What CT would be good to measure 250w (approx)

Hi was wondering what ct would be good to measure 250w (approx) and what changes do I need to make on the equipment from the store. I got two of the emontx and a emonpi.

Current transformers do not measure watts, they measure current in Amperes. So to convert your 250 W to amps, I need to know the system voltage and power factor. Then to be sure it will fit on the cable, we need to know the overall cable diameter.

That’s doing it properly. Your problem is current transformers are generally designed to handle large currents - in the tens to thousands of amps range, so finding one for one or two amps is hard. Licking my finger and holding it in the air to see which way the wind is blowing, I think the lowest possible current rating you can get would be the one for you. Bear in mind that you need one capable of giving a voltage across your chosen burden resistor of about 1.1 V rms at your maximum current, and that limits your choice significantly. Can you break the circuit to install it? If yes, then a ring core CT is much better for your purpose, and probably easier to find.

You could look at the 5 A CCS or the 10 A Sentran one on this page. The CCS one will need the burden resistor in your emonTx/emonPi removing, and the Sentran will need another resistor adding in parallel (as that page states).

Our standard 100A SCT clip-on CT sensor used with an emonPi / emonTx will measure 250W just with reasonable accuracy (4%-6%, in our testing).

However, if you are after higher accuracy then a different CT could be used, as Robert suggested. This will require a hardware modification.

Oh wow so based on the number of turns you looped the wire you were able to decrease the max error

Yes, you can have a multi-turn primary winding, but depending on the wire size and the location, it might not be possible to have more than a single turn. If you can have multiple turns, then the effective (as far as the transformer is concerned) primary current is multiplied by the number of primary turns.

Hence in your case, it will be working nearer to its design current where the accuracy should be better.