Half of the equipment on my server cabinet (data servers, networking) are critical and need to be backed up by UPS's, while the other half of it is non-critical (high-load simulations that only take 3-5 minutes per job and can accept downtime). My PDUs have 4 outlets while my UPS's have 8, which is plenty. Usually, I'd do this:

  • Mains1 -> UPS1 -> PDU1 -> Extension bars -> All equipment (primary)
  • Mains2 -> UPS2 -> PDU2 -> PDU extension bars -> All equipment (secondary)

However, I'm thinking I want to spend more resources on backing up the critical equipment instead of the simulation servers, and there are 2 approaches:


  • Mains1 -> PDU1
    • -> Extension bars -> Non-critical equipment (primary)
    • -> UPS1 -> Critical equipment (primary)
  • Mains2 -> PDU2
    • -> Extension bars -> Non-critical equipment (secondary)
    • -> UPS2 -> Critical equipment (secondary)

Q1. If my UPS's only draw 10A at full load, which each PDU's outlet does supply, is it OK to put the UPS behind the PDU like this?


  • Mains1 -> PDU1 -> Extension bars -> Non-critical equipment (primary)
  • Mains2 -> PDU2 -> Extension bars -> Non-critical equipment (secondary)
  • Mains3 -> UPS1 -> Critical equipment (primary)
  • Mains4 -> UPS2 -> Critical equipment (secondary)

Q2. What about this? Do I still need a PDU behind a UPS, even if (i) my UPS already has plenty of outlets and (ii) I don't need any of the special PDU features such as switching, metering, network-based monitoring, for my critical equipment?


If the outlets are rated for 10A each, AND the UPS is rated to only draw 10A max, you can power it through the PDU. Be certain to check the rating of the UPS. Bigger ones can draw a lot of current, and won't be able to be powered through the PDU. You might need a different cable for the UPS to connect to the PDU.

For your second question - you never NEED a PDU. I've done plenty of smaller setups that didn't have them. PDUs give you the switching, metering, etc that you said you don't need. They also let you make the wiring a lot tidier. If you can manage the spaghetti monster of cables coming from the UPS, it will work just fine.

Are you certain the 8 ports on the UPS are all battery backed? Some models have all outputs on the battery, some have a few that are just surge protection. IF your UPS has any that are just surge protection, you should put the non-critical equipment into those ports (or PDUs connected to those ports) so they can still benefit from the surge protection and line conditioning of the UPS.

  • Thanks so much! Answered my questions. It seems like all the ports on the UPS are battery-backed (I think I'll go with strategy 2 and use this), but points about the surge protection and line condition are noted... The PDU's are HP modular PDUs, which only seem to have overcurrent/undercurrent protection, I will see if there's something to do that will benefit them. – elleciel Feb 9 '14 at 3:45
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    Also some higher end UPSs let you control the outlets invididually so you can program them to shut down nonessential loads once the battery drops to a certain percent. Or the ups software can be installed on all the servers, with the nonessential ones set to power down after being on battery for more than a few seconds. – Grant Feb 9 '14 at 3:50

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