How to Install Current Transformers?

Tips for Properly Installing Current Transformers to Maximize Results

You’ve thoroughly researched submeters, current transformers, and communications options to determine the perfect equipment for your project. Next, it’s time to consider the current transformer installation process. Even if your project doesn’t rely on revenue grade accuracy to be successful, there are simple steps that you can take to ensure that current transformer installation goes smoothly and that you are maximizing performance.

Keep in mind that current transformer and meter installation should always be completed by a trained, qualified professional and follow all local electrical codes.

Make Sure the CT is Properly Sized

Project parameters are constantly evolving, so before current transformer installation, it’s important to verify the CTs on hand are still a good choice for your application and that they are correctly sized for the load you wish to measure. There are two key factors in sizing a CT: the rated current and the physical size.

The rated current indicates the amperage that can be measured by a particular CT. In most cases, a CT remains at its specified accuracy from 5% to 120% of its rated current. This means that if the rated current for a CT is 100A, it can be used on services from 5A to 120A. At lower than 5A, the CT will not be as accurate and at higher than 120A, the CT may begin to saturate which will, again, product inaccurate readings.

In additional to current rating, a CTs physical size is important to consider. Installing a current transformer on a conductor that is too large for the window means that the CT will not be able to close properly, producing incorrect readings. Oversizing a CT is also not recommended and, in general, a CT’s window should not exceed the conductor size by more than 50% to maintain best accuracy.

Check the Wire Polarity

After the CT is installed around the conductor, it’s time to connect it to your power meter. If your CT includes lead wires, the color coding will indicate the wire polarity. In general, the white lead is positive while the black lead is negative and must be connected to the meter’s positive (+) and negative (-) terminals, respectively. If your CT uses an alternative color scheme, such as white and brown, check the CT’s documentation before making this connection.

Much like the phase orientation, if the wires are connected to the meter “backwards,” the meter will measure negative energy for that phase, so it is important to take time to make the proper connections.

Make Sure the CT is on the Correct Conductor

It may seem like an obvious detail, but for the meter to collect accurate data, the CTs must be installed on the same phase as the voltage input. Mismatching CTs and voltage inputs will produce erroneous measurements, so it is especially important to verify this step of the current transformer installation prior to leaving the job site.

It can be a helpful best practice to label the CT leads to ensure the CTs are properly connected to the correct terminal on the meter. For crowded panels or switchgear, this can be essential to keeping track of which leads go to which terminal. It can also be a timesaver if troubleshooting is needed after the installation.

Don’t Overextend CT Leads

In general, it’s OK to extend the leads of the CTs using similarly gauged (usually twisted pair) wiring. To minimize noise or interference and ensure the CT’s maximum burden is not exceeded, it is important to not extend the leads beyond what is realistically needed for the installation. In other words, if the distance between the meter and electrical panel is 20 feet, there is no need to have 50 feet of lead wire, which can negatively impact accuracy (and add unnecessary cost). To maintain peak performance, shorter cable runs are always better.

Check Your Work

The installation process can be time consuming but pausing a moment to verify the installation is done properly is the best way to prevent costly trips back out into the field to correct problems. Before you leave, check that meter readings “make sense” for the load you are measuring. This step will often catch current transformer installation issues, meter setting problems, or other configuration pitfalls.

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