Two separate 120VAC services, one neutral from one 3-phase, 480 to 208 Transformer

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dan62

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I have a 3-phase, 480 to 208 transformer in the first compartment of a control panel enclosure that has three compartments. For simplicity I'll call these Panel 1, Panel 2, and Panel 3, with Panel 1 being the one housing the transformer and a Main disconnect switch. The output of this transformer gives me 3 hots and one neutral which I can use to supply 120VAC to three circuits in Panel 2. The neutral would be bonded to earth ground at the transformer.

The problem is the third compartment, or panel 3. Panels 2 and 3 have to be able to turn off independently because they will power two different machines. If one panel is turned off, the other should keep working. I want to supply 120VAC for outlets along the machines to both panels, yet keep one live if the other is off. Each panel has its own disconnect switch. PANEL 1 WOULD ALWAYS BE ON. It would be turned off only if both machines need to be off (like during maintenance)

I thought of a way of doing this by using 3-pole contactors mounted inside Panel 1; one for Panel 2 and one for Panel 3. If Panel 2 is turned on, the 24VAC from that panel would energize the coil of the contactor and supply 120 VAC to that panel only. Same goes for Panel 3 if I turn on that panel.

However, the neutral wire would be shared. Will there a problem there?. Should I use a 4-pole contactor or transfer switch? Any transients to worry about?. Please see attached image.

Thank you
 

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Thank you Don. The panel is not built yet. It's still in the design stage. The rotary switches look like an option, however, both panels will be on at the same time. If there is any maintenance to be done to one of the machines, they will turn off only that panel. The other will still be fully functional. Once they finish maintenance, they would throw that panel's switch to ON. This will provide 480VAC to the motors, and 24VAC to the contactor for that panel to energize its coil and feed 120VAC to the outlets.
These panels are for motor control, but I need 120VAC in each panel to plug in other controls along each machine.
 
Yes n1ist, I'm working on that option as of the time of this reply. I wanted to use only one transformer to make things easier and not use much space (each 15 KVA transformer is about 3 x 3 x 3 feet). I might go with the two transformers as I posted my question everywhere with very little to no response at all. Either it's a big no no and nobody wants to take a chance answering, which I understand, or it's a somewhat difficult problem that requires a lot of analisys and checking of NEC codes. Anyways, I'll have both options ready just in case.
 
I don't see any code issues with your original proposal for the contactor with a common unswitched grounded conductor. I just see it as overly complex when you can accomplish the same thing with a multipole disconnect.
 
Unless you need to switch remotely why not just use a 208/120 3 phase panel for panels 2 and 3 with a disconnect. Other than not having a code book handy-- electrically it will work fine--either run 2 neutrals or size the neutral big enough to handle the total load of both .

panels--also, I believe, the 480/208 xformer is a seperatly derived service and needs bonding/grounding done throughout the run

Unless you are energizing/de-energizing under load there shouldn't be any spikes or transients.

Now that I have stuck my foot(feet) in front of my mouth I'll let some of the more code guru types tell us where we both are wrong and insert my feet for me!!!!!!!:ashamed:LOL
 
There could always just be procedures written such that only one panel can be taken out of service at a time. Seems like a lot of trouble to rig up a control system for it.

Also, the way that I read it, what is wrong with just having a breaker for each section of the panel? Unless the disconnects need to be somewhat remote?
 
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