Generator sizing

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mikehughes8

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NC
When sizing a generator for a load how should lock rotor amps be considered. I have ten buildings, each with (2) large AC units with dataplate indicating 67 LRA. Do I size my generator for the worst case scenario and assume all AC units will start at the same time? If not whats the best way to go about this. I'm in a very hot climate. The AC units will be cycling often.
Also, is anyone familiar with RLA. I saw this on the dataplate and I'm not familiar with it.
 
RLA = Running Load Amps This would be the load once it has started and settled into its normal operation.

I would try putting your loads into the generator sizing software that one of the gennie vendors (ie CAT, ONAN, etc) puts out. This will size your generator, engine, etc, and it allows you to account for elevation, ambient temps, and the like
 
mhulbert said:
RLA = Running Load Amps This would be the load once it has started and settled into its normal operation.

I would try putting your loads into the generator sizing software that one of the gennie vendors (ie CAT, ONAN, etc) puts out. This will size your generator, engine, etc, and it allows you to account for elevation, ambient temps, and the like
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I don't suppose they supply this software for free on one of their websites?
 
Usually you would add the minimum circuit ampacity of all the units except the largest. Then for the largest one, you add the starting amps.

If you think they will be cycling a lot, you might even want to substitute the starting amps for a couple of extra units. But at any rate, I wouldn't use the starting amps for all of them. They won't all start at the same time. And if there are 20 units, odds are they won't all even run at the same time unless it is very hot. And if that happens, they won't be cycling on and off, they will just stay running.

And you probably can find the software for free. Also try Generac. They just had a web seminar on sizing commercial generators, and the instructor said they would have another one coming up on residential generators (See the NEC part of this forum. I think Roger posted the link.)

Steve
 
Sizing

Sizing

Mike,

See if this link helps. I didn't have too much time to search, sorry.

Hope this link works
GMc
 
How many transfer switches are you going to use? If just one, the loads will start together, i.e. in 1 step. If you can use time delay relays or programming in the A/C units to stagger the restarts you will be able to use a smaller genset.

You total the running amps and the starting amps for step one. Then for step two. But the starting amps for step two = the running amps for step one PLUS the starting amps for step two. etc. The genset must have the capability to handle the max starting amps and running amps encountered in any step. Starting the larger loads first will help.
 
sparkie001 said:
How many transfer switches are you going to use? If just one, the loads will start together, i.e. in 1 step. If you can use time delay relays or programming in the A/C units to stagger the restarts you will be able to use a smaller genset.

You total the running amps and the starting amps for step one. Then for step two. But the starting amps for step two = the running amps for step one PLUS the starting amps for step two. etc. The genset must have the capability to handle the max starting amps and running amps encountered in any step. Starting the larger loads first will help.
Click to expand...
In my situation the generators are not being used in a backup capacity. There is no prime power hooked up to these buildings. Therefore the generators(s) I use will be the sole source of power. I'm actually in IRAQ therefore my resources are limited and I'm dealing with High Ambient temps. The lighting loads etc. are easy to figure. I'm just concerned about the AC units.
 
OK, then if you start the AC units manually, one at a time, you can get by with a smaller genset. Start the largest units first, if not all the same size. Good luck!
 
Having sized many generators over the years (some a large as 2.7 megawatts) I would strongly suggest speaking to a sales rep from two or three of your favorite generator brands and have them size it for you. If you let them know that they are competing for the job, they will not oversize it nor will they undersize it since they are on the line to make sure its gonna work. I have used the factory sizing software for Cummins and Cat. They are a bit complicated....as they should be since sizing a generator is no simple task. What I have found is that the sales reps generally know the right questions to ask and can guide you through the sizing process quickly (and for free).
 
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