Understanding the VA Rating of a Current Transformer

In this blog I will try to answer these 3 questions:

1. What is a VA rating and when does it apply to a CT?
2. How can I figure out the VA rating for my application?
3. What happens if I get this wrong?

What is a VA rating?

Power monitoring includes two important readings: voltage and current.  Both are needed in order to calculate the various parameters we might be interested in, including active power, reactive power, apparent power, power factor, power demand, and so on.

The voltage measurement is rather straightforward; the voltage lines connect to the power meter and internal circuitry measures the voltage and waveform.  As long as the voltage doesn’t exceed the maximum voltage the meter can handle, or fall under some minimum threshold, the meter will be able to get its voltage reading.

Current Transformers on the other hand, are a bit more complicated.  In order to measure current, a CT is typically used (other options exist, but are less practical).  In order to work properly, the CT must “wrap” around the primary conductor(s) of each phase, i.e. each phase must pass through its own CT.

Today, there are multiple types of current sensors; this blog will focus on all “true” CTs, i.e. any CT that has a current output.  Common current outputs include 5A, 1A, 0.1A or a low mA output. These CTs are prolific, and remain the most-widely used type worldwide.  Each of these current sensors is affected by a VA rating.

*Note that CTs with a voltage output (regardless whether AC or DC) are burdened with a resistor inside of the CT, and do not have VA ratings.

So what’s a VA rating?  The VA rating of a CT is an indication of how much power the CT is capable of producing.  CTs with larger cores are generally capable of transferring more power.  If we are outputting current, one might wonder why we care about power.  Well, the wire we use to connect the CT has some resistance to it.  In addition, the current output of the CT gets “burdened” by a resistor at some point – usually within the meter – in order to convert the current into a voltage, which is required for measurement.  Collectively, these resistances must be “overcome” by the current output of the CT, which is where the VA rating comes into play.

What will happen if the VA rating is wrong?

If the VA rating is too low (relative to the entire resistance of the circuit), current will drop as it travels through the circuit, resulting in under-reporting the actual current.  Can the VA rating be too high?  From a technical standpoint, there is no such thing as a VA rating that is too high.  From a practical standpoint however, large CTs are more expensive than smaller ones, and they are also bulky, making them more difficult to install.

How can I figure out VA rating?

This is where our calculator comes in handy.  It calculates the resistance of the circuit, which consists of 2 parts:

1. The resistance of the wire in the complete (roundtrip) circuit.
2. The meter’s resistance (the burdening resistor).

If you have already chosen a meter, #2 has been decided.  However, #1 is determined based on the wire conductor type (the type of metal), the length of the wire, and the gauge of the wire (thicker gauge wire has less resistance).

After you enter these parameters into our calculator, it will indicate how long the leads of the CT can be (and you can see the result of switching wire gauge).  Alternatively, if you plan to use a certain length and gauge of wire, it will tell you the minimum VA rating need from the CT in order to maintain the CT’s stated accuracy.

If the resistance of your circuit results in a required VA rating greater than your CT can provide, the result will be reduced accuracy.  The further these two numbers are apart, the worse that accuracy will be.

VA Calculator

Understanding the VA Rating of CTs

You can use this calculator to do a few things:

1. Determine the max length of wire you can use with a meter (w/o losing accuracy).
2. Determine what VA rating you would need for a CT given a required distance.
3. Determine the effect of different gauge wire on the CT circuit.

Choose an option: