First of all collect all the data from mechanical and other process engineers regarding the type of equipments including static and rotating machines : obtain preliminary information about their perfomance and duties
Determine the continuous running equipments, critical loads,
determinine the intermittent loads
determine the inductive , resistive loads,capacitive loads,
Fix the diversity factor
find out the maximum demand after determining the connected load
Another way to determine the load schedule is to go to each circuit breaker panel and count the total amps the panel could deliver. Add up the total number of amps that panel could provide and multiply by the voltage it provides.
That would provide the peak load schedule. A more realistic picture could be obtained by measuring the current for each breaker while everything is in use. You will need a current clamp to get that. Some circuits will be switching on and off while you are measuring so take the highest reading you see for each circuit. That would provide your actual usage load schedule. Make sure to record which breaker number and the reading together for each panel.