Understanding Amperage Ratings and Burden Resistors
Amperage ratings are used to guarantee a certain output at a given input. For example, a SCT-0750-100 will output 333 mV when 100A is “sensed” on the primary conductor. A SCT-0750-150 is an identical product, except for the burden resistor inside the two devices. The SCT-0750-150 will output 333 mV at 150A (and 222 mV at 100A).
Many customers are initially surprised to find out that a 3000A rated device can cost the same as a 250A device. This occurs when the only difference in the devices is the burden resistor, e.g. the RCT-1800-250 versus the RCT-1800-3000.
With most current transformers the product is guaranteed to be accurate with a given range of the rated current. Often this range is from 10% to 130%, although this varies slightly depending on the product type. Using a SCT-0750-100 as an example, the product will be 1% accurate measuring from 10A up to 130A.
Understanding this range is particularly helpful when purchasing a CT where you expect to measure at lower ranges most of the time, but will occasionally see higher amperage inputs. For example, say you are measuring current up to 500A, but you are most concerned with a much lower amperage, let’s say in the 50A range. One could purchase a SCT-1250-600, but the 1% guarantee of accuracy cannot be reached at 50A. Instead the best choice would be a SCT-1250-400. It can measure down to 40A with 1% accuracy and yet will still be 1% accurate up to 520A.
Another concern of customers is safety. Often the question is whether the device will be safe when operating at a higher amperage than the rated amperage. Safety and the amperage rating of a device are two different concerns. The products are of course guaranteed to be safe when operating at the rated amperage, but just because a device is rated at 100A does not mean it cannot be safe at a higher amperage (although it typically won’t be as accurate). Instead, what determines safety is the internal wiring of the device, i.e. how thick are the wires winding the core. Devices that are UL listed have gone through testing that determines this upper limit. If the same-model device is sold with an amperage rating higher than the one you are purchasing, you can be assured that the device is safe up to that higher amperage. For example, a SCT-1250-050 will be safe up to 800A because a SCT-1250-800 is also sold.
Once an inspector found an SCT-0750-100 installed on a 200A power panel line and gave the installer a bunch of grief. It took some time to convince the inspector that the SCT-0750-100 was safe up to 200A. Although the SCT-0750-100 probably wouldn’t product 666 mV at 200A, it certainly wasn’t going to melt or anything of that nature. In this case the installer knew that the customer would want to measure accurate down at 10A (and seldom if ever at 200A), so it was more important to have accuracy on the low end rather than the high end.