Select the right size of emergency generator

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    Juan J.Isdray

    How to select the right size of emergency diesel generator of 3 ph 480 V 60 HZ considering plus 10% for future load considering motor starting current and voltage drop about 15% during nomal operation



    From the information you have given, you are obviously talking about an industrial type of device.  Consequently you will need to consider other things such as an automatic cross over switch and other items.

    The units of measure will generally be in KVA.  Note that the K means “thousands”, the V stands for voltage which you have specified, and the A stands for current in Amps.  If you already know what devices must have power, they will likely be part or all of a substation near your switch gear (breakers, etc.).  Frequently you can observe the typical current being used at that point.  Understand, however, that many of the devices, lights, and more, may not be in use at the time of your observation.

    In order to leave a little room for the unexpected, you need to consider the maximum current needed at any point right now, add 10% for future growth and make sure that figure does not exceed 80% of your generator.  If you run above 80% of the full load power, you will generally wear out the generator before it should be.  Remember that some loads require a momentary higher than normal current to start up.  That has to come from somewhere!




    As NYOJ says make sure that you account for transient loads such as starting currents, particularly if you have one or more loads which consttitute a significant proportion of the generator capacity.

    Another point to consider is harmonic current. Today, installations often include many non linear loads such as Variable Speed Drives, switched mode power supplies etc. These all produce harmonic currents. You will need to measure, or make an assessment of, the harmonic current content in the system and advise the generator supplier of this as he may need to make an allowance for the additional heating these will cause in the generator.



    I agree with your both comments Smithy and NYOJ referred to not exceed the 80% of the EDG size and the other about to take into account the non-lineal load notifying  the suppliers.

    Thank a lot



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