120VAC for two panels with one transformer

[dtlxii]

I want to use a control panel enclosure with three compartments to house the main power in one, and two independent panels that get their power from the main compartment but have their own disconnect switch. They will power VFDs but also provide 120 VAC for other controls along the machines.The main panel would be Panel 1, the others Panel 2 and Panel 3.

I also want to use only one 3-phase, 480 to 208 transformer to provide the 120VAC to the two independent panels (Panel 2, Panel 3) This transformer would be connected to Panel 1.
When Panel 2 is switched on, Panel 2's 24VAC transformer will energize the coil in a 3-pole contactor (housed in Panel 1)) and power would be transferred to feed the three 120VAC lines for Panel 2 only. Panel 3 would have its own dedicated contactor, also in the Panel 1, that would energize only when panel 3 is on and provide three lines.

My question is: The secondary of the 480/208 transformer has one neutral. How do I handle this?. I think that if the neutral is shared by panels 2 and 3, even if one panel is off, the neutral will still be carrying current. Should I use a 4-pole contactor and run one neutral wire to each of the contactors? Is it better to use one 480/208 transformer for each panel? Please see attached drawing.

Thank you.

 

[davidbeach]

A single neutral bus in panel 1 is sufficient. But, how the heck do you get the 24V to pull in the contactor? You've go a chicken and egg problem; no 24V until the contactor is pulled in, no contactor pulled in without 24V.

 

[dtlxii]

Panel 1 provides 480 VAC to Panels 2 and 3, but these two panels don't do anything until their own switch is turned on. If you look at the drawing, when panel 2 (in the next compartment) is turned on, it feeds a small 480/24 VAC control transformer in that panel. It is this 24VAC that then energize the coil for the contactor. Same goes for Panel 3.

 

[waross]

David. No sense wasting time on something that will soon become apparent. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Is this like a Schrodinger's control panel here? The boxes exist in a state of being both energized and de-energized simultaneously?

Putting aside the obvious conundrum, I'm scratching my head trying to figure out a purpose for all this complexity. You don't want power to go to one panel unless it is going to the other? Are you trying to create an interlock, i.e. Motor A must not be running unless Motor B is available and vice versa? There are a lot simpler ways to accomplisgh that if true.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

You're asking about switching the neutral. That is usually only done in a transfer switch that needs the neutral isolated. You should have the neutral grounded at the transformer.

Why not just put a 3-pole disconnect switch in panel 2 and panel 3 to turn-on the power to panel 2 and panel 3?

 

[waross]

With higher power systems, that include ground fault monitoring and or tripping it may become necessary to switch the neutral. There are specific code and technical requirements for equipment used to switch a neutral.
I don't see that any of that applies here. Use a solid (unswitched) neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dtlxii]

My bad!! I apologize for the confusion and insulting everybody's intelligence. By trying to make a simpler version of the circuit in MSPaint I got things confused. Please see the new drawing.
So, here is how it works:
Panel 1 would be on all the time. It contains a 3-phase, 480 to 208 VAC, 30KVA transformer and other supervisory circuits. out of this transformer, 120VAC go to the inputs of two contactor, one for Panel 2, and one for Panel 3.

If I want one machine 2 to run, I turn on Panel 2. This feeds the motors for machine 2, but also, through a 24VAC control transformer, energizes the coil for Contactor for Panel 2.
When this coil is energized, 120VAC would transfer to the outlets
in machine 2.

If I want machine 3, same principle as above. Panel 3 also has a 24VAC transformer. I can have one machine or two running at the same time.

My concern at this point is that both machines outlets would have the same neutral, so if they want to service one machine, even though is off, there might still be current flowing from the other machine.

Again, I apologize for the confusion. I did not mean to waste everybody's time. You guys are a great source of knowledge and I appreciate your help.

Thank you.

Pls. see new drawing.

 

[dtlxii]

If I use a 4-pole contactor instead of the 3-pole shown in the drawing so I can switch the neutral as well as the hot legs, would that work? Would it violate any NEC codes?
Thanks for your help.

 

[jraef]

So let me ask you a question as a way of helping you think of this another way.

If you are going to work on the lights in your house, you go find the breaker for those lights and turn it off, correct? That of course only opens the Hot side of the circuit right? Is not the neutral for those lights then also connected to the neutrals of every other circuit in the house? So by your thinking then, you have potential "current" flowing in the neutral wires in your light fixtures while you are working on the circuit that you have de-energized with the open breaker, is that right?

If you think you do, describe the path of current flow to someone who accidentally makes contact with that Neutral circuit.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[John2025]

I'm probably not the guy to answer this but here's my take. Put the 24V transformer in panel 1 and use it to feed the low voltage in panels 2 and 3. The neutral doesn't need to be switched with the caveat that things need to be setup so that when anyone is working on any of the circuits, all sources of power are positively disconnected. You don't want a scenario where a break in the neutral somewhere, combined with power going to anything that uses that neutral, produces a hazard.

 

[John2025]

jraef,
It took me so long to post that I didn't see yours. I'm thinking the situation is analagous to a multi wire circuit in a house. I'd want both breakers open when I'm working. I'm no expert, so forgive me if I've got it wrong.
John

 

[dtlxii]

Thanks guys. Maybe I'm looking at this wrong. Neutral should be 0 volts because it's bonded to ground at the transformer, so there should not be an issue then. I got confused because I read somewhere
that if you share a neutral, and you switch off one of the circuits, all circuits using that neutral should be switched off. I don't remember if this is for residential or control panel wiring.

 

[waross]

There is an issue with a shared neutral and only one breaker turned off, when someone opens a connection on the neutral. Then the energized circuit will backfeed 120 Volts to the open connection.
This should not be an issue with separate panels. Don't work on the panel feeds unless both feed breakers are open.

Bill
--------------------
"Why not the best?"
Jimmy Carter