Current Sensing Circuit
The current sensing circuit is based on the design from OpenEnergyMonitor. You can find details and the explanation behind the design on their site: https://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface
Here is the diagram of the circuit I built with suggested resistance values based on the SparkFun kit:
The circuit is very simple: we are just creating a 2.5 volt reference point half way between the 0 V ground and +5 V supply voltage on this board. One wire from each current sensing transformer is connect to an analog input on the Arduino, and another wire is connected to the 2.5 volt reference. Between the two transformer wires is a 47 ohm burden resistor; the Arduino will measure the voltage created in this resistor based on the current from the transformer.
To keep the diagram simple I have only drawn one current sensing transformer, you can see where additional transformers would be connected to their respective burden resistors in the exact same way. I have also only drawn 3 burden resistors, but as the Arduino has 6 analog inputs you could use up to 6 current sensing transformers with their matching burden resistors.
Make sure your Arduino power supply has enough power to run the board plus the relays, I initially had reliability issues with my setup due to inadequate power supplies. I was running a 12 V 750 mA supply but it seems that wasn't enough, now that I'm using a 12 V 1.5 A power supply the board is working as expected.
You can ignore the reasoning for the capacitor unless you want to actually understand the circuit. The 10 uF capacitor allows the AC signal from the current transformer to bypass the 100k ohm resistor to ground as if it was a wire, but prevents the 2.5 volts DC from passing through, allowing the center point to stay at the 2.5 volt reference.
I soldered this all in the most compact possible form to save room on my Arduino prototype board for any future improvements, but you can also spread the circuit out more to make the soldering easier.