is the intention that the generator only powers these loads, or supports the whole building during any power cut, and then as an aside, if there is a fire this extra  load is added ?

The reason I ask is that 'sizing a generator' is not just a case of adding up all the loads for worst case and buying the next size up.

There are almost no generators that will start properly under significant load and one that will tolerate a 100% load step has to be designed specially, normal  generator controllers will only stay in control if the load appears in small steps, perhaps 1/4 to 1/3 of the full load. If too much load is applied at once, there is no time to increase the fuel supply to the engine, and generation stalls either totally, requiring a re-start, or partially where the load drops out due to under-voltage and then things cough and retries, perhaps repeating that cycle several times - neither is good.

Unless you have the exact data at your fingertips, and especially for something that may become safety of life, it would be as well to liaise with the generator makers, and involve them in the sizing calculation.

Getting it wrong, could be very expensive,

Mike.

The Standby generator is to only power the life safety systems. I allow for a 60% step load to the generator, as per manufacturers requirements. There will also be time delays applied to the ATS's however all life safety loads need to be online within 15s from generator cold start, so there is possibility of overlap, hence including for starting currents and DOL current for Smoke fans?