The best way to minimize the size of your emergency generator, is to try and place the various loads into steps, or a sequence of operation, rather than all in one step.
The sequencing can be accomplished by the use of transfer switches, time delays or building automation systems, or a combination of them all. Usually a delay of at least a couple of seconds between each step is enough for the generator to recover and take on the next load.
How do you decide what loads go first through last?
If your project has ‘life safety’ and/or ‘critical branch’ loads, these must go first in your step as they need to be on within 10 seconds. These should be on their own transfer switches.
Other various loads can be placed in the next step. These could be loads such as air conditioning, ventilation, pumps, UPS, non-emergency lights, etc.
Large motors are the key factor in driving up your generator size.
If you have a large motor that comes on and stays on continuously, it could be placed in one of the first steps. By setting it up this way you have more generator available to handle the ‘in-rush’ before other loads are applied, reducing the total generator size required.
However, the most motors are not continuous in a typical design. Air conditioners, water pumps, elevators, and fire pumps, do not come on as soon as power is returned via an emergency generator. Therefore the best plan is to put the largest motor in the final step.
A ‘soft-start’ mechanism should be used on large horsepower motors to reduce the ‘in-rush’. These come in different forms, reduced voltage auto transformer (RVAT) wye/delta, solid state, to name a few.
Last thing to remember are the allowed voltage dips. Fire pump, no more than 15%, booster and well pumps, typically 20%, medical imaging equipment 10%.
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