3 Phase Delta - Wiring a New Machine
OK at 220V your RLA (run load amperage) is 39.2A, NEC Table 430.250 says you can calculate 22A for a 7.5 hp @ 230V. You are saying that the new machine would run on 30A so that would fit the RLA for a 7.5 HP motor. Using that as our guide your minimum circuit amps would be 27.5A. If you machine is actually 15KW then your RLA is 39.2 and your minimum circuit amps would be 49A. 

So you either need a circuit that can be fused at 30A (7.5HP) or 60A (15KVA). Your #6 conductors would work just fine. Running a quick voltage drop calculation you are under 3% but you will drop a volt or two. Besides most motors prefer to run a little hot (higher voltage)rather than under voltage. 

Because of the discrepancy of the information, I would install a 60A disconnect for the machine, since you can always fuse down, while fusing up could be a problem. Also your machine disconnect must be within line of site, maximum 50' away in an unobstructed pathway and lockable. Whether you want to fuse it at your disconnect is up to you, but never fuse it higher than the main or any upstream over current protection. 

Also pay particular protection to @ThreePhaseEel. If it's a 100 year old facility it could very well be a corner grounded delta. You might verify that with a local professional before proceeding.

Stay safe and good luck.