Better results up to 35 A

I would like to measure currents up to 35 A as accurately as possible with the emontx. At the moment I use the 100 A SCT-013 current transformer. If I understand that correctly, then I could exchange the 22 Ohm load resistors for some with 68 Ohm. Are there other possibilities? Which procedure is recommended? Are there alternative current transformers that measure better than the SCT-013 in this range and are not much more expensive?

For any c.t, the best accuracy is when the c.t. is under the least load, which is when the burden has the lowest resistance. Having said that, the SCT013-030 is almost the same, but with slightly fewer turns (I’ve not done the maths, but that might be so that they can use standard value resistors as burdens), therefore they think that the accuracy is acceptable.

You could look at the Use in N.America page for some ideas. We have not tested these, but the data sheets indicate that they can be used with the emonTx with little modification (no more than you’re suggesting). I don’t know the prices of these in Germany, it could be that some are quite expensive.

If you are able to break the circuit to install a ring core (solid core) c.t, that will normally be much better than a split core one. Most manufacturers will have a larger range of ring core than split core.

From the conversations that @Bill.Thomson has had with Magnelab, theirs seem to be very tolerant of the choice of burden, so I would be inclined to look at Magnelab first.

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Hi Robert,

thank you very much for your many useful hints! The availability of the CT’s mentioned under Learn | OpenEnergyMonitor is unfortunately somewhat limited here. But I asked YHDC for the transformer winding ratios and the burden resistor values (because I couldn’t find that information in the data sheet) and received the following information:

SCT013 has ratio 1000 ,2000 and 3000.
The conventional output voltage model has 1800 turns.
SCT013-015 resistance is 180 Ω and 390 Ω
SCT013-030 62Ω

So you’re right, the SCT013-030 actually only has a few turns less and a relatively high resistance. Is my assumption correct that I would hardly achieve any improvement over the SCT013-000?

Hi Elias,

Did you check ?

They’re Magnelab’s Europe distributor.

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The advantage that the 30 A : 1 V c.t. has is certainty of calibration. It does not depend on the value of your burden resistor, because it has been factory-calibrated for the internal one.
The advantage of the 100 A : 50 mA c.t. is you can choose your value of burden, so adjust its maximum current to what ever you want (within the expected limits).

I have never tested a SCT-013-030, so I cannot comment on the phase errors. I would expect that they will be worse at 30 A than a SCT-013-000 at 30 A when burdened for 100 A.

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