Directions for the dimmer say that you can use only one dimmer per circuit. What defines a circuit? Does each breaker control one circuit? Or is a circuit one complete element - one switch and a light fixture(s)? Specifically, I have installed a dimmer which controls a light. It is not a 3-way because there is only one switch. Controlled by the same breaker are several other switches, including a 3-way that I would like to install a dimmer on? Can I?