Standby loads are loads required to be operational during power outages of their redundant loads. For the purpose of electrical calculation, let us say you have 3 500hP compressors, two of them running all time and one is standby. During power outage all three of them will be running at the same time for a certain time,2 hours, once the third compressor is running for two hours the first one turned off.

So during your system study especially for the worst case are all three considered loads or only two to be considered. Note: the mechanical system is capable of using only two at a time.

Please provide a basis for your answer including codes and standards?
.