First off, by the NEC the maximum load on a breaker
shall never exceed 80%. That, of course is if a licensed electrician does the job. If the owner does it, without inspection, the NEC is more of a guideline. However, the NEC is a good guideline. If you follow it, you're sure of the safest install available. And, with electricity, that is not something to sneeze at. It really matters
... ...
Now, 30 amp is about the minimum that
I would use for myself. Think of it as the breaker protects the wiring, not the load. My compressor is set up on a 30 amp plug. It's only a 5 HP (compressor load). The motor has a thermo-disk in the end bell to protect the motor. The machine you're setting up
should have an overload protection device integral to the controls. If not, there are many sources for such an animal.
Then, there is the 4 vs 3 wire business. If you are getting the 4 wire service, it
probably is a sub-main. For that, I would want at least a 50 amp circuit. My shop has a 70 amp. With a seperate building and all the electrics in it, there is actually a 150 amp panel with a 70 amp feeder. Look at
http://www.hudsontelcom.com/ , down the page a little for "
Home Shop Electrics". It is lengthy, ~50 pages of PDF, but is well worth the time to read. For a non-electrical person, it should give you enough insight to keep from burning down the shop. An important point because the heat would damage the truth of the machine(s). And make the spouse a little angry, too.
Bill Hudson