Answered pretty quickly earlier, now that I looked at things again - I change my line of thinking on my initial reply.
Actually read 450.4 this time also, which says the transformer must be protected at no more then 125% of input current rating of transformer.
NEC is not all that clear on exactly what that means, as the autotransformer used to go from 240 to 208 volts here is not going to be a 12 kVA transformer but rather a 2 kVA transformer
2 kVA high voltage coil of that transformer will only see 8.3 amps at full load rating - but the 32 volt coil of that transformer can see put to 62.5 amps at full load rating.
I presume NEC intends input rating for this 2 kVA autotransformer to be 62.5 amps 125% of that is just over 78 amps - so it could have up to 80 amp protection. Actual load in OP was only 58 amps - so in that case 60 amp protection and 6 AWG conductor is most practical for the load - unless it is continuous load then we jump back to 80 amps and 4 AWG conductor needed.
Now the fact that the actual load of 12 kW is only going to be 50 amps on the 240 input - as long as it isn't continuous load I think you could still provide 50 amp overcurrent protection and 50 amp conductor as the input conductors. Output load is 58 amps though and you would need 58 amp conductor for the output portion of the circuit. It is still protected by the input overcurrent device and will hold even though the output current is higher then input curren.
