240VAC ckt , battery backed up by two (2) 120VAC UPS units (one for each pole/ leg)

[JimmyCornholio]

Hello,
I am reviewing a plan for 240VAC double pole 40A circuit, backed-up by two individual 120VAC On-line UPS's (one for each leg). Both of the UPS outputs tie back into a double pole 40A on the load side. My concern is if either of the UPS were to fail before the other, only 1 of 2 legs would be energized creating what seems to be a safety code violation, but which one?

Thanks,
JC

 

[GoldDigger]

Hello,
I am reviewing a plan for 240VAC double pole 40A circuit, backed-up by two individual 120VAC On-line UPS's (one for each leg). Both of the UPS outputs tie back into a double pole 40A on the load side. My concern is if either of the UPS were to fail before the other, only 1 of 2 legs would be energized creating what seems to be a safety code violation, but which one?

Thanks,
JC

Even before that I would ask whether the two UPS units are designed to be linked together (stacked) to produce 240V. If their outputs are not synchronized you cannot drive a 240V load.

 

[JimmyCornholio]

GD~Thank you for the feedback (has helped me go further with the problem). The double pole breaker/ output from the combined (2) individual 120VAC UPS inverters, feeds a 120/240 LoadCenter containing only single pole circuits. This panel/ LC will not be outfitted with any double-pole circuits. So, I suppose the out-of-sync inverters will not cause any noticeable equipment operational issue for the 120v devices. Seems like this may only be an El Cheapo way of backing-up a system without the cost of a 240V UPS and since it will be maintained and accessible only by qualified personnel it may not pose a safety concern either.

Thanks again,

JC

 

[GoldDigger]

If there are only 120V loads and no circuits are wired as MWBC, you could also, in theory us NJe only
one double sized UPS. But that would require adding an external transfer switch instead of running just the UPS. And it would keep you from taking full advantage of a dual conversion UPS.
In any case, clearly mark the panel for no 240V loads, since it will appear to be an option to anyone who looks while the grid power is on.

 

[jap]

Hello,
I am reviewing a plan for 240VAC double pole 40A circuit, backed-up by two individual 120VAC On-line UPS's (one for each leg). Both of the UPS outputs tie back into a double pole 40A on the load side. My concern is if either of the UPS were to fail before the other, only 1 of 2 legs would be energized creating what seems to be a safety code violation, but which one?

Thanks,
JC

How are the output neutrals of the (2) separate UPS units connected to the load?


JAP>

 

[JimmyCornholio]

Reply to JAP>

Reply to JAP>

Individual UPS output neutral conductors are terminated at the 120/240 VAC LoadCenter neutral bar

 

[iwire]

In my opinion your plan results in a violation of NEC 110.3(B).

There is no way these units are designed to have the output neutrals combined or to feed a panel in this way.

 

[infinity]

In my opinion your plan results in a violation of NEC 110.3(B).

There is no way these units are designed to have the output neutrals combined or to feed a panel in this way.

I agree, but would it work?

 

[iwire]

I agree, but would it work?

I think Golddigger covered that aspect of it and the hoops you would have to jump through to make it work.

I can wire a 200 amp service with 14 AWG, it might even work. That does not mean it is a great idea.

 

[texie]

I agree, but would it work?

My guess would be no as many of these types of small UPS have circuitry to monitor basic wiring faults. But as others have said, I think this is a horrible idea even if it functioned. It brings to mind a whole host of NEC and NRTL issues.

 

[JimmyCornholio]

Thank you for your responses. I agree that the UPS(s) are being used in a way likely not in accordance with their listing.

-JC

 

[JimmyCornholio]

Illustration of UPS design

Illustration of UPS design

 

[ggunn]

I agree, but would it work?

I seriously doubt it. I do not believe that the inverters in the UPS units would have any way of synchronizing with each other. The phase angle between their voltage waveforms would be completely random and their relative frequencies would probably drift.

 

[iwire]

I seriously doubt it. I do not believe that the inverters in the UPS units would have any way of synchronizing with each other. The phase angle between their voltage waveforms would be completely random and their relative frequencies would probably drift.

There are no 240 volt loads, only 120 volt so the lack of sync would not be an issue.

 

[GoldDigger]

There are no 240 volt loads, only 120 volt so the lack of sync would not be an issue.

With only 120V loads, the lack of sync will only be an issue for current in common neutrals, including the neutral bus bar and wires of the panelboard.