With the firmware version 1.3 or below, 1 PSU can operate the UCS B-Series chassis with "Non-Redundant mode", if the power consumption in chassis is less than 2500w.
http://www.cisco.com/en/US/docs/unified_computing/ucs/hw/chassis/install/5108_HIG.pdf
P.1-13
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Smaller configurations (requiring less than 2500W) can be powered by a single power supply.
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Thus, the required PSU number for each power redundant modes are as below:
<Chassis power consumption is less than 2500W>
No Redundant => PSU * 1
N+1 => PSU * 2
Grid => PSU * 2
<Chassis power consumption is 2500W or abve>
No Redundant => PSU * 2
N+1 => PSU * 3
Grid => PSU * 4
However, starting from firmware release 1.4, the required PSU number is always same as below regardless the power consumption of the chassis.Please note that this currently(6/24/2011) IS NOT documented.
No Redundant => PSU * 2
N+1 => PSU * 3
Grid => PSU * 4
Otherwise, "Power Redundancy Failed" error will be logged in CIMC.
