### 200 AMP current sensor

Hi,

I am a newbie to this project, and also to electricity in general. I would like to however learn more, and I would also like to learn if the current sensors in use by the project will work on a 200 AMP line.

Thanks,

Mike

### Re: 200 AMP current sensor

Certainly not. The recommended current transformer (stocked in the shop) has a maximum rating of 100 A. However, there is nothing to stop you from using a different one that is rated to 200 A. The point you need to watch is that it is capable of developing a voltage of  1.1 V rms across the burden resistor, the value of which you'll need to calculate when you know the secondary current that the c.t. supplies. So unless the c.t. that you choose has a secondary current of 50 mA @ 200 A (i.e. a turns ratio of 4000:1) you will need to change the value of the burden resistor supplied in the emonTx kit.

### Re: 200 AMP current sensor

Can you suggest a supplier for the sensor?

I too have 200 Amp service.

Changing the resistor should be easy.

Al

### Re: 200 AMP current sensor

Unfortunately not. Most of the 200 A c.t.'s have a maximum rating that is too low for the voltage required by the emonTx. Their maximum output voltage is usually 0.333 V rms, whereas to use the full input range of the ADC, you require about 1.1 V rms.
If you are happy to accept the reduced resolution (= greater error at low currents), then that's fine and no damage will be done.

However, if you increase the burden resistor to increase the voltage and exceed the c.t.'s rating, the output waveform will be distorted (hence your measurements will be wrong) and you may overheat and damage the c.t.

It is possible that the Magnelab SCT-1250-000, the burdenless variant out of a range that is rated to 600 A might be suitable, but the information is sparse and I have not tested it.

If you do track down a suitable range of c.t's for currents greater than 100 A, do let us know!

### Re: 200 AMP current sensor

A few Magnelab ones are listed here.

http://openenergymonitor.org/emon/node/2701

They are very helpful if you require details of the split core burdenless ones.

### Re: 200 AMP current sensor

One solution could be two CTs in cascade or simply an op amp buffer to boost the voltage from a 200A one.

Bob

### Re: 200 AMP current sensor

I have a much better solution. Jeff Lawson, V.P. of Sales and Support at Magnelab, Inc. has confirmed this:

"If the SCT-1250-000 is burdened at 37.5 Ohms, it will output 1.0 V ac at 200A.  The ratio of all the SCT-1250 units is 7,500 turns.
Please note that Magnelab adjusts the burden resistance of each SCT to compensate for small variations in the core performance.  This allows for the precise tolerances expected."

It would seem that your best option is to use two 75Ω 1% resistors in parallel for your burden, giving you 1 V, or a single 39 Ω 1% resistor would give you 1.04 V - both comfortably within the emonTx input range, and the winding and core tolerance that Jeff mentions can be taken care of when you calibrate the input.

### Re: 200 AMP current sensor

I guess Jeff must know his product but a little surprised that a commercial item that is designed around 333mV output would have sufficient design margin to behave linearly at three times the output voltage.

Maybe it is cheaper to have one core in mass production for all currents up to 600A rather than a range?

Bob

### Re: 200 AMP current sensor

He's only specifying it up to 200 A for a 600 A c.t. Don't for a minute imagine there's enough headroom to run to 1 V out at 600 A. It's all about the VA rating. Remember a c.t. behaves as a current source?

Maybe it is cheaper to have one core in mass production for all currents up to 600A rather than a range? I think what you mean there is "it's cheaper to use the same c.t. and fit different burden resistors to give 0.333 V output at each current up to 600 A"  and that is surely true. (He did write that they trim the burden resistor on test. What it means for the firm is they make and stock one core with its secondary wound on, then customise a small batch or maybe each one as they fulfill orders. It means they need much lower stock levels, etc, etc.)

### Re: 200 AMP current sensor

I suppose that I am more used to CTs as applied to moving iron meters which tend to be 5amp fsd and the CTs are not fitted with burden resistors and have winding ratios to match the meter movement to the desired range.

Not too many analogue AC ammeters around these days. Now many more applications must rely on voltages derived from CTs and so the single 600A core with a fixed turns ratio (which I had not appreciated before) and varying burden resistors satisfy the requirements.

Bob

### Re: 200 AMP current sensor

I suppose that I am more used to CTs as applied to moving iron meters which tend to be 5amp fsd  Me too! Though I've run into problems there with VA rating - by the time you've got about 3 in series (kWh, p.f., kvar) you need a pretty hefty c.t. Obviously this was in the days before all-in-one electronic instruments.

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