ECN Forum Forums Code Related Discussion NEC & other Code issues 2 power sources in one building?

I got a call to give an estimate to run a 240 volt line to a detached utility building. The homeowner has already run a 120 volt line to the building and terminated it in a 6 circuit panel. He wants me to run another line from his house and terminate it in another panel so he can run a 240 volt air compressor. One of the exceptions for this according to a old code book ( don't have a new one yet) if there are diffences in voltages this would be allowed. Could 120 volts verses 120/240 volts, be used in this exception, justifying another feeder??