[Need TECH Help!]

Are you using a RPC or have 3 phase delta in your shop?

The common way of isolating the 120V devices is with transformer. The main switch will break the circuit to all 3 legs. I normally connect the 2 ''hots'' to L1 and L2, then the wild leg to L3. The only place the L3 goes is to the 3 phase devices. L1 and L2 would connect to the transformer which would be wired for 240V in, and 120V out. I would not bring a neutral into the cabinet from the wall. I also identify the wild leg with yellow or orange tape on both ends. With transformer isolation, it really doesn't matter which legs are connected.

I have a American Rotatory 20HP CNC duty RPC.

Your main service wiring procedure is what i practice, but was wondering if there was a device to protect against the (L3)240V wild leg being wired at main disconnect, to L1 or L2. Like a relay that would disconnect the L1 and L2 lines if either one was over voltage. As for transformers, would you use a Buck/Boost transformer for the 120v devices such as PC,Monitor,DC power supplies, other Digital devices? And a Control Transformer for things such as mechanical relays, contactors, motor starters,etc.? Or just one big transformer for all?

Thanks for the advice,
Troy