Calculatinf System HVAC Load

[Ramdeom]

In a communication with Prof. Spitler, author of ASHRAE "load Calculation Manual" he states that this manual addresses the room loads only.
ASHRAE Standard 183 (Calculating Peak Cooling and Heating Loads) states that it is intended to address the zone load and not the system load although both are linked.
Does ASHRAE have any documentation for calculating the system load and determining the required plant capacity?
My idea was that we calculated the room loads, then the zone loads (for sizing the air handling unit) for a number of related rooms, then the block load for that floor. This is repeated for each floor and the loads for the different floors are added together, a diversity factor is applied and the system or plant load is arrived at.
I am not sure whether this method is correct and how the diversity factor(s) are obtained. Does ASHRAE have a documented and detailed method for this process whereby the entire system load for a facility is obtained and the plant capacity is determined?

 

[EnergyProfessional]

A proper load calculation software determines at what time and date you have peak for the system. I also takes into account thermal mass and obviously all the instant load factors. the load report will show that it was July 7th at 2 pm or whatever it determines when thee is peak for each system.
the software does that on system level (i.e. AHU) and total plant level since the sub systems could peak at different times.

The how to determine that isn't really the problem. The problem is a human would have a really hard time doing that, which is why a software is used that calculates load over all 8760 hours.