Determining Generator Loading

[rockman7892]

We have a 750kW 480V generator here at our plant to provide emergency power to certain loads in the event of a power loss. I am trying to determine weather or not we have room on this generator to add additional loads which we have determined should be on emergency power.

I plan on adding up all of the loads from a theoretical standpoint, as well as take some power recordings of these loads in the field during normal operation.

What is the best way to determine how much a generator can be loaded? I have seen other threads that have referened free software from manufacturers for determining these values? What are typical factors or calculations that can be used from a theoretical standpoint with particular loads?

 

[itsmoked]

The best way is with a powermeter. You can calculate all you want it will still be wrong. You should test the generator as it will normally run, if at all possible.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Highly dependent on the load, the governor, the engine itself (turbo or not), the AVR, and so on. Rules of thumb are inherently conservative because if they weren't then they wouldn't work in the majority of cases. If you're up near the rating of the set then you need type-specific data. Cat give out copies of their rather good Specsizer software without much of a fight, Cummins UK are being their usual pain-in-the-ass selves although I think it's easy enough to obtain in the US (please God, could you send down a lightning bolt and obliterate Manston?) and most of the other manufacturers will run the calcs for you if you ask nicely.


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

Also depends if the authority having durisdiction will force you to use connected load for designing the load on the generator. If you know that actual load factor of the loads you can design and engineered solution, but is the generator continuously rated? seldom is, so that will be another factor.

 

[rbulsara]

There is a reason for the nameplate on the generator. It means something.

Rafiq Bulsara

http://www.srengineersct.com

 

[rhatcher]

For generators providing power to motors, the motor starting KVA must be considered when sizing the generator. If you size the generator nameplate KVA to equal the motor full load KVA, then you will not be able to start the motor.

This FAQ explains it better than I can:

http://www.eng-tips.com/faqs.cfm?fid=766

 

[zade]

I would recommend you take a reading at the worst time and add to it %25 as code recommends when you want to add a new load to an existing service. Use software to see if generator will take it. That %25 will give you latitute for during emergency loads will be shifted to emergency, (those that you did not measure for they were not plugged to emergency outlets) and also it sort of meets the code for you can consider generator service as an electric service

 

[waross]

I don't understand this:

If you know that actual load factor of the loads you can design and engineered solution, but is the generator continuously rated?

Standby generators are.
Prime power generators are not only continuously rated but allow a 10% overload, 1 hour out of 12 hours.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Waross,

Is your 10% overload (1 hour out of 12 hours) relates to the engine ekW or Generator kVA?

 

[waross]

A standard condition of many manufactures is that a 100 kW prime rated power generator is capable of outputting 110 kW for 1 hour out of 12 and such use shall not void the warranty.

Bill
--------------------
"Why not the best?"
Jimmy Carter