CT’s and PT’s









CT’s, or current transformers, and PT’s, or potential transformers are used in metering to step down current and voltage to safer and more manageable levels. Many people want to know what is a current transformer and potential transformer. Here I will try to demystify the CT PT confusion. One thing that I want to note also is that CT rated meters are not only used as a secondary electric meter, they are also used as a primary electric meter as well. CT rated meters are also typically demand meters as well.

When CT’s and PT’s are used in a metering installation, the installation is known as being transformer-rated. Some people refer to the meters that use a CT PT combination or just CT’s as a current transformer meter. Transformer-rated services run in parallel with the service. This means that unlike self-contained services the customer’s power is not interrupted when the meter is removed. The reason that they are needed is that either the current and/or voltage of the service to be metered is too high. This also depends on the policies and procedures of the utility. For example, some utilities require anything over 480v to be transformer-rated. While other utilities do not.

Also, some utilities do not use PT’s in 480v services at all. I recommend against this practice for the safety of the meter tech or lineman who may need to install or remove these meters from service. Read why you should be using PT’s here.

So, what do CT’s do? As stated before they serve to step down high current to a safe a manageable level. Revenue grade CT’s are engineered to produce 5 amps when the amps on the service are at the rated value. For example, a typical installation in a 120/208 service 400 amp service contains 200:5 CT’s. When 200 amps are flowing through the primary side of the CT, 5 amps are coming out of the secondary terminals.

CT’s have nameplates and ratings just like any other piece of electrical equipment. The most important things to note on the nameplate are the ratio and the rating factor. The ratio will be printed in large letters on the side of the CT. Typical ratios are 200:5, 400:5, 600:5, 800:5 and so on. Again, what this means is that when the stated value of amps is flowing through the primary side of the CT, 5 amps is flowing through the secondary side.
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The rating factor is used when determining what size CT to use in a particular installation. Some CT’s have a rating factor of 4, 3, 2, or 1.5. What this means is that the manufacturer says the CT is accurate beyond its nameplate value. For example, a 200:5 CT that has a rating factor of 4 will accurately measure a service up to 800 amps. So, if that particular service were to have 800 amps on it, there would 20 amps coming out of the secondary side of the CT and in the meter base. This is important because we want to size our CT’s so that they are fully saturated. Meaning that we want a 200:5 CT to be sized so that the amps flowing through the primary side has as close to 200 amps as possible. When the core of the CT is fully saturated it is the most accurate. CT’s tend to lose some of their accuracy at lower amp levels.

Most transformer-rated meters today are class 20 meters. This means that the current coils inside the meter are rated to carry a continuous 20 amps. You do not want to overdrive the meter by placing more than 20 amps in the meter base because you sized the CT’s incorrectly. For example, you would not want to place 200:5 CT’s in service that you know will be pulling 1000 amps on the primary side. This would place 25 amps in the meter base going over the meter’s rated capacity. This leads to lost revenue.

To properly size CT’s it is important to know what the actual connected load will be. The best way to do this is to consult with the engineer. If the CT’s are to be placed in a pad mount transformer or on the pole and there is only one service coming off of those transformers, it is best to size the CT’s to handle the maximum amps that the transformer is good for. This does two things, one, it makes sure that your CT’s are never overloaded and two, it is a way to find overloaded transformers.

Another thing that many people want to know is what is the current transformer sizing calculation. I know that I said before that you should consult with the engineer and you should but the formula that we use for current transformer sizing for a single phase transformer is:



KVA x 1000

line to line voltage

Now, to find the correct current transformer size for a three phase service we use this current transformer sizing calculation.

KVA x 1000

line to line voltage x √3

This is actually the formula to find the maximum ampacity of transformers. With this information we can then size the current transformers based on the information that is given.

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Enough about CT’s, let’s talk about PT’s. PT’s are potential transformers. They are also called VT’s or voltage transformers. They are used to step down the voltage to a safe level so that it can be metered. PT’s are typically used in any installation where the voltage on the service is 480v or higher. Some typical PT’s are 2.4:1 and 4:1.

Now that we know what CT’s and PT’s are, we can talk about meter multipliers. Meter multipliers are used when meters are installed in transformer-rated installations. If the CT ratio is 200:5, then the meter multiplier is 40, which is simply 200/5. If a service has both CT’s and PT’s then the two values are multiplied together to give the billing multiplier. For example if a service has 200:5 CT’s and 2.4:1 PT’s, the multiplier will be 96. This is because 40 x 2.4 = 96.

We also know much about CT’s and meters because of Blondel’s Theorem. Follow the link to find out more about this theorem.
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58 Comments

  1. Thanks for the helpful overview… I’m a utility guy (legal side) and wanted to know the difference… Great overview…

    • If a CT operated energy meter is installed at the transfer secondary and another meter is installed 5 meters away from the other meter (parallel I.e same conductor and same transformer), which of the meter readings will be higher. Kindly explain in detail

  2. No problem Rich. I am always here to help. If there are any other questions that you may have please let me know.

    • Hi just want to know i installed 3 600amp meters on a 3 ph disel genarator the amp meter tels me when i use a 220v grinder its pulling 150amps must i install cts if so what size will work

  3. My electricity is 3phase smart meter, 8000 khw,3 * 230 / 400 v, 3*5 amp,my consumption per month is always less when I read the meter, how can I work it out to be accurate?

    • You will need to make sure that you are reading the meter on the same days that the company is reading the meter. Or, alternatively, you could look on your bill and look at their readings and use those for your calculations. You will also need to keep in mind that you may have demand charges, base facilities fees and taxes. Another thing that companies are also adding on are charges for their renewable energy portfolios. The first thing that you want to do is find out your electric rate as this will have the information you need to calculate your bill.

  4. Thank you , for this great review .
    It is of great help .
    Regards..

  5. If ct ratio is 600/5 and meter ratio is /5. What is multiply factor

  6. Great information, thank you.
    How would we calculate the voltage induced into the secondary circuit?
    Assuming we have a 120/208 supply, and a 200:5 CT.
    Would it remain the same @ 120v/line?

  7. Thanks for response, and confirms my thoughts. I read your email also, very informative with fitting example. Your site has good common sense information, without going off the rail. I will now look closer at PTs next time I am working in a metering cabinet
    Cheers

  8. 2kw-hr power read from (present minus prev.)
    ang CT multiplier is 160
    assuming $9/kw Hr

    2k x 160 =320
    320 x $9 = 2,800 bill
    is this correct?

  9. I love the information on your website. Thanks a lot!.

  10. Our hydro supplier never took actual meter reading for 21months and recalculated and sent a bill for 24k$!!! We had to pay it or we would’ve got cut off. Our monthly bills are the same as they have always been! We fought and argued that they were wrong. Any advice?

    • My advise would be to contact your utilities commission. They should be able to guide you on what your options are. Sorry for that happening to you.

  11. hi,
    We have a 3 phase machine with 100KW load capacity installed what CT ratio energy meter will best suit the requirement ,

  12. christian dean rosales

    Example: Primary Metering for 3-phase, Delta connection
    Will use 3 each – Potential Transformers rated at 14,400 V.
    & 3 each – Current Transformers rated at 400:5 amps.
    What is the Multiplier ?

    • You take the PT ratio and multiply it times the CT ratio. I am assuming that you 120:1 PT’S. That would mean 400:5 = 80 x 120 = 9600. So 9600 would be the multiplier. If your PT ratio is different you would need to use that ratio instead of 120 and multiply it times 80.

  13. my ct/pt is 1/1A but i m using meter with Ib 5A are they compatible.

  14. I installed transformer of 500kva . Now i want to know what rating of current transformer and potential transfomer i should install

  15. what is the effect of using a ct with ct ratio lower than that of the load to be metered?

    • This can actually be desirable depending on the rating factor of the CT. The rating factor means that the CT is capable of handling more than its nameplate value. For example, a 400:5 CT with a rating factor of 4 means that the CT is capable of accurately measuring loads up to 1600 amps (400 x rating factor of 4 = 1600). Now, you must also consider the meter that is being used with the CT. Now most transformer rated meters are rated as Class 20 meters. This means that their current coils are rated at 20 amps. However, there are still some Class 10 and even Class 5 meters still out there. So, you can overdrive and damage the meter. Remember that standard CT’s are most accurate at their nameplate rating up to their rating factor. So, check the rating factor of the CT and the current rating of the meter.

  16. Sir plz tell me about window current transformer

    • Window type CT is a term used to describe the CT. This means that there is a hole in the middle to pass the wires through. The other type of CT is known as a bar type CT. Bar type CT’S have a buss bar that passes through the middle. This allows you to bolt wires to them.

  17. Thanks. I currently have PQ meter installed in my generator switchgear compartment with PT (due to 13.2kv) and CT’s. I want to duplicate the pq meter with a ISO compatible meter due to requirements of the utility for generator metering. Im assuming the PT can be just duplicated to both meters, but how do we duplicate the CT without adding another set of CT’s?

    • Yes you can use more than one meter on one set of CT’s as long as you do not go over the burden rating of the CT’s.

  18. Hi, I have a customer who employs a old CT revenue meter and wants to have it upgraded, I know our local supply company supplies the revenue meter but the consumers wants to order a new enclosure with three CT’s etc, can you please recommend any NZ or Aussie suppliers.

    • Normally the utility supplies the CT’s as well. I am sorry, I am in the States and I do not know of any suppliers in NZ or AUS. Is there a particular reason they want to replace the CT’s? Have the been tested and shown that they are no longer accurate?

  19. Hi,

    This is extremely useful so thank you. I was wondering whether you can work out how many amps a supply is by knowing the CT ratio.

    So I know that the CT ratio is 500:5 and it’s a 3 phase 4 wire supply 3 x 230/400V

    Does this tell me how many amp supply it is?

  20. Good day! how can we know the standard CT to be install. what should be our basis on that?

    please give us example.

    many thanks!

  21. What’s the math on CT metering on the spades in a 3 phase pad 120/208 with 200:5 CT’s if running conductors back thru the CT’s to meter another service?

    • The wire running backwards cancels itself out so as not to add to the amount of current registered from the spades.

  22. Am I able to power up two tenancies off one CT?

    • The short answer is yes. The longer answer is that while it is possible, I advise against it. You will be better off if you meter each tenant separately.

  23. Hi,

    There are CT’s installed at my place but i strongly suspect there is something wrong with the connections of the CT’s or with the meter. Rating of CT’s is 600/5 and we are using multiplier of 120. Transformer size is 150 KVA/415V. My running load is around 110 HP and my bill comes very high compared to other plants who have the similar machines. I am in dilemma what is going wrong. If the bill was 10-15% high, i can understand but its coming more then double compared to others. Please assist me on trouble shooting this.

    Rgds,

    • There are several things you can check but the first is to examine your bill carefully and check the rate you are on.

  24. Hello,

    Can you recommend quality + value make/model CTs and VTs for implementing a Schweitzer 735 meter to a 550V 3-ph ungrounded wye circuit carrying 1000A

    Thank you,

    • ABB, Peak Demand, and Ritz all make good CT’s and VT’s. They differ a little in their specifications but products from any one of those companies should meet your needs. For some people it all depends on how the connections are made to the transformers.

  25. what happens if CTs of 200 ams installed to a 400amp meter and vice versa

    • I hope that I am understanding your question correctly. If you install
      200:5 CT’s on a 400 amp service then as long as the rating factor of your
      CT’s is at least 2 you should be able to meter the service properly.

      If you are asking if you can install 200:5 CT’s with a 400 amp meter then
      no. That is because a 400 amp meter is a self-contained meter and does not
      use CT’s. Also, if you are using a 400 amp meter you have no need for CT’s
      that are rated for less current.

      If you go the other way and install 400:5 CT’s on a 200 amp service then
      you will most likely lose some revenue. The reason is that CT’s are most
      accurate at their nameplate values.

  26. Hi
    I need to replace a meter of a transformer (the secondary side) The secondary supplies a 450v 3 phase load, the secondary side of the transformer has no neutral. The existing meters seem to use 2 CT’s and only 2 voltage supplies, there is no access to the meters.

    What would be the best way to meter this using CT’s?
    Would it be 2 CT’s and 3 voltage connections or 3CT’s? Or alternative?

    Any help would be appreciated.

    • Sounds like a 3 phase 3 wire delta service that would typically use two CT’s and possibly two PT’s depending on what voltage you want in your meter base.

  27. We have a standard 3ph and N supply.since the power Co installed a new smart meter our bills have almost doubled. They say that our old CT rating was 200:5 and they have changed this to 400:5. and we have been substantially under paying. Could this be correct?

    • Unfortunately this could absolutely be correct. Power companies have a couple of different ways of putting the muliplier into their billing systems. One is to program it into the meter and another is to put the multiplier into their billing system. This is all done by a human and sometimes we get things wrong. If they found that the multiplier was incorrect and have now corrected it then yes your bill could double in this case.

  28. Is C.T ratio is considered while installing a energy meter?
    Let I have a energy meter and I don’t have information regarding its C.T ratio configuration, then what should I do?

    • Yes the CT ratio is considered. It tells you what the multiplier will be in the circuit. If you do not know that the ratio is you need to look on the nameplate or use a CT ratio tester to figure it out. Beyond that you can use an ammeter. Just make sure that you are using equipment that is rated for the voltage and current on the circuit and that you are wearing all necessary personal protective equipment necessary.

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