can pack in more of them. Or high output T5 tubes, thinner and already overdriven to get more light from the same length of tube. Or compact fluorescents, thin fluorescent tubes bent into a variety of shapes to fit in a smaller space than the equivalent straight tube.