How to Evaluate Emergency Generator Capacity in an Existing Installation
 
There is often a big difference between a planned generator application and what we find in the field. Equipment gets added or removed, fuel systems are often modified, and generators are even traded from site to site without close evaluation of load. This makes it difficult to evaluate the appropriateness of an application. I have, therefore, come up with a few quick procedures that will give you an accurate indication of your generator's ability to perform.

First:

You will need a clamp on AC ammeter with the capability to record peak current and peak voltage. (We have these meters available to loan)

Go to the automatic transfer switch during a time of day when your facility will use the most electrical current. This is usually 4 to 6 PM for telecommunications buildings, when traffic is high and the greatest heat load is on the building. Set your ammeter for AC amps and peak load then clamp it on to each leg of the utility power (A, B, and C) in turn. Leave the meter on each leg for as long as possible; thirty minutes to an hour at least. Record each reading.
 
You will now have readings something like this:
 
A = 145
B = 178
C = 124

Add them up and divide by 3:

145 + 178 + 124 = 447

447 divided by 3 = 149

The number you now have represents the average current you need to provide to start and run the load you have. Notice I emphasize start. By setting the ammeter to peak, you will measure the maximum current your system needs. Be sure the air conditioning units cycle on and off while you are measuring peak current.

(By the way, if you have a single-phase system rather than a three-phase system, you will only measure A and B phase and divide by 2)
 
Second:

Compare the current you need with the capacity of the generator.

Your generator will have a nameplate. The nameplate will give you the current rating for the voltage you are using. For example a 30kW generator wired for 120/208 three phase will produce 83.4 amps. This will be on the nameplate. Obviously 83.4 amps won’t reliably do the job when you need 149 amps. But a 60kW generator, wired for 120/208 single phase, will produce 166.7 amps. Therefore, the 60kW generator will handle the load.
 
If you cannot find the nameplate you can use the following formulas to figure current.
 
Single Phase
kW = (Volts x Amps) / 1000

Three Phase
kW = (1.73 x Volts x Amps x 0.8) /1000

KVA = kW / P.F.

Keep in mind, I said "83.4 amps won’t reliably do the job when you need 149 amps." The key is the word reliably. It just might work sometimes; but if you have more current available than your measured peak you can be assured of consistent performance.

If you would like additional information or specific motor starting data and generator capacity please call:
1-800-888-4594

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