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#53962 07/13/05 02:55 PM
Joined: Jan 2005
Posts: 61
jkochan Offline OP
Smell rubber burning? It's my brain... Here's the thing. Commercial building. We are using on average 280,000 Kwh per month. 480V, 277V, 120V, 240V, single and three phase scattered about the building. I'm trying to figure out what reserve capacity we may or may not have on the standby generator when running in the emergency mode. I have the panel calculations from the prints and some panels aren't on generator power. Not much has changed in the building since the original build-out, and I have redlined the prints as the changes occured. I need just a general ball park caculation to let the powers that be know when we need to upgrade. I recomended that we have a load survey performed, but $$$ is always a problem .... until they have to throw it at a problem that could have been solved or at least identified much earlier. Can I add up the panel loads, subtract the non supplied panels and get close? The Generator is 1875 kVA 1500KW 277/480.

Joined: Dec 2003
Posts: 751
The best way is to measure the current at the service during operation, and during emergency operation at the generator. Recording voltmeters are best, as they record over a week (or more) to give you a graphical representation of your power comsumption. The bean counters love this.
No money for these beasts???, Use your Amprobe, and take readings at different times of the day over the week, keep a log, and present your hand-drawn graph to the bean counters.
Simply adding the panel loads is never accurate.

Joined: May 2003
Posts: 2,876
e57 Offline
Just go ahead and forget about kwh. If you are concerned with the capacity of the generator, focus on the loads it serves. And, and this is a BIG 'AND', capacity will be will be limited more by how the loads react at start up, than the 'paper value', or normal operation of the load. (LRA, excitation current of transformers, etc, etc.) Suddenly drop too heavy of a load on the generator too fast, or all at once, and you could have the gennie stall, or drop the load as an over-load. Talk to your generator Rep. to see what that maximum current, and duration is. In fact, you may find some, if willing or suspect that you are interested in the possible sale of a generator, might just do this whole calculation for you. Or be able to suggest eqipment that can better mange, or increase the load capacity.

Mark Heller
"Well - I oughta....." -Jackie Gleason
Joined: Feb 2005
Posts: 219
You can contact the local Utility Co and request a peak demand for this customer, then convert it to kW.


Joined: Jul 2005
Posts: 3
Junior Member
dean is right on the money - no pun intended. If a volt/current data logger is out of your price range: like : - Before the self contained dataloggers were commercially available, I stuck an A-D card in an old PC - wrote a very simple BASIC program that sampled the A-D card with an Amp probe attached - download the text file to Excel and analyze it.

Joined: Oct 2004
Posts: 55
There was a time, pre PC, when load calcs were done with a hand held ammeter and readings were logged by hand in a paper notebook. For the situation you discribe, the man hours required to create the log would not be more than an hour a day for about 3 figure 20 to 25 manhours to do a load assesment by hand.

e57 has the best option...check with equipment reps and you can probably get it done for the price of a sales pitch.
In a refinery the AVO rep demonstrated a data logger on a piece of equipment. He was going to leave it run for a couple of days and then forgot to come back to get it for a couple of months.

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