Two circuits to one device

[ejb1556]

Normally I would just get my EE to answer this question/concern but he is on vacation currently and I am not one to disturb those on vacation.

I have been asked by a client on a project we are trying to quote as to whether he can have two 120VAC feeds, powered at all times from separate power sources, on one set of contacts of a device. Basically, a direct single phase feed of 120VAC connected to (L1) (N1) and (G1) of a device as well as a UPS feed of 120VAC connected to the same (L1) (N1) and (G1) of the device. It is a 120VAC device.

To me, this sends off all sorts of red flags, but my basic understanding of electrical systems does not produce a reason as to why. I would think that this would be against NEC (plus it wouldn't be 120V would it?) and my initial thought is "auto-transfer switch!!"

Am I thinking correctly on this?

Thank you,

Evan Briggs

 

[tallgirl]

Yes, you'd need an auto-transfer switch. One side would be the master (grid side) and the other the backup (UPS) side.

So long as power is present on the first side, that power source is used. When the power goes away, the other side is used.

If both go ... well ... surprise!

This type of solution isn't all that uncommon in circuits that need to be backed up. The other solution -- a pass-thru UPS -- is also common. In that solution, the UPS has an integral transfer switch. But some applications require more power than is available from that type of UPS so an ATS is used.

One of my clients has triple-redundant power -- solar, grid, generator. In that case, solar is primary. If it drops, the grid is the next source. When the first ATS is in "use the grid" mode, if the grid drops, the generator is called. If the solar comes back, everything else is turned off.

 

[rcwilson]

All kinds of things wrong.

Backfeed is bad. Imagine going to the normal power panel and turning off the breaker. Voltage is still on the load and at the load side of the breaker.

Why even have the UPS if this is how they will use it? When normal power goes away, the UPS will try to power up the normal power circuits also since it is hardwired to the normal power feed. The UPS will overload and shutdown so the load is lost anyway. If they still want to connect it this way, make sure they understand that the UPS will never work to supply power during an outage.

There is no guarantee that the UPS output is in phase with the normal power source. Connecting the two circuits together could cause a fault that will trip both breakers (Normal and UPS) and probably damage the UPS.

With two circuits feeding one device, how is the overcurrent protection going to work? If both have 20 A breakers do you wire for a 40 amp circuit?

I'll leave to it better experts to list all the Code violations.

If this equipment has a DC power supply it may be possible to use two DC power supplies, one fed from normal power and one from UPS. Diode isolate the outputs and the end result provides the reliability the client wants.

Bottom line- never connect two circuits together.

 

[BJ Conner]

Normally I would just get my EE to answer this question/concern but he is on vacation currently and I am not one to disturb those on vacation.

I have been asked by a client on a project we are trying to quote as to whether he can have two 120VAC feeds, powered at all times from separate power sources, on one set of contacts of a device. Basically, a direct single phase feed of 120VAC connected to (L1) (N1) and (G1) of a device as well as a UPS feed of 120VAC connected to the same (L1) (N1) and (G1) of the device. It is a 120VAC device.

To me, this sends off all sorts of red flags, but my basic understanding of electrical systems does not produce a reason as to why. I would think that this would be against NEC (plus it wouldn't be 120V would it?) and my initial thought is "auto-transfer switch!!"

Am I thinking correctly on this?

Thank you,

Evan Briggs

Your thinking is dead on.
No only are you mixing sources. The sources may not be in sync. A Y-Y transformer upstream in one branch and a Y-D upstream in the other can give you a 30 degree difference in voltage.
The systems can also both be Y-Y (or D-D) and they can be off by 120 degrees if the transformer are connected incorrectly on the secondary ( like A,B,C to X1,X2,X3 on one and A,B,C to X2,X3, X1). IT's not supposed to be but the case I saw couldn't have been the only one in the world.
What kind of device is it?
It is pretty common for servers ( data servers ) to have two plugs. You plug one into one outlet the other into another outlet. The Outlets are from two seperate sources.
The switching or selection system is built into the server. Someone is probably making ( if not maby they should ) one in a black box.

 

[Pharon]

One of my clients has triple-redundant power -- solar, grid, generator. In that case, solar is primary. If it drops, the grid is the next source. When the first ATS is in "use the grid" mode, if the grid drops, the generator is called. If the solar comes back, everything else is turned off.

This seems unusual to me. Why not just parallel the solar array to utility? This way they will always be able to maximize the solar component, no matter what its generating capability is, and maybe even get net metering credits from the utility company for when it exceeds the load. Then you'd only need one transfer switch to start the generator if the grid power becomes unavailable.

 

[steve66]

Normally I would just get my EE to answer this question/concern but he is on vacation currently and I am not one to disturb those on vacation.

I have been asked by a client on a project we are trying to quote as to whether he can have two 120VAC feeds, powered at all times from separate power sources, on one set of contacts of a device. Basically, a direct single phase feed of 120VAC connected to (L1) (N1) and (G1) of a device as well as a UPS feed of 120VAC connected to the same (L1) (N1) and (G1) of the device. It is a 120VAC device.

To me, this sends off all sorts of red flags, but my basic understanding of electrical systems does not produce a reason as to why. I would think that this would be against NEC (plus it wouldn't be 120V would it?) and my initial thought is "auto-transfer switch!!"

Am I thinking correctly on this?

Thank you,

Evan Briggs

As others have said, you can't do exactly what you said.

However, there are UPS's made to run in parallel. These UPS's usually communicate with each other, so their outputs stay sychronized. If one uPS dies, the other takes over, so the load usually doesn't even see a glitch.