My newly installed Insinkerator Evol Plus garbage disposal runs when switch is turned to off, trips breaker when switched to on. I tried it on a shared circuit. Now it's hardwired to it's own dedicated 15 amp circuit. Replacing the switch didn't help. Is this brand new disposal defective?
You incorrectly wired the switch into this device. What you've created is an always on device and a short in the middle of the circuit:
Black --------+-----------+ | | line switch disposal | | White --------+-----------+
In this scenario, when the switch is flipped, you create a short circuit that trips the breaker, and there's nothing preventing power from always going to the disposal.
Instead, what you need is to switch the hot, so that power is turned off to the appliance when the switch is opened. Note, you don't switch the neutral since that would result in power going to the appliance and becoming a shock hazard should anything make a path available to ground.
Black =---+-switch-+------+ | line disposal | White --------+-----------+