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CT Sizing

CT Sizing

CT Sizing

Im curous about CT sizing for a 347/600V service entrance. It being 600A 100% rated, would a 600/5 CT become saturated near the full 600A mark? Would this application require a 800/5 CT instead?

RE: CT Sizing

We generally use the rule of thumb that we size our CTs (on new construction) to 80% of the upstream breaker capacity.

RE: CT Sizing

80% of the upstream? I have a 600A 100% rated breaker, that would leave 480A at 80%, you're suggesting a size it at 480/5, which I dont belive is available? Im not sure if I follow you.

RE: CT Sizing

Sorry, what I mean is that a 600:5 should be okay for your application.  When we size our CTs, we assume that in general (it depends on the application and load) we won't be seeing more than 80% of the rated current of the upstream breaker. After calculating what 80% of the rated current is, we choose the next available standard size CT for our application.

Upstream breaker = 600A @ 100%
80% Load = 480A
Next high available size CT = 600:5

RE: CT Sizing

Are the CTs being used for metering or protection.

CTs have a rating called "rating factor"...official name continuous thermal rating factor. This is how many times the rated primary current that the CT can operate at continuously without thermal damage.

For metering CT, it also defines the top limit of the accuracy class.

For instance, a 600:5A CT rated RF=2.0 and accuracy of 0.3B0.5 (IEEE rating) is guaranteed to maintain a 0.3% accuracy performance from 600A to 1200A (rated current up to rating factor) and 0.6% accuracy performance from 60A to 600A (10%rated to rated current). For metering applications, it's a common mistake to use to high of a ratio. 600V CTs for metering normally come with rating factors of 3.0 or 4.0. If the max current is 600A, the best way to meter is with a 200:5A CT with a rating factor or 3.0.

If the CTs are for protection, then you need to educate yourself on the protection ratings of whatever standard region you're in. But in short, no a protection-rated CT won't saturate at or near rated voltage, assuming the rated burden is not exceeded.  

RE: CT Sizing

These CT are used for metering.

Interesting post scottf, Im interested in reading more regarding this. Mostly for metering as all of the protection relays have been specified by OEM.

RE: CT Sizing

if you used a 200/5 ct for a 600A service, what would be the secondary output be at 600A? 15? How would you configure your meter for this?

RE: CT Sizing


In the IEEE world, most meters used are Class 20 meters, meaning rated for 20A continuous input.

I'm not sure, but I think in the IEC world, with 1A rated CT secondaries, the meters are Class 2.  

RE: CT Sizing

Just tell the meter you have a CTR of 40, in that case 600A is completely irrelevant.  

RE: CT Sizing

I have some questions around the same guys... Retrofitting power monitoring to a sub-station which only has protection ct's fitted. It has been decided that 3 measurment ct's will be installed. Substation 66/11kV. Going on tx size, 400Amps seems to be max constant secondary. Protection CT's are 600:1 and the guy asking me to sort this for him has asked me to try and find 600:5 split core measuring ct since the sub will only be down for a short time - i am not sure if such a beast exists. He is hoping to hook it to a satec PM175. (the existing protection ct's are hooked to a satec PM171E - off drawing)
The PM175 specifies a burden of 0.1VA. I was wondering what voltage the PM175 would see at its input if 600:5 ct hooked up assuming primary current did get as high as 600amps? 20mV?
The other thing is i have had some guy tell me that a split core is available but isolation level is only 7kV - can someone guide me here - the cable it is going around is insulated etc photo attached
Thanks in advance ...

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