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I have a number of Dell 2950 servers with two power supplies. In normal usage how are the PSU's utilised?

Do they both draw '50%', only switching to 100% if one of them dies? Does one supply provide most of the power whilst the second is more or less redundant until the first fails?

The reason for the question is thus: I have a couple of APC UPS units in the bottom of the rack and currently both power supplies for each server are plugged into a UPS unit. Am I better off plugging one of the power supplies into the UPS and the other onto filtered mains power? My thinking is that if the power goes down and the UPS picks up, will I get a longer grace period if only one of the PSU's is drawing power?

If both PSU's draw '50%' then it would make no difference obviously, but if there is a power overhead for having two PSU's then it becomes relevant.

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6 Answers 6

up vote 2 down vote accepted

I can't seem to find this documented but my experience (with a clamp-on ammeter I might add) is that they do balance the load.

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Both PSU's draw roughly 50%, and will ramp up as required.

The main reason to plug each PSU into a different supply is one of redundancy - if your UPS has a fault and stops providing power, then everything plugged into it is going to be affected.

If you have one PSU plugged into the filtered mains, then you are protected in both situations - loss of the UPS and loss of mains.

Thats a lesson I learned the hard way a few months ago.

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3  
So, I shouldn't be using that silly "Y" power cable Dell ships with some servers????? :-) –  BillN Jul 29 '09 at 13:41
    
If you do this won't it mean the the UPSes load and runtime calculations are all shot? –  Christopher Edwards Oct 11 '11 at 7:31

I believe the draw is proportional to the load. So beware total loading -- if one UPS dies before the other, the survivor will have to be able to carry the full load on its own.

Learned that the hard way, myself.

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its intended for redundancy. get a second ups and plug the second psu into it.

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I don't know for sure about the power draw per PSU, but you might consider splitting it just because UPS's go bad...you'll thank yourself for splitting the power sources if it happens to come in handy down the road. It just gives you more options.

You could also test if you have UPS's with load meters (or finer metrics from a management console) by just seeing what one looks like if both are plugged into it, then remove one and place it on another UPS and see what the loads do to even out.

I'd recommend having both go into some sort of UPS anyway just for redundancy; I don't think you'll see much in the way of savings or gain by having only one power supply on battery backup and you risk losing something if there's a power outage or failure in the power supply (or UPS) that you're getting feed power from, but maybe I'm missing some logic here that someone else will correct me on.

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I don't know the actual answer, though I have verified that all our servers, from 2500's up through R805's will run on one PS. Having the sources split also comes in handy when having to move servers/racks/UPS's and generally untangling the Gordian knot of cables the previous admins left for you (me) to deal with. (Though you have to untangle it a bit first to be sure the sources are split to begin with...and you can't untangle this one like Alexander did. :)

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