OpenEnergyMonitor Community

Larger CTs on EMONTXV3


Is there an out-of-the-box split-core CT that is suitable for EMONTXV3/EMONPI up to say 200A or larger with the correct burden resistor integral and correct plug?


is there a device that will take any standard 5A CT and convert it to something that an EMONTXV3 can make use of?

Your starting point is the “Use in North America” page. There is a wide selection there, some are easier to use than others.

The YHDC 100A/200A/400A Split core current transformer SCT023R is the easiest, and is a direct replacement for the “shop” c.t. Despite what others recommend, you should choose a current rating that is closest to but above the maximum current that you think you will ever need, plus a small margin for contingency. Although your service is rated at 200 A, it’s unlikely that you will ever demand that.

The next easiest is the Sentran - MODEL 4LSF. This requires a small modification to your EmonTx or emonPi: You need to add a second 22 Ω resistor in parallel with the existing burden resistor. (There are holes ready to accept a wire-ended component.)

The remainder are harder, and need you to remove the existing burden resistor to substitute a higher valued one.

A few words about sizing your c.t: US 200 Amp Service explained or Do I need a 200 Amp CT?

The answer to the alternate question is yes, it’s called an “interposing c.t.”, so you use a 5 A : 50 mA c.t. on the secondary of your 5 A c.t., feeding into the emonTx/emonPi. But of course, you have the errors of both to take into account. What is possible, but I don’t recommend it, is using an external burden on your 5 A c.t. to give you the required 1.1 V at 5 A - it will need to dissipate 5.5 W at peak load.

Thanks Robert,

That is most helpful. One final question - if I use say the 400A version of the SCT023R, how do I tell the EmonTXV3 that it has a 400A CT connected and not a 100A one? Is there a way to set this via the serial programmer?

Appreciate your help,


If you’re happy to edit the sketch and reload it, that’s the safest and to me the “best” way to do it. If not (and you can’t do it by writing to the EEPROM at startup), then what you do depends on where the data is going. Basically, the currents (possibly) and powers coming out of the emonTx are 25% of what they should be, so at the first convenient point outside, you multiply by four. That might be, if you’re using an emonPi, the “scales = …” line in emonHub. The numbers there are multipliers, and they relate 1:1 to the names a line or two above. Or you can leave it until later in the chain. The downside to leaving it later is if you send the data to more than one place, when you might have split the feed ahead of the multiplier.

If you do edit the sketch, bear in mind that, if you draw more than about 130 A, you will have problems fitting the power value (which is in units of 1 W) into a short integer for transmission, so multiplying later is probably the best way to go. (400 A × 230 V = 92 kW.)

Perfect, thank you Robert.

I will modify the multiplier on the emonHub of the emonbase. Should be straight forward.

Thanks again,