I have a 30 amp double breaker in my main home panel feeding my detached garage 100amp sub panel. Can i run a electric furnace with a 90 amp breaker in the subpanel. Or is this setup all wrong
If the second panel is fed by a 30 ampere breaker, then the maximum capacity of the second panel is 30 amperes. If you try to draw more than 30 amperes, the breaker in the main panel will trip.
You may be able to increase the size of the feeder breaker, but only if the wires feeding the second panel are large enough (which is not likely). You could probably increase the size of the feeder breaker, and the feeder wires. That would increase the capacity of the second panel, and possibly allow you to supply the heater.