I suppose I’m partly at fault, didn’t read through the specs thoroughly, and assumed a CT input was a CT input.
The EmonTX units indeed have four CT inputs, but only three are apparently fully rated (I need them to handle up to 200 amps), the fourth is labeled 4.5 KVA max.
I also ordered a separate EmonPi and its inputs are apparently limited to 100 amps.
I’d appreciate info on how to move on from this point. As it is, I’d need to pick up an additional 2X EmonTX units to for all the circuit monitoring I’ll be needing, and have to hang them all off a USB hub. I’d also have to pick up another (and an additional Rpi) for the other location. Can these devices be modified to take the additional current?
The c.t. input is actually a 50 mA input, so you can connect any c.t. with a 50 mA secondary current, or with a small change of a resistor, the c.t. burden, any c.t. capable of delivering about 1 V into that burden.
(I’d question why you need 4 rated at 200 A - surely only your service entrance wires need to be that size?)
You also need to change the calibration accordingly in software.
Therefore, to convert the emonTx input no.4, you need to open the case and solder a 27 Ω resistor in parallel with the existing burden resistor. Two holes are provided on the p.c.b. for a wire-ended resistor.
If you look at the page in the ‘Learn’ section about using the emonTx in North America, you’ll find a selection of c.t’s that are suitable - some will require the burden resistor to be changed. Everything above applies to the emonPi, as that uses exactly the same input circuit.
Bear in mind the basic emonTx does not communicate using USB, the USB socket is only for power, either when you’re not using the a.c. adapter or if more power is required than the a.c. adapter is able to supply. To have it communicate over your LAN, you need the ESP8266 Wi-Fi adapterand USB power.
Simple - it is UK-designed and it’s specifically for the standard UK PV setup, which is limited to 16 A. That leaves a bit in hand with clipping happening at 18.3 A with our standard 100 A c.t.
Is it within the realm of possibility you might actually draw anywhere near 200 Amps?
I can’t remember for sure, but I think you mentioned the transformer feeding your place is a 50 kVA pot.
If that’s the case, and your place is the only load on that pot, your max continuously available current is
~208 Amps. Would you actually have that many loads powered on simultaneously?
HVAC to this building all by itself will need ~125A (at 240V) worth of breakers. I think I’ll break the 100A limit easily on that, much less the entire building. And yeah, I’ve got an undersized service transformer and will probably need to upgrade it.
The CTs I planned to use for the 200A circuits are SCT019 units purchased from openenergymonitor.org, and are apparently rated for 33ma at 200A. These will work in both EmonTX and EmonPi devices without needing modifications, I hope?
The ones I planned for the 100A circuits are SCT013-000, which are apparently 50ma at 100A.
Okay, so I’ve ordered some 27-ohm 1% resistors. (100 of them, seems to be a minimum order, lol.) I don’t need higher precision, do I? Not sure where to find 'em.
Texas, actually, outside of El Paso, but NM is close. You must’ve had fun out in the desert.
They will work without any changes, but as it’s limited to using the bottom ⅔ of the input range of the emonTx/emonPi, so the accuracy at very low currents will be worse.
If you want to regain that upper ⅓ of the measurement range, but almost certainly with some degradation in performance of the c.t. itself, you could again change the burden resistor so that you get 1.1 V at 33 mA, i.e. 33 Ω instead of (not as well as) the fitted 22 Ω
Those are the standard ones for use in the UK, the ones everything was designed for.
Plus shipping ,and the shipping itself would be higher than the cost of q100. I’ll get free shipping from Amazon when I bundle it with other purchases.
Thank you. I’ll probably either stick with the loss of accuracy, or pick up different CTs.
