If I have a rack full of servers using redundant power supplies, and I'm keeping the amps at 80% (16A of 20A), isn't this still asking for trouble in the event of a power loss on one of the circuits?
If power is distributed 50-50 (e.g. an average of 1A per power supply on a server that would use 2A when using only one power supply), when one circuit fails, the load is going to pull from the other circuit (and will likely throw a breaker).
I guess I've always just gone the 80% route without thinking too much about it, but does this mean I should only be "maxing out" our circuits to 40%?
Best Answer
Running dual-PSU servers in a balanced power distribution mode doesn't mean that they each use exactly 50% of the power that a single PSU would use. It's usually horribly inefficient to balance the load. I don't have the exact numbers, but afaik it's about a 20% waste in power.
However - running them in balanced mode and then push each PDU up to 80% is begging for trouble. I see absolutely no chance in the remaining PDU surviving if you're already that far up in usage with balanced power distribution.
One other thing to keep in mind is the most catastrophic scenario in this situation - a site-wide power failure where both PDU's goes offline, and one of them breaks it's circuit because of overvoltage or similar (not that uncommon). What happens when all your servers power on at the same time, with a single 20A circuit to share? I'm sure I don't have to explain it further..
If I were you I'd start making preemptive changes right away.. set the servers to high efficiency power (if they support it), which usually gives you a 2-3% usage on the standby PSU and 100% load on the other. You'll quickly see if you're maxing out one of the PDU's. Also - make sure that the servers have power-on delayed in clusters to make sure that they all don't power on at the same time in case of outage.
Edit: Oh, and of course - order bigger PDU's and bigger circuits :-)