2 minutes reading time (300 words)

Removing the Mystery from Current Transformer Classes

Current transformers are an important part of power transmission and control panel applications today. They have many ratings, and some can be customized to your business. However, there are letters and numbers printed on current transformers, and it is important to know what they mean. This brief explanation can help you get a better understanding of the subject.


The difference between secondary (Is) and primary (Ip) currents is the ratio. For instance, when you order your current transformers from Midwest Current Transformer, you have to specify the amps for primary current rating. Secondary current ratings are either 1 or 5 amps. 5 amp CT applications are common. However, when circuit relays are at a distance, 1 amp CT applications are best because of factors like voltage drop.


CTs are rated by accuracy class. It is very important because there is potential for error between the primary and secondary currents. Accuracy is dependent upon: 

  • Saturation – this is the point where a core is completely magnetized and producing the most flux.

  • Burden – load of the secondary side of the transformer

  • Load – equipment connected to circuit and drawing current. It can vary a great deal, but there is a maximum the CT can safely handle.

  • Frequency

  • Temperature


Suppose you have a CT class 5p20 current transformer. The first number represents how accurate it is. Most transformers are between five and ten. The letter “P” stands for protective device. It is meant to prevent overcurrent conditions to protect circuits and equipment. The next number, “20” is the accuracy factor. For example, the 5p20 CT can do its job even when there is 20 times the current flowing in the secondary.

If you are unsure of your CT needs, call Midwest Current Transformer today. Our toll-free number is 1-800-893-4047.

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