Hi I have fitted out several set ups similar to yours & have found that putting more than 3 per circuit causes problems.
These fittings are very inductive so have very high inrush currents at switch on, this can cause switches to burn out and even C curve MCB's to trip.
Now I wire them in pairs on 3 circuits via a 3 pole contactor, then you can use a single weatherproof switch to control all 6 fittings.
This keeps wiring runs short as don't have to loop down to switches, I fit contactor next to dist bd take Neutrals straight to each pair of lights, lives/lines via contacts and control circuit from board to switch and back to contactor coil, normally get away with C10 mcb's, wire in 2.5mm2 if long runs,
I had similar problems to what Sparx relates I used three phase lighting track which allows a fuse in each lamp supply and I used a contractor with C16 MCB feeding track. Think they were 6 amp fuses in each lamp. They are heavy units and plugged in hanging on jack chain seemed easy way to fit allowing everything to be pre-assembled. I wired all lighting track clips did not trust the spring clips. Cables to track was SWA
All best Eric
PS are you sure High bay lights for cow shed? I would but uncertain on using even low bay lights in most cow sheds I have seen!
A quick check at 60 degrees spread to get 0.66 watts per square foot they will need to be 24 foot from work surface and at 45 degrees 34 foot from work surface. Seems a little on high side for cow shed?
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