All it is saying is that if it is over 250V or over 40A, then the "Single Motor Unit" method of determining the sizing of the branch circuit conductors and overcurrent protection can't be used.
Because motors briefly draw a large current at startup, and then much less current while running, the code in some circumstances will allow a bigger-then-normal circuit breaker on a motor circuit to account for the startup surge. The 250V, 40A test is simply a test to determine which rules you are required to use to determine the wire and OCPD sizing, not a limit on what voltage can be used to power an A/C unit.
[This message has been edited by SolarPowered (edited 09-25-2006).]
