I have a detached 350 square foot workshop about 70 feet from my main service panel. I've buried 3-#4 Type USE-2 for 240v and 1-#6 USE-2 for ground. I've also driven a ground rod near the workshop's sub-panel and tied that into the ground-bar. The neutral and ground are not bonded in the subpanel.
I've been using this configuration for a couple of years, with double 60a breaker on each end to protect the feed. The service to my home is 200 amps, so I think I have plenty of room there.
I plan to add a hot tub, in the backyard, about 30 feet away from the workshop. It requires a 60a GFCI and breaker.
I currently have a 3kw electric heater in the workshop, which runs less than 3 hours at a time, a 2.5 hp table saw (seldom used), 320w of flourescent lights, and a shop vac, as well as some low power items in the workshop. The max current on the hot tub will be 46 amps. It has about 5hp worth of motors in it, in addition to a 5.5kw heater. It will continuously pull 1/2 amp for the circulation pump. It is conceivable that the heater (12.5a) , table saw (13a), lights (1.5a), shopvac (11a) and hottub (46a) would all be at full tilt at the same time (84a).
1) Would it be okay for me to upgrade my 60a breakers, currently protecting the workshop's subpanel feed, to 90a or 100a? The #4 USE-2 has an ampacity of 95 amps, according to NEC 310-16. I think the 2008 NEC 310.15(B)(6) code changed, so that sub-panels cannot be protected with the next higher amp breaker.
2) Would I be overloading this 90amp circuit, is it borderline, or should I just run the hottub wiring directly to my main panel?
Any help, advice, or opinions are all appreciated.