Uncle, if you plan on adding more 3 phase machines, the 7.5 HP will serve you well. The largest (HP) machine I run is a 5 HP Scotchman FI-51 iron worker. The 7.5 RPC has no problem under load. I have turned on all machines (one at a time) with no problem. I used #8 wire in conduit to all 3 ph receptacles allowing future expansions if needed.
The RPC is very quiet unlike some other I've seen but I started out with a very nice idler motor. I used this motor on my first scratch built system in my old shop and it saved me well for 10 years. One day I started up the system and it chattered badly. I was in the middle of a time sensitive job and ordered a Phoenix 7.5 HP to complete the job. It worked well but was much more louder than I was used to. When I built the new shop I emailed Phase Craft (eBay) my motor specifications and they built a control system for $200 delivered.
When I built the original system, I had well over $500 in parts from Grainger (not including the idler) so the Phase Craft was a no brainer and the phases are balanced nicely (I confirmed). I plan on adding a pilot light to the system as its ultra quiet. FWIW, I purchased the Howell motor for $50.00 from a local motor service center. Apparently, they rebuilt it for a customer who never came back, so after 3 years on the shelf, I walked in looking for a specific size, rpm and service factor. All they wanted was the cost they had in rebuild parts. I have more in wiring, conduit, receptacles and cord caps then the system itself.
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As mentioned, the 3 phase is in conduit and supplies power to machines throughout the shop including overhead drops. The runs are somewhat long and might get longer with possible machine additions (you never know haha) so #8 wire was used.
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Unless you buy a ridiculously large machine, your 7.5 RPC system will serve you well and have plenty of growing room.
I also wanted to mention the use of VFD's with a RPC system. With Mark's (mksj) help, we where able to add more control to the plate roller and the lathe. Something a RPC does not offer. If you find yourself wanting more control for features such as motor braking, adjustable rpm's on the fly, proximity carriage stop and more, you can always add a VFD.
Many experts on this site (I'm not) can help you with that when and if the time comes. Keep us posted on you power progress and as always, best of luck.
Turn and burn,
Paco
Edit: When you mount your RPC, use good vibration isolators to prevent resonance especially if you wall mount.