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Anonymous Poster

# How to Calculate CT Ratio and PT Ratio

10/16/2009 1:08 AM

Please tell me how to calculate the CT ratio and PT ratio for 11kV?

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Guru

Join Date: May 2009
Location: Richland, WA, USA
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#1

### Re: How to Calculate CT Ratio and PT Ratio

10/16/2009 11:47 PM

There may be variations, but usually the desired output from a CT is 0-5 amps. Determine the amps of the current passing through the center of the CT, and select the ratio accordongly. For PT, determine the voltage applied to the primary coil, and the output voltage you need. Select a ratio to match.

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#3

### Re: How to Calculate CT Ratio and PT Ratio

10/18/2009 12:14 AM

Friend,

When you are connecting to a meter, you also would need the meter's faceplate to be printed with the proper scaled values. On our switchgear, the incoming voltage is nominally 12.5kV and the PT ratio is 12,000:120 or 100:1. This gives a secondary voltage that is well-within the normal insulation range of building wires. Before applying any PT's, it would be well to determine the actual type of connection for the incoming power--Wye (Star) or Delta, because this will also dictate the ratio and connections for the PT's.

Similarly to this, for CT's the same scaling of the meter faceplate is needed. It may be ordered with a 0-5A movement, but scaled so the 5A corresponds to the full-scale primary current of the PT.

--JMM

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#2

### Re: How to Calculate CT Ratio and PT Ratio

10/17/2009 5:16 AM

CT output is 0 to 5 amps, if you are connecting to a map chart recorder such as a Bristol. The idea is to keep the output from CT to chart recorder at 2.5 amps, so with that in mind read the following...........................Current Transformers Electronic and mechanical measuring devices monitor the amperage in the switchboard. To monitor the motor current, it must be reduced to a level that is easier to measure. If the amperage drawn by the motor is not reduced, measuring devices have to be large and cumbersome in order to house components capable of withstanding high currents. Most submersible pump system measuring devices (motor controllers) are designed to operate from 0 A to 5 A. To reduce or scale-down the actual current in the switchboard to an acceptable level, current transformers are used. Current Transformer Ratios The current transformers should be sized so that under normal operating conditions, secondary control amperage levels are between 2.5 Aand 3.5 A. This is done to obtain the best motor controller performance and to have the pen track in the center of the recording ammeter char t. The current transformer size calculation is — current transformer size = expected amperage through the switchboard x 1.6................next part............Example Motor and Switchboard Information 39 A, 990 V required motor surface voltage; MDFH size 3, 1,500 V switchboard. Using current transformer size = expected amperage through the switchboard x 1.6, current transformer size = 39 A x 1.6 = 62 A Choose a current transformer with a ratio as close to 62:5 as possible. (In this case, a 75:5 ratio.) (39/75) x 5 = 2.6 A hope this helps

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Anonymous Poster
#4

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 7:55 AM

Hi, Could you please tell me where the 1.6 factor (which is applied to the load current in your calculation) comes from? is this just applied to allow for flexibility in full load current? or have i missed something?

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#5

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 8:25 AM

it is the co-efficient used to calculate the correct answer. I asked that question years ago, that was the answer, so I used it. hope this helps

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Anonymous Poster
#6

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 9:21 AM

HAHA, thanks, about as much help as everyone else who i have asked. (None).

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#7

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 9:30 AM

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Anonymous Poster
#8

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 9:45 AM

Dont get me wrong, i appreciate the reply, but as an engineering student, submitting technical information to professional institutes to demonstrate an understanding of basic engineering theory, i am trying to gain an understanding of WHY a factor of 1.6 is applied to a current, not simply "that it must be done". Where this information is sourced is not important, but understanding it is key.

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#9

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 9:51 AM

that my friend is a FAR FAR better reply, rather than the previous answer. I thought you might be in college/university, so give me time and will TRY and get a FULLER answer for you. However I would ask this.... why do you apply 3.14 to every equation when calculating the circumference of a circle? Because it's "Pie" and we know it works, same theory, but I will look for answer for you OK?

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#12

### Re: How to Calculate CT Ratio and PT Ratio

11/08/2009 2:02 AM

Guest,

In reading the exchange you are part of, I believe the answer of the 1.6 factor is embedded in the original answer. IF you are wanting to get the current reading marginally above the midpoint of the meter range (as suggested for keeping a chart recording pen near the middle of its full scale), then the 1.6 is simply a scaling factor for shortening the range of the full-scale reading. That may be a valid approach, depending on the nature of your data needs. Problems with this approach include an increased % reading error and decreased ability to read very low loads.

All-in-all, your approach needs to be determined problem being solved. If accuracy is important, scaling the reading near full-scale is better. If ability to read overloads is important, scaling for the normal load at (say) 60% of full-scale is more reasonable. Whatever you do, make sure that any assumptions and limits to the validity of the readings are included in your documentation. That also is good basic engineering practice.

--JMM

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#10

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 10:31 AM

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Anonymous Poster
#11

### Re: How to Calculate CT Ratio and PT Ratio

11/06/2009 11:46 AM