Is it a mistake to tap 2 legs from AD-20 RPC to supply single phase 240V?

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.

two of the legs pass through from the single phase pannel so it will present no load to the RPC motor and will not affect the voltages.

gmoushon said:

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.

Click to expand...

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.

Thanks for the reply. That is a clever way of doing things.

I guess my biggest concern was somehow throwing the balance between the 3 legs off and messing up my CNC. I don't think it would be a problem if I used the 2 legs that pass through, but I've heard guys say they've had odd things happen in that regard. I would think it would be safe...just not sure.

I am a bit late to this one. I have an ADX 10. Even though the 220 is pass through I "pretend" every line is coming from the ADX 10 complex. I use a 3PH breaker on the output of the ADX 10 as input to my control/distribution panel. So all the feeds going to tools are protected by that 3ph breaker. So if any line "shorts" all legs trip. Result no hot at any tool.

Unless I am missing something, this approach resolves the issue with one "pseudo main" 3ph breaker. Each tool also has a fused disconnect box matching the power requirement of the tool. In the last 15 years for this shop the only fuse trips I had are one of the legs in the disconnect box.

Just another approach

Bob

You really need two distribution panels. One panel is for 3 phase only and one for single phase. Each would have its own main breaker and individual breakers. The RPC is just a single phase load and thus would have its own breaker. The output of the RPC would go to the 3 phase panel with just the individual load breakers.

Incoming power wires from the outside would be protected by the power company. Remember, the breaker (or fuses) is to protect the downstream not the upstream.



Tom

piedmontg said:

I am a bit late to this one. I have an ADX 10. Even though the 220 is pass through I "pretend" every line is coming from the ADX 10 complex. So if any line "shorts" all legs trip. Result no hot at any tool.

Unless I am missing something, this approach resolves the issue with one "pseudo main" 3ph breaker. Each tool also has a fused disconnect box matching the power requirement of the tool. In the last 15 years for this shop the only fuse trips I had are one of the legs in the disconnect box.

Just another approach

Bob

Click to expand...

..A "common-trip" 3-pole breaker is what makes it OK. ANY application with more than one pole, and internally tied. Not just pinned or clipped.

Many externally-tied ones earned a bad reputation for the clip or tie-bar having enough lack of reliable motion conveyance - or too damned SHORT a travel - that one pole COULD trip and NOT release the other one (1-P 2XX) or 3-P-wotever.