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Hi Bill,

1. There are no serious disadvantages, both are pretty accurate. The v3 was designed to reduce "noise" based inaccuracies at very low measurements, has an additional 4th CT circuit, requires just 1 ac adapter and has a more refined "retail" finish with a nice case etc, but the v2 has a distinct connectivity advantage with the exposed connectors etc and at half the price 3/4 of the CT circuits is still a saving.

2. The calculations are documented in the building blocks under calibration theory if you search the forum for Magnelab you should find several discussions on larger ct's

3. You can only connect 1 serial device to a Pi's gpio serial port, so you would need to use I²C or something to interconnect the Tx's and have at a single serial connection. MartinR's full fat 3phase uses I²C to interconnect 3 emonTx v2's and then transmit RF from the master Tx to Pi, this last stage could be a direct serial connection instead. Alternatively you could make the Pi the master and all the tx's slaves.

4. definitely

Paul

hello all i am new to here

can anyone tell me is it 400a per phase that the limit is

so for instance can i use 400 amp cts

if not what is the highest rating that can be used

what needs for to be done for to increase the ct size

Gerry

There is no limit as such. Tell us what your needs are, and we can recommend a suitable CT.

hello Robert

thank you for the reply

ideally i need a ct for to measure 100, 200,400,and 600 amps

i am looking for to measure voltage and kwh and pf if possible

i am looking at multiple boards in a stack so i can measure many 3 ph circuits

what would be the best way to incorporate a stacker for them

Gerry

If you can build the emonTx V2, then it's very simple to stack 3 units, see MartinR's Full-fat 3-phase monitor. You can of course do a similar thing with the V3, but physically it's not so neat.
Note this uses one emonTx per phase, as that allows for the voltage of each phase to be accurately measured. You'd monitor 3 x 3-phase circuits with 3 stacked V2s, or 4 with 3 stacked V3s. More than 3 stacked units could share the voltage references, and then you'd probably want a common 5 V power supply feeding everything (which you'd need anyway with V2s). Using one emonTx to monitor one 3-phase load means you estimate, not measure, the voltage on the second and third phases.

For CTs above 100 A, I usually recommend the Magnelab range, though there are others. These are designed for a 0.333 V output whereas we require a little over 1 V (I usually say 1.1 V is the working maximum), so you need to choose the model without the internal burden resistor, and derate the CT by a factor of 3 (so for 200 A, you need a 600 A CT). You'll need to know the turns ratio (I know some - I must ask Magnelab for a full list) and then calculate a burden resistor to give you the correct voltage at your maximum current. (And that explains why I wrote earlier that there's no maximum - it depends entirely on the CT you choose.)
But you want to steer clear of CTs with 1 A or 5 A secondaries, as you then need a high power off-board burden resistor, and the high VA rating means dangerous voltages and currents should the burden be disconnected for any reason.