CT sizing query for UK domestic main ring circuits


Apologies in advance though can anyone advise the correct CT sensor needed for a typical Ring Circuit using 2.5mm T/E cabling in the UK ?

In my consumer unit, 2 x 2.5mm cables terminate in the same circuit breaker which is rated at 32A.

I can see that there is a 20A Max (6mm square cable opening) CT sensor option, and a larger 100A Max (13mm square cable opening) though i don’t know if the larger version would be too loose so to speak around both wires?

I did read somewhere on here that this shouldn’t be a problem though I just wanted to verify if this is the correct approach for obtaining power measurements on ring circuits ?

Any advice greatly appreciated.



Being loose doesn’t matter. The important thing is not to clamp or wedge the c.t. onto the cable(s), because the ferrite core is very brittle and can easily snap if stressed.

The 20 A (SCT006?) isn’t a good idea because it will saturate and not read correctly above 16 A, when used with the 22 Ω burden resistor in the emonTx or emonPi. (It’s rated to 20 A only with a 5 Ω burden, which doesn’t generate enough voltage for the input to have a decent range.)

The downside of using the SCT-013-000 is its size, especially if it needs to go inside the consumer unit.

Thank you Robert, that’s really helpful, i will take a closer look at the diameter of the SCT-013-000 variant with consideration of the internal spacing in my consumer unit.



If the ring main tails are long enough, you could loop the pair of wires to pass through the CT 3x, that would give you a full scale of 33.3A and 3x the resolution (and perhaps fill out the CT a little :slight_smile:) . The resulting value will need to be scaled down again, that can be done in emonhub by using a scale of “0.333” in place of the existing value of “1”.

[Correction] - These comments were made with and emonTx/emonPi in mind based on the title and thread content rather than the category title which I evidently missed. While the numbers may differ the concept remains. This is not “necessary” with any of the hardware mentioned, but maybe desirable/useful and will increase the resolution with any of the hardware options.

That’s not necessary with IoTaWatt. The accuracy at low power is very good without amplifying the current. Also, the SCT013-000 is full scale at 83 amps in IoTaWatt, so for a 32A circuit, it would be prudent to only use one loop if so inclined, and the output to Emoncms would be scaled using the calculator in the Emoncms server configuration of IoTaWatt. There is no Emonhub when using IoTaWatt.