More likely an electrician who had the wire on his truck, or assumed that ALL 220/240 circuits needed a neutral, the same as electric ranges and dryers. Few would ever think ahead for that purpose, since a subpanel at an A/C unit would be a decidedly rare installation. Not all 6/3 wires have grounds, so until you specified that yours did it was not obvious. WHERE is that third wire connected now, since it should have no purpose in life, or need to exist.
Sorry, I forgot to add after "planning ahead"
The NM-B terminates in a junction box, then jumpers for the hot pair run through a mud ring and bushing into the back of the disconnect box mounted outside. The neutral was capped off with a wire nut in the junction box.
Last edited by jeffeverde; 12-12-2009 at 08:08 AM.