Knowing how to calculate the full load ampacity of a transformer is a very important calculation to have in your bag of tricks as a meter technician. Here I want to show you how to do the calculation as well as provide why you want to calculate the full load ampacity of a transformer. Finally, I will show you how you can use the calculation to troubleshoot a transformer-rated metering installation.
How to calculate the full load ampacity of a Transformer
There a couple of things that you need to know before you start to calculate the full load ampacity of the transformer in question. First, what are you even calculating? You need to know what your answer represents before you go punching numbers into your calculator. The full load ampacity describes how many amps the transformer is designed to handle. This is important because it helps determine what size transformer is needed to handle a particular load.
Many times we receive information about load in terms of amps. Well, most transformers are sized based on KVA, kilovolt-amperes. Since transformer are sized based on KVA we need to convert this number to amps in order to know what the transformer is capable of handling.
Next we need to know a couple of other things. One is the line to line voltage of the secondary output of the transformer. You also need to know if the transformer is a single phase transformer or a three phase transformer. Once you know all of this information you can start with the calculation. For simplicity we will start with 100 KVA single phase 240v transformer. To calculate the full load ampacity use the following formula:
KVA x 1000
Line to Line voltage
So, for a 100 KVA transformer we will multiply 100 x 1000 and then divide it by 240v.
100 x 1000
That gives us 416.67 amps. So, for a 100 KVA 240v single phase transformer the full load ampacity is 416.67 amps.
Next let’s calculate the full load ampacity of a three phase transformer. There is one more step that you have to do in order to find the full load ampacity and that is to use the square root of 3 which rounds out to around 1.732. Let’s do the same thing for a 120/208v three phase transformer. Use the following formula:
KVA x 1000
Line to Line voltage x 1.732
For a 120/208v three phase 100 KVA tranformer we calculate the full load ampacity as follows:
100 x 1000
208 x 1.732
That gives us 277.58 amps. So, for a 100 KVA 120/208v three phase transformer the full load ampacity is 277.58 amps.
Why Calculate the full load Ampacity?
Now that you know how to calculate the full load amps of a transformer you probably are wondering why in the world you did that in the first place.
One reason specific to metering is that it tells you the number of amps a transformer is capable of producing so you can size your CT’s accordingly. In both examples above you can get away with using 200:5 CT’s with a rating factor of at least 3. This covers the entire operating range of each transformer.
Another reason to know the full load amps is that it ensures that you are not over or undersizing your transformer. An undersized transformer is one that is going to have a shorter life span because of the excess heat that is generated due to being overloaded. An oversized transformer is a transformer that is being under utilized. This adds up in the form of increased system losses because even though the tranformer has plenty of capacity the coils still have to be energized and this can be thought of as waste.
Knowing how to calculate the full load ampacity of a transformer can help you troubleshoot the entire installation. You as the meter tech will most likely be going out and testing transformer-rated metering installations. Many of these will be installed on tranformers that are serving only one customer. When you test the site you will find out how many amps are on the service. You can then take this information and compare it to the full load capacity of the transfomer.
Another thing that you will do is look at the demand on a transfomer by looking at all of the meters being served by a particular transformer. Looking at each meter individually will only let you know what each service is pulling on its own. If you add each of these service together you will be able to tell whether or not the transformer is sized properly.
Finding the full load ampacity of a transformer is a very useful calculation to have on hand. It can alert you to problems that may arise on your system as well as help you install the right size CT’s.