CT Metering Calculations

[ousnjie]

Dear All,
I want help in calculating the correct multiplying factor for maximum deman CT meters. For example, I have a scenario where the CTs are rated 200/5 and and the meter 300/5. What multiplication factor should I use? What would be the resulting factor if both CTs amd meter had the same rating (300/5).
I have tried dividing the meter ratio by the CT ratio giving 1.5 but am really not sure.

Pls assist!

 

[waross]

If the meter is marked 300:5 it was probably last used with 300:5 CTs.
The multiplier is the CT ratio times the PT ratio.
If you are not using PTs the multiplier is the CT ratio.
For 200:5 CTs your ratio and your multiplier will be 200/5=40 Change the meter to read 200:5 if possible.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

And if you're stuck with this meter and CT combination then your meter will indicate 1.5x whatever the true value is.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty;
I don't know where the poster is residing, but in North America, Most CT rated kWHr meters and combination kWHr/demand meters read the input values directly.
However, for ease of meter reading, the overall multiplier is added to the mater nameplate.
When a meter is re-installed, the multiplier or ratio shown on the meter is changed to indicate the new value.
If this meter was in Canada, the meter would be changed (erased and re-written) to show the new value. eg: 200:5
Some meters do have a multiplier when used in special applications but in this case it may be 1.5 or 2. Ususlly a small enough number to avoid confusion with the CT and PT ratios.
Some multiplier numbers are lithographed onto small plates and the plates are affixed to the meter nameplate at the meter shop. Such plates sometimes look as if they are an integral part of the meter and it is not obvious that they were added to the meter in the meter shop.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[epepito]

"And if you're stuck with this meter and CT combination then your meter will indicate 1.5x whatever the true value is."
I think the opposite is true. When your 200:5 ct is maxed out your meter will read 300 if it was previously calibrated to work with a 300:5 ct. So you would really divide by 1.5.

 

[davidbeach]

Think a bit more. You confirmed that the meter will read 300 when it should read 200, so it is reading 1.5x the true reading. You are also correct that to get the correct reading you need to divide by 1.5; you need to divide because the reading is 1.5x what it should be.

 

[waross]

I have seen quite a few meters but never one to take 300 amps.
Neither have I ever seen a 300 amp meter socket. The most that I see are CL 200 rated for direct readng of up to 200 amp services. I installed a new direct reading 200 amp meter last Tuesday. The multiplier was 1. That's one.
I see a lot of current-transformer fed meters. (Wednesday, Thursday and Friday) The meters typically read the 5 amp outputs of the CTs. If the meter is used with 300 amp CTs it will often be labeled 300:5 so that the meter readers get the right multiplier.
With 300:5 CTs the multiplier would be 60.
Scratch out the 300:5 and write in 200:5. Now the meter reader will know that the multiplier is 40.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Morrand]

For what it's worth, we do install some 300 amp meters (class 320, actually) on single-phase 400A services, usually big residential services.

For example:

http://www.geindustrial.com/cwc/Dispatcher?REQUEST=PRODUCTS&pnlid=5&id=kvcl320

 

[epepito]

Wow, sorry that was kind of dumb on my part.

 

[waross]

Thanks for the heads up, Morrand.
The utilities in this part of the world have not yet moved to class 320. Class 200 is the highest rating without CTs here.

Bill
--------------------
"Why not the best?"
Jimmy Carter