Redundant Power Supplies Active / Passive or Balanced Load ?

On the PowerEdge 2850, 2950, R510, R515 & R710 are the redundant power supplies active / passive where only 1 is in use at a time or are they balanced and both distribute the power draw / load evenly between them?

We are trying to figure out how to setup our power rails in our server racks and cant find any documentation on how the power supplies work.

Thanks