CT and PT Calculations

So I have been getting a lot of requests about how to do CT and PT calculations. This page is dedicated to showing examples of CT and PT calculations. There are a few examples with CT calculations only and a few examples with both CT and PT calculations. Some of this information is repeated from the CT’s and PT’s page. So, if you have not already checked it out head over there for an overview of what CT’s and PT’s are and their role in metering.

You may be asking yourself why you need to know how to do these calculations. These calculations are very important to the accuracy of your metering equipment. One of the mistakes in metering that causes the largest loss of revenue or over billing is incorrect multipliers. These incorrect multipliers can come from poor calculations.

Also, it is important to know how to do these CT and PT calculations to make sure that the CT’s and PT’s are sized correctly. This ensures that they are sized to the service correctly. It also ensures that they are sized correctly to the meter that is being used. This means that you will not be over-driving the meter. It also means that you will have enough current going through the CT’s so that are measuring current values close to their nameplate ratings. And so with that, let’s do some math!

CT and PT Calculations Example 1

Ok, for the first example let’s say that we have a service with 200:5 CT’s and when we check the wire that is going through the CT with our ammeter we get 100 amps. How many amps will we get on the secondary side of the CT?

First we need to remember that the ratio of 200:5 can also be written as 200/5 or 200 divided by 5. This is the first calculation that we need to do as this will give us the multiplier. So 200/5 = 40. Thus, our multiplier is 40. Now we can divide 100 by 40 and get our answer of 2.5 amps. So, if there are 100 amps on the primary side of the CT there should be 2.5 amps on the secondary side.

Example 2

For our next example, let’s start in the meter base. Let’s assume that we also are using 200:5 CT’s in this service. We are checking the amps in the meter base and find that we have 3 amps flowing on the secondary side. How many amps should be on the primary side? Remember from before that our multiplier for a 200:5 CT is 40. In this example we need to multiply to get the answer instead of dividing. So, 3 amps x 40 = 120 amps. If 3 amps are flowing in the secondary side then there should be 120 amps flowing in the primary side.

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Example 3

For the third example let’s incorporate the rating factor of the CT into the equation. Let’s say that we have a 400:5 CT with a rating factor of 4. What does this mean? This means that the manufacturer says that the CT is rated to handle 4 times the rating on the nameplate. The nameplate rating is 400 amps. So we would multiply 400 amps x rating factor of 4 = 1600 amps. This is the maximum this CT is rated for.

Example 4

Example four will include CT’s and PT’s. Assume that we have 400:5 CT’s and 2.4:1 PT’s on a 277/480 three phase 4-wire wye service. We have 250 amps flowing in the primary side of the CT. How many amps are on the secondary side of the CT? Also, how many volts should we expect to see in the meter base? Remember however, that we need to find the CT multiplier to find the current first. So, 400/5 = 80.

Next we need to divide 250/80 = 3.125 amps. To find the voltage in the meter base we simply need to divide the voltage by 2.4 since 2.4/1 = 2.4. So, 277/2.4 = 115.42 and 480/2.4 = 200. This is the voltage that we would expect to see in the meter base. We need to remember though that 277/480 is the nominal voltage and that many times it can be a bit higher. Also, remember that the multiplier for the entire service in this case is 2.4 x 80 = 96. This is the billing multiplier.


Hopefully this helps clear up a few things about CT and PT calculations. Remember that these calculations need to be memorized. You should be able to do these calculations on the fly very easily. Also, make sure to leave any comments below.

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