N+1 or N+2 setup

[Guibao]

Hello all!

Please enlighten me on a matter we´re going through at work.

This is not regarding one specific UPS model but it´s for the setup itself.

We must choose between a N+1 or N+2 config, where the N+1 would have 2x 30Kva devices and the N+2 would be 3x 20Kva devices. The load is 30Kva. 

This is a new scenario for me so I ask: which setup is better? In case of one device failure on the N+1 I´ll stress the only UPS left to the limit, whereas on the N+2 I have 40Kva total on the remaning two UPS. 

Which option should we pick?

I thank you for your input.

[BillP]

@Guibao 

N +1 equals the number required plus one redundant.

N +2 equals the number required plus two redundant.

The recommendation is all redundant UPS have the capacity to sustain the load. So, if you go with N + 1 and split the load between two 30 KVA UPS if one fails the second will hold the load allow time to correct the issue. 

If you go with three 20 KVA UPS you can split the load and loose one the other two can sustain the load. However, if a second fails the load will crash. In this scenario the recommendation is to decrease the load and then fix the initial issue. 

[al_buffington · Accepted solution]

@Guibao  I'm assuming these UPS units will be operating in parallel with a common output?  Typically parallel UPS systems are either designed for either redundancy or are capacitive.  If your load is 30k, a 2 x 30 installation would not leave you with any buffer in the event that one UPS has a problem or is shut down to perform maintenance. If your load won't exceed 30K then it won't be an issue to have all the load on one system - however if you have slightly underestimated your load, or the load fluctuates,  then a single UPS might not be able to carry the full load. This would likely cause you to run in overload bypass in the event one UPS is offline. A 3 X 20 would afford you a bit more leeway. In this scenario since 2 units would have a capacity of 40k you would not run into this problem. It would also afford you some amount of capacity for future expansion. Hope this helps. 

[Guibao]

Thanks BillP and Alviin. 

That´s the answer I was looking for.

Take care!