Wiring dual CT sensors on 220v feed

I live in the US where we have 3 wire 220v feeds. I was wondering what was a good way to wire up 2 CT sensors on the 220v input. A few sites have 2 CT sensors wired in series, one sensor mounted on each of the 220 leads. Other sites show the sensors wired separately into separate analog inputs of the Arduino.

Seems like the CT sensors wired separately would be better and then just have the Arduino add the results together. If you wired them in series, since the same current flows through the first 220 lead as the second lead for 220v appliances, wouldn't you actually be measuring the same current twice if you wire the sensors in series  (which adds the voltages of the burden resistors together)?



Robert Wall's picture

Re: Wiring dual CT sensors on 220v feed

A current transformer works as a current source, and a current source is the dual of a voltage source: if you wire two in series it's like wiring a 1.5 V battery in parallel with a 9 V battery - not good.

Basic wattmeter theory (because a c.t and v.t working into an emonTx or Arduino is effectively a wattmeter) says that you need one wattmeter less than the number of wires, and you add the result. Simple. It doesn't matter which pairs of wires you use.

In practice, it is usually more convenient to treat the earthed neutral as the common wire, and measure the voltage to each line with respect to that, and measure the current in each line. Then you multiply V x I for each pair to give the power and add the two powers.  But there's nothing to stop you treating one of the lines as the common wire, and putting c.t's on the other line and neutral, and measuring voltages L-N & L-L.

Wiring the c.t's in series or parallel or anything else will give you wrong answers except in very special circumstances that you're unlikely to get in practice - namely everything on the two halves of the system is exactly balanced. And if that were the case, there would be no current in the neutral and you wouldn't need a neutral conductor.

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