How To Choose The Right CT For Your Application
Current transformers (CT’s) come in many shapes and sizes for power metering devices. While having a lot of options can be a nice thing, it can also present a challenge when deciding which one should actually be used for your application.
Knowing whether the application calls for a 333mV or rope style Rogowski coil CTs determines which Power Meter model number is selected (KW350 and KW320 single circuit meters only). If the application requires 333mV CTs then the model number would end with an “-SC”, and if Rogowski coils are utilized then the model number would end with “-RC”. Note: The KW1850 multi-circuit meter is compatible with multiple CT input options, however, 333mV CT or Flexible Rogowski Coil CTs should not be intermixed within a specific meter.
If you ask yourself these five simple questions, selecting the best CT’s for use in your sub-metering application will be easy.
What is the goal of the power monitoring project?
For contractors, understanding the overall goal for the installation of the Power Monitoring equipment is important in the selection of the proper equipment. The overall cost and selection of the meters and CT’s are all predicated upon how you are planning to monitor all of the data. Questions like "do I have a building management system?" "Is this a permanent install or temporary install?" "Do I want to use a Power Meter with Data Logging capabilities to download the data into a .CSV type file for use in a spreadsheet?" "Am I going to be using the information for Tenant Sub-Metering or billing application?" "Will I be monitoring a single load or multiple loads?"
What is the amperage that I will be monitoring?
Each CT has a useful operating current range, therefore understanding the minimum and maximum operating current for each of the loads to be monitored is important in order to properly select the best CT for your application. You can safely use a CT up to the maximum useful operating current as long as it does not exceed that limit. Keep in mind that the minimum useful operating current range is important since this is the point at which the CT will not be able to monitor the current anymore, thus causing the Power Meter to “Snap to Zero”. This means the Power Meter will output that “0” current is being used, even if there is a small amount of current flowing. So in applications where the maximum operating current is below 600 Amps, a standard split/solid-core style CT is recommended due to the increased resolution of the CT, smaller size, and overall cost savings. Why use a Rogowski Coil CT with a 5 to 5000 Amp useful operating current range to monitor an electrical panel rated for 100 or 200 Amps? The smaller, solid and split-core CT’s have a useful operating current range of 1 to 200 Amps (100A Midi CT) or 1 to 300 Amps (200A Midi CT) respectively. Note that Rogowski Coils should be used when the wire size or buss bar is too large for the use of the standard solid core or split-core CT’s aperture or window as well as in applications with multiple conductors and panels rated above 600 Amps.
How accurate do my CT’s need to be?
For contractors, ease of installation, stocking a single CT, and shorter install times can often lead to the choice of using Rogowski Coils over the smaller split/solid-core CT’s. Determining whether or not to use a standard accuracy CT or a Revenue Grade CT has both accuracy and cost implications. The Revenue Grade CT’s have been designed and tested to meet a higher accuracy when used with a Revenue Grade Power Meter and should be used with projects like tenant billing where accuracy is of great importance. Other than the CT's accuracy, the other main difference is the additional cost of the Revenue Grade CT’s versus that of the Standard Grade CT’s. In a vast majority of applications, using a Standard CT will meet all performance requirements. As discussed above, the key in selecting the proper CT should be based upon the minimum and maximum load size, selecting the CT that will give you the highest amount of sensor resolution. Using a 100 Amp CT on a 100 Amp Load will give you a better sensor resolution when compared to a CT that is rated to 5000 Amps. Each CT type has a place, and knowing when and how to properly select and install them will aid in the overall performance and accuracy of the data.
How much space do I have for the CT’s?
Let’s be honest, there’s not much space in a typical electrical panel. By the time you get everything, you need in there, an inch of free space becomes a valuable commodity. In some cases, the mindset that “one CT fits every application” will lead contractors to select Rogowski Coils, and their larger sizes often take much-needed space. A helpful tip is to make sure that your CT matches the size of the circuit breaker or load being monitored.
Can my CT leads be extended?
Yes, all of the Standard CT’s and Rogowski Coils may have their leads extended from the typical six to eight-foot length up to a maximum of 100 feet. Typically you can do this in the field or you can custom order it with the proper lead length from the factory. For the best results, ACI recommends ordering them with the proper lead length from the factory where there will be no external splices (which can take 3-4 weeks time). If you want to do this in the field, it is imperative to use the same wire type specified by the manufacturer, as well as using the Sure-Connect sealed butt splices as recommended by the manufacturer to ensure that they are properly sealed. If you ever have any questions regarding lead length or extension, make sure to contact an HVAC professional.
Knowing the answers to these five questions will not only help you select the proper CT for your application as well as to enable you to greatly reduce the overall cost of the power monitoring system. If you still have further questions about what is right for you, please feel free to contact one of ACI's sales engineers at 1-888-967-5224.
