50:5Amp CAT. 2 SFT-500

[cizzle21p]

How far can this CT transmitt a reliable amp reading? I cannot find the literature. We are reading a motor that needs to run under 50 Amps. This will be connected to a analog simpson meter 0-100 ampos about 50-60 feet away.

 

[kwired]

How far can this CT transmitt a reliable amp reading? I cannot find the literature. We are reading a motor that needs to run under 50 Amps. This will be connected to a analog simpson meter 0-100 ampos about 50-60 feet away.

My guess is it depends on length, voltage drop, and size of conductor, all of which will effect one another. Prevention of picking up induced currents from other circuits will likely make some difference also, which can be greatly reduced by not running in same cable or raceway as other circuits.

 

[jim dungar]

I don't recognize the number of your CT.

Have you asked your vendor?
Is your meter analog or digital?
Why are you using a 50A current transformer with a 100A meter movement?
How much 'less than 50A' is your load?

 

[gar]

120622-0807 EDT

cizzle21p:

Current transformers are a current source, not a voltage source, over some reasonable range of output voltage. Thus, a change of load resistance on the current transformer makes little change in the current reading.

A 50:5 current transformer will have 10 turns on the secondary (5 amp side). The primary is 1 turn of the single wire thru the current transformer once. If you looped that input wire back thru the current transformer a second time, then it becomes a 25:5 transformer.

It is intended that a low impedance current meter be the load on the secondary of the current transformer. These meters might be in the range of 50 to 250 millivolts full scale.

You would use a 5 A meter movement on a 50:5 current transformer, but the scale would be calibrated 0 to 50. If you used a meter not matched to the current transformer and it was a 0 to 5 A meter and the scale read 0 to 5 A, then you would need to multiply the meter face reading by 10 to get your circuit current measurement.

Suppose the ammeter is 50 millivolts at 5 A, then its impedance is about 0.05/5 = 0.01 ohms.

Now suppose the burden rating of the current transformer is 10 VA. then the maximum load impedance you should place on the current transformer is 10/5² = 1/2.5 = 0.4 ohms to remain within the accuracy rating. Copper #14 wire is about 2.5 ohms per 1000 ft. To stay within 0.39 ohms the total wire loop length is 1000*0.39/2.5 = 156 ft. Thus the meter could be up to 75 ft from the current transformer with #14 copper wire.

Check my math for any mistakes.

See http://electricalnotes.wordpress.com/2011/04/16/current-transformer/

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