A maximum of six power supplies can be installed in the Enterprise Chassis. Either 2500W or 2100W power supplies are available. Two 2500W power supplies are standard in model 8721-A1x. Two 2100W power supplies are standard in model LRx. Additional power supplies are orderable per the following table. All installed power supplies must be the same.
Redbooks - http://www.redbooks.ibm.com/abstracts/tips0863.html?Open#power