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Will the following wiring diagram work? This 240v single phase NEMA L14-30R receptacle is right off the main panel with a 30amp breaker. I want to obtain both 120v single phase and 240v three phase off of this receptacle using 2 separate circuits in which only ONE of the TWO will be used at a time.

The first circuit having a NEMA L14-30P plug will utilize both hot wires and a ground wire, leaving the neutral not connected. It will feed into a VFD (assuming already sized for the application) which will provide 240v three phase.

The second circuit again having a NEMA L14-30P plug will utilize only 1 hot wire, a ground, and neutral leaving the 2nd hot wire not connected. It will then be connected to a 20 amp breaker. From the breaker, it will be connected to a 120v single phase receptacle.

Assuming all wired connections are made properly at both the receptacle & main panel, all wire is the correct guage, and that the VFD & 20amp breaker are in a proper NEMA electric box, will this set up work safely?

I'm sure many will think this is an odd set up (because it is) but I want to have the 240v single phase receptacle as my starting point instead of the main panel. Again, please note the assumptions made.

Thanks for looking at this thread but I would only like answers to my question leaving out the personal comments of why you guys would never do something like this even though it may work.

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    Is there a reason you don't have the whole industrial control panel on a single plug? Also, is the 30A breaker in the main panel a common trip two pole unit? – ThreePhaseEel Sep 28 '17 at 3:19
  • I find it simpler and safer to have it on 2 separate plugs. The 110v plug will be mainly used for a hobart handler 140 welder and the other plug will be for a 5hp milling machine. And Yes the 30A breaker is a common trip two pole unit. – C. Fox Sep 28 '17 at 22:38
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I do not see where you will have any issue with this arrangement - I am assuming your VFD has a means of control for motor speed ; whether that be a pot on the front, a PID loop control sensor or a PLC connected to it. Not really something in Home Improvement as most homeowners don't have a VFD .

Since you are not going to use them at the same time - I think your setup will work with out issue.

You might want to look at the VFD specific requirements on grounding and neutrals.. depending on the VFD you might need to do some things different regarding your ground and neutral connections, also if using a PID loop control you might have some specific instructions..

I assume you know this info because you are using a VFD - but maybe not ..if you don't what make and model VFD ?? Yaskawa , Rockwell other ??

  • 220V 4KW 5HP VARIABLE FREQUENCY DRIVE INVERTER VFD (on ebay if you want to know the details)...the instruction manual that came with it show to wire it the way I've drawn in the diagram, its on the cheap end of VFD's but it works for me – C. Fox Sep 28 '17 at 22:23
  • Also, I appreciate your answer & advice. – C. Fox Sep 28 '17 at 22:40
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Looks great to me.

The system will look for a longer answer than that, so let me say it's nice to have someone arrive so clear on what they are doing and so on-top-of the requirements.

The only thing that concerns me is you won't be getting a whole lot of total power out of this -- 7200W peak and 5760W continuous. I'm surprised anyone bothers with 3-phase at such a low power level.

  • A lot of it is probably for the soft-start/control functions of the VFD :) it's more power than an inverter mini-split, for one... – ThreePhaseEel Sep 28 '17 at 11:42
  • @ThreePhaseEel Forgot about the VFD, that alone is the reason to go 3-phase. – Harper - Reinstate Monica Sep 28 '17 at 15:50
  • 4000W VFD that will power 5hp motor – C. Fox Sep 28 '17 at 22:25
  • Also, thanks for your answer & advice. Very much appreciated. – C. Fox Sep 28 '17 at 22:41

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