Existing Duct Systems
The (CEC) California Energy Commission requires a minimum of 300 CFM per Ton of A/C for existing or altered duct systems. That is because the installer cannot be held responsible for ducts he neither installed nor has access to for repairs. A system that has less than 300 CFM per Ton is very inefficient and may lead to freezing up of coils and even compressor failure. New duct systems.
The (CEC) California Energy Commission requires a minimum of 350 CFM per Ton of A/C when the duct system is "All New" *.
A new duct system's blower motor must use < 0.58 x CFM (measured) airflow. Example; A two ton A/C system has a measured airflow of 400 CFM per ton = 1000 CFM. We take 1000 (CFM) x 0.58 = 580 maximum allowable fan efficacy (watt draw) for this system.
Unfortunately many existing systems move just 200-300 CFM. This is yet another reason that HERS testing is now required for existing and new duct systems.
* All new duct system is defined as; 75% (or more) of the ductwork is new and all ductwork is accessible. Existing cans and register boxes can be part of an All New duct system.