Ok, so it's not like halide lamps which the ballast puts "X" watts into the bulb. T5 draw the power they need rather than receiving whatever is given.
Straight from the Fulham website, I can find wiring diagrams for the Work Horse 5 for both 2-39W bulbs and 3-39W bulbs (not for just one). I don't see anything about their recommended combination - maybe I will email them.
Given the information you guys have provided though, I would be best off running 3-39W lamps on each of these ballasts?
What do they mean by "for linear lamps use a starting aid"?
