I wouldn't screw around with any fuses in the Buck-Boost transformer at all - just feed it from a 2-pole 240V 20A breaker if you're using all 12-gauge wiring, 15A breaker if some is 14 or16-ga cords.
If you want to save yourself some heartache, get panel-mount fuseholders and 3A or 5A fast-blow fuses, put one on the side of each fixture connected to the Hot lead to each ballast.
With a dozen fixtures in a Carport mounted at 8' - 9' it's not a huge problem to track down a shorted ballast...
BUT it's a good habit to fuse 277V lights, because when you can put 30 to 50 High-Bay fixtures on the same 277V leg in a Warehouse, tracking down a single shorted fixture will take you all day (or more) and drive you nuts as they yell "What's taking you so long? We can't work with the lights off!"
Whereas the one fixture with the burned ballast will pop it's fuse and isolate itself - You find that fuse blown, check the ballast before you replace the fuse.
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