Thanks all for a lot of good ideas. The reason for my over engineered
project using an Arduino microcontroller is to have a programmable
thermostat with programmable current sensing and hysteresis, plus time
delays, etc. Also, there are arduino libraries for communicating X-10 over
the power lines. I'm not sure if X-10 modules will be powerful enough for
my heaters but I can use them to switch on a solid state relay.

If I can use the current sensing relay idea, or the comparator idea, I can
have my current sensing on digital input pins, saving some analog pins for
temperature sensing.

My current idea is have the current transformers and Arduino at the breaker
box. When the current goes too high (or temperature setpoint), the
Arduino sends an X-10 command to shut off the heater on that circuit.
In short, it will be a multi-zone programmable thermostat that tries to
prevent breaker tripping.

This project came about because of the price of propane, I want to have an
alternative if the propane runs out while the price is outrageous.

RogerN