Extending CT wires

I'm new here so apologies if this has already been answered and I've missed it doing my search. I'm new to this so please be nice :)

I'm planning my first install of OEM and would like to know more about extending the wires for the CT sensors on EmonTX V3.

I understand that the sensor wires can be extended using shielded twisted microphone cable or similar, and that the longer the cable the more likely it is to pick up interference along the way. I also understand that the signal will drop over longer lengths of cable and that it should be run as far away from mains cables as possible.

I would like to know what sort of length the wires can be extended to. I would likely be buying 2 or 3 EmonTX V3's initially and would like to keep them somewhere central in the house to keep it as tidy as possible (and keep the mrs happy) in one place to be able to monitor several separate sockets/appliances as well as the main circuits at the fuse board. 

How much would, for example a 10m extension affect the readings? Is there a maximum distance? If the readings are noticeably inaccurate, is it possible to correct the resulting figure if I took an accurate reading with a multimeter? I would be using the CT sensors supplied from your online shop if that makes any difference.

I was also considering using shielded cat5 twisted pair cable for the extensions as I have an abundance of it in my line of work (IT), running it to each room and then splitting off to each CT sensor. Assuming they just use 2 cores each I could potentially run 4 CT cables over one piece of cat5. In this case, would the twisted pair from each sensor interfere with the other readings at all? The cable would be shielded from external noise but would there be any crosstalk between them?

Edit: also I should add, what size cable are the CT sensors designed for? Will they clamp round standard 2.5mm² cable as used in UK wiring or is that cable too thin to work?

Robert Wall's picture

Re: Extending CT wires

You're right, you have missed the answer! You're also right about interference and staying away from other cables, but you're wrong about the signal dropping - it won't. The CT is a current source and all the current has to go into the burden in the emonTx, so unless you have really ropey insulation there's nowhere else for it to go. (That's not true for "voltage output" CTs, but even so, the voltage drop would be minimal.) You should be OK with 10 m, and the way in which CAT5 is twisted should mean there's little interference - for exactly the same reasons as when it is used for data.

You can use any size cable as the primary winding of the CT, so long as the ferrite core isn't stressed, as it is brittle and will snap easily (i.e. up to 25 mm2 meter tails).

sovking's picture

Re: Extending CT wires

Same problem for me. I would like to monitor production of PV and total energy consumption of my house.

PV inverter and the meter are outside home, under rain and wind, and I prefer do not put other electronics in such heavy conditions.

I would like keep the EmonTX and the raspberry inside my house, leaving the CT only outside. I've already one pipes where inverter data are read through rs485 by a UTP cat5 cable.

If it is reasonable, I would extend the two CT by using another UTP cable. The cable should be long about 20 meters. It's too much ?

How many UTP wire do you suggest to use for each CT wire ?

If it's just 1 UTP wire for 1 CT wire, I would reuse the cable used for reading inverter data. (2 wire x each CT + 3 wire for RS-485 = 7 wire used of 8).

Thanks for your advice!

Robert Wall's picture

Re: Extending CT wires

If you have the cable installed already, it is worth trying it to see if (a) the CT output interferes with the RS485 data, or (b) if the RS485 data interferes with the CT signal, or (c) you pick up interference from somewhere else. The maximum current from our standard CT is 50 mA, so one pair will have no problem with carrying the current, nor is there any problem with voltage drop because the CT is a current source and it will generate whatever voltage is needed.

If you take the CTs off their cables and you read zero or only a very small current/power, then you don't have a problem with interference into the emonTx. And if the inverter data (into the RPi?) is OK when the CTs are on their cables and working, you don't have a problem there.

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