# Electrical System Redundancy



## gregdzip (Nov 16, 2006)

Does anyone know or have a link to the exact defenition of N, N+1, N+2, and 2N electrical distrubtion for a data center?

Thanks


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## dewd (Nov 17, 2006)

N stands for needed. If it takes 2 generators for needed back-up, N+1 would be 3 generators. Similarly, 2N would be 4 generators.


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## gregdzip (Nov 21, 2006)

Thanks, But I belive there is a little more to it...


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## dewd (Nov 21, 2006)

Uhhh... If your question is what the definition of those things are, there is nothing more to it. If you want to know how to apply that definition to a data center installation, I suggest you contact a qualified electrical engineer or read one of the thousands of articles written on the subject.


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## grover (Jan 6, 2007)

That's pretty much to it. N is defined as the number of (whatever) you need to run the site. If you have a 300kWload and use 200kW generators, you need 2 generators running to carry the load and N=2. It's all written alegbraically, nothing tricky about it.

Note that this refers generally only to the generators and UPS, and not to the switchboards, as N+x generators often tie back into a single switchboard.

Personally, I don't like anything less than N+2 for critical loads- allows 1 down for maint (scheduled or not) with N+1 running to allow for failure. Gives a nice comfort buffer for unplanned growth, too! At least for generators- UPSs not so much, as you can always run the generators while one of the UPSs is down for maint. NEC only requires N+0 generators for emergency power though... boggles the mind


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## twada (Feb 8, 2007)

gregdzip said:


> Does anyone know or have a link to the exact defenition of N, N+1, N+2, and 2N electrical distrubtion for a data center?Thanks


Are you taliking about Teir 4 UPS system?


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## twada (Feb 8, 2007)

Check www.apc.com


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## grover (Feb 13, 2007)

twada said:


> Check www.apc.com


APC's Symettra series doesn't really do true N+ redundancy, though, they have a quasi-internal redundancy which does offer redundancy, but not in the conventional sense. It offers some advantages at a reduced cost, but lacks some of the advantages of a true N+1 system; namely that you can secure one half of a parallel-redundant UPS for maint while the other assumes the load. They have some excellent white-papers on their website though.
N+1 UPS and N+2 Generators are generally sufficient for tier-4 quality power. Ideally, N is as small as possible, generally 1 unless your datacenter is so ginormous you can't find one large enough.


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