I recently added a sub panel to my house It contains 3 15 amp breakers, 2 15 amp tandem breakers, 1 20 amp breaker and 1 2 pole 30 amp breaker for a water heater, all the 15's and the 20 are for general purpose receps. How do you calculate the size of breaker to put this sub panel on. Taking 80% of all the breakers added together just doesn't seem right, maybe it is though. Please help me. Thank you.
You got the cart before the horse. First, you compute the combined loads, using diversity factors allowed by the NEC. Second, you choose a panel of a size adequate for the loads just computed. Third, you choose a feeder of ample size. Fourth, you choose a circuit breaker sized to protect the feeder and panel.
Or, take the shortcut: size the breaker for the size of the new panel (usually 100 or 60 Amps)and feed with 100 or 60 Amp wires.