Have a 42'x56' shop with white metal in the inside walls and ceiling. Believe i want to go with t5's in a 6 bulb 4' fixture. If you think this is a bad idea please let me know!
As Charles alluded to, much depends on the ceiling height and thus the effective mounting height of the fixtures. But regardless, six F54T5HOs in one fixture is a
LOT of lighting power packed into one relatively small area; and that in turn means that getting even light distribution throughout the space may be more difficult than it needs to be.
If this will be alright how many?
In broad strokes: Assuming you want the typical ~100 lumens/ft.^2 at working height, let's target 125-140 lumens/ft.^2 at the sources, with fresh tubes, before fixture losses, and all that rot. So... 42' x 56' = 2,352 ft.^2; multiply that by 125-140 lumens/ft.^2, and we get 294,000-329,280 total source lumens. Let's throw a dart and say 300,000 for round numbers. A typical F54T5HO tube is rated at about 5,000 lumens when new; so that's 60 tubes. If you put these into six-tube fixtures, that's only ten fixtures to cover the entire space. I would be VERY leery of this. In fact, I would go so far as to say it's a prescription for disaster.
Stepping back to four-tube fixtures based on F32T8 tubes lets us use 27 fixtures to cover the same space, at essentially the same average intensity; but it will provide MUCH better light DISTRIBUTION, with far fewer and far less severe "hot spots" vs. dim/shadowy areas.
I have a sissor truss 15'8" in middle to 13'6" on sides.
Are those figures to the bottom of the trusses, or the total height to the underside of the roof? Either way, this pretty much confirms what I said above about 6-tube T5HOs being too much power in too few places. Even 13-15 feet is just not all that high as compared to the sort of spaces where such high-power fixtures really would be appropriate (think aircraft hangar and/or large factory floor).
