Have an American Rotary AD-20 supplying a 3-phase sub panel for the shop. Will using 2 of the legs from the sub panel to power a single phase 240v grinder cause any issues with the balance between the 3 legs for my 3 phase machines?
Is this considered "bad practice"? I know it can be done, just don't know if it is advisable.
Thanks.
It's not unusual for a guy to set up his single-phase shop with a three-phase panel, where the two 'mains' feed straight in, and the RPC feeds the third. I've helped friends do this, and 'my' standard practice is to reserve BLUE as the generated 3rd leg, and the panel's red and black feeds are the standard mains.
Doing this, means that any single-phase loads on the panel tie to red and black bus. Only 3phase loads use the red.
Keeping that in mind... your breaker panel has 'more' than one main... it has main breaker for the single-phase coming in... EVERY load (single or three) is being powered through that breaker. The third (blue) however, is 're-fed' through a separate breaker that protects your 3rd phase current.
It COULD be described as bad practice, because... you could put excess load on the RPC, trip that blue leg, and the two hots to every RPC load would STILL be live. If every machine had phase-loss-protection, you'd be 'sorta' okay, but it's not in any way ideal... because you'd have two hots still live in the machine... and you COULD have an unprotected machine continue to motor on single-phase.
I prefer to set up my panels side-by-side- one that is 3-phase only, and the other is single. Basically, the 3ph panel is a sub-panel to the single, BUT... there's a 3ph contactor on that 3ph panel, with a Phase Loss Relay in series with that contactor, so that if there's ever a loss of 3rd leg, the panel drops out.
The clever way to do this, is to incorporate the 3ph panel's output control and phase loss, into your RPC's start and fault detection circuit... you have a start sequence for the RPC, and once the RPC is up and generating the 3rd leg, the load contactor feeding your 3ph panel pulls in.