1) Depends on the speed of your S9s, to a small degree.
Presuming 13.5TH, they draw 0.098W/TH, so 1323 Watts FOR THE CHIPS ONLY. Bitmain lies when they say that is "At the wall". Add another 10% for power supply inefficiency, control board power, fans, etc. So lets call it 1455 watts each.
Blokforge recently posted this video:
https://youtu.be/gMZSEV6H-_4, which shows 1458 watts on a 13.5TH unit, so 10% is close.
2) At 208V, each miner is going to draw about 7 amps (6.995 something per my calc)
2a) A 20 Amp breaker can be continuously loaded at no more than 80%, so 16 amps.
2b) Therefore you can run (2) S9s per 20 amp breaker
2c) Therefore you have enough breakers to run all the S9s, but not the L3s.
4) Built mine out of Unistrut (well, the similar material available from Home Depot). Much stronger and re-designable vs. commercial shelving. Likely much cheaper. My setup has (5) 10' shelves that each hold (10) units. Those are replicated as needed.
Is there any negative to increase the breaker to 50amp( 50 x .80 = 40/7 = 5.6a) for 5 miners and one PDU per breaker? I wonder if they have a 5 (c13) PDU at great price?
) that for every 11amps of three phase power you can run three miners one for each phase? If true then, 50a x .80 = 40a/11a = 3.63a x 3 miners = 10.9 miners?