I am responsible for several racks worth of gear, and as part of a datacenter efficiency project, I need to move some gear around.

One constraint I'm running into juggling these servers around is PDUs loading. One particular part of the plan involves moving about 5 amps of equipment into a rack with two 120V PDUs drawing 9.0 and 9.5 amps on independent 20 amp breakers. I'm concerned because the total draw in the rack is above 80 percent of 1 breaker already, and if we add 5 more amps, when one circuit fails the increased load would trip a breaker (aka cascading failure).

My director says it'll be fine, because server PSUs are more efficient when they only draw from one PSU. So that we can load up both PDUs to 15 amps without sacrificing N+1 redundancy.

Is the director is right? Why? Given a pair of 120V racks, what would be a good threshold to stay below during normal operations?


Your Director was correct 10 years ago. (maybe reword that 3rd paragraph) Now a days (do a google image search) a modern PSU's are ALOT more efficient at low loads than older PSUs.

Speaking of the 3rd paragraph. Depending on how your servers are configured, sometimes the PSU's are actually NOT redundant, its very possible to have a server where the PSU's are in tandem NOT redundant. And many server can run on one. I've seen alot more single PSU machines in the datacenters recently. I wouldn't trust older servers tho.

Also when spec'ing Rack/PDU load you need to go by what is on the PSU's in the server. Most servers can idle down to save power (and idle down alot), but this often tricks people into overloading their PDU. I've seen circuit breakers pop and whole racks go down because " well the display said only 12A when I plugged them all in" ... "yes that was until you pushed work to the servers then they spooled up an, well it looks like there is 20+ amps of server here" Lesson: Don't look at the display.

There is an cost efficiency as well(hopefully what bossman was thinking). Most datacenters charge by power. so If you have 3 x 2.4kW racks then that's what your getting charged. Even if nothing is plugged in and your drawing no power. So load those PDU's up to the 80% limit. 15 amps on a 20A. 11A on a 15. 45A on 60A ;-) (always be aware of single(some will incorrectly say dual because 2 hots) or three phase and the RMS voltage. Don't worry about PF. also if redundant or primary)

Your smart to be aware of a cascading failure... You may have todo convincing and load test (some linux boot disc and run some server stress tool. ) Then drop a PDU watch the servers jump to the other PDU and drop that PDU. Also to show that even tho the servers don't draw their rating all the time you still need to account for it in the PDU load. Plan this and purpose todo it on a sunday. IF the director still insist on overloading the PDU. For added drama make step 1 "check fire extinguisher charge(s)" it helps with the convincing ;-)

Volts kinda matter but, Watts and the melting part: Amp's does. Take what your paying for and times by 0.8; 20Amp * 0.8 = 16, I like 15 for some buffer. Now 120V rack * 15 = 1,800 Watts (2.4kW rack). Now find the Wattage on the Server PSU's add them up and hope its less than 1,800 watts (or whatever you calc). That is what you should load the rack to and don't forget to include switches and anything else pulling from the PDU(s).

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