Not sure if it has a standard on that but if you look at the spec sheet for the power supply some times it lists by the dimensions what clearance it requires. I just had a PLC with a PSU and some other cards and it required like 6" on each side from any other equipment or enclosure walls for proper ventilation. My coworker (since I am new) just has me look at dimensions of the unit going into it then with the back panel taken into consideration make sure it is clear of the door even with wiring onto the top of the unit etc. (hopefully there is no exact code for it that he didn't tell me about)