That has been a matter of some debate. My opinion is that working clearance is not required, and that the only space that needs to be maintained is whatever the manufacturer says about giving it room to conduct its heat away. I leave a minimum of 6 inches for that purpose. Others will contend that you can't do thermographic studies unless the covers are off and the transformer is energized, and they would invoke 110.26 on that basis. This might have to be a question for your local AHJ.