GeneratoR CapacitY

How to size a generator?

Usually, books tell us to use rule of thumb. Simple formula, but I forget it, hahaha... If I'm not mistaken, then it will be the total load (not connected load) plus at least 1.2 times the largest standby motor capacity. Why do you need the 20%? Well, it's not for future load. It's for the compensation of motor starting.

You see, when a motor is started, it draws current from generator much higher than normal. It can reach 500-800% of its normal current. As the consequences, the sytem voltage will drop. How much will the voltage drop? It depends on the reserve power the system has.

One thing to remember, the system itself has a limitation of voltage drop. Usually, the running motor will loss its power when the motor terminal voltage is lower than 80% of its rating voltage. The started motor will be failed to be started. And the contactor in motor starter or feeder will open if the voltage is lower than 70% of its rating voltage.

It is troublesome to see that up until now I can't find some formula in books or standards to calculate how much the reserve power is needed considering this voltage drop on motor starting.

The formula I use now? Well, since I can't find it in any literature, I believe that it's kind of company belongings. I just wondering, is there general formula? I mean, a formula all electrical engineers know and use.

Anyone can tell me the formula?


@layla, 1:37 AM 3/22/2009

thEsE days

New habit. Trying not to get involved any further. But how can I? It's my drugs. The more I consume, the more I want it.

My prediction, this thing will hurt me in some months. And again, I don't avoid it.


~@layla, after idle months~