Network Appliance: Determining The Quantity Of Rack-Based PDUs

The following table shows the calculations I will use to determine the quantity of rack-based PDUs to purchase for a remote data center. Depending upon the available power either a 20 amp PDU (16 amp max) or a 30 amp PDU (24 amp max) will be purchased.

Component Power Supply Min Amps Max Amps 16 amp Max 24 amp Max
FAS960 PS 1 1.25 2.5 15.0 22.5
PS 2 1.25 2.5
DS14 – Shelf 1 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf2 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf3 PS 1 1.25 2.5 15.0
PS 2 1.25 2.5
DS14 – Shelf4 PS 1 1.25 2.5
PS 2 1.25 2.5 22.5
DS14 – Shelf5 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf6 PS 1 1.25 2.5 15.0
PS 2 1.25 2.5
DS14 – Shelf7 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf8 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf9 PS 1 1.25 2.5 15.0 22.5
PS 2 1.25 2.5
DS14 – Shelf10 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf11 PS 1 1.25 2.5
PS 2 1.25 2.5
DS14 – Shelf12 PS 1 1.25 2.5 15.0
PS 2 1.25 2.5

I was asked to purchase rack-based PDUs for a remote data center so that the Netapp filer and shelves in that location could be remotely shutdown in case of a power outage. The remote data center is on UPS power and we have approximately 1 hour of battery life if there is a power outage.

We needed PDUs that can be managed remotely especially so that we can power off the Netapp shelves and controller in the event of a power failure. I determined that the company had already standardized on Cyclades PM8 and PM10. (I found this out by searching the main data center at the company headquarters and by looking at recent purchases made by the IT department. )  Since the Cyclades PM10 allows for remote control of individual receptacles this seemed to be the best choice in order to maintain a consistence standard.  Armed with this information I was almost ready to purchase the necessary PDUs but I still needed to determine the QTY necessary to do the job.

There are a couple different models of Cyclades PM10s that I know of and based on the type of power available.  I needed to purchase the appropriate model and that would also determine how much load I could put onto each PDU. There is the 30 amp model PM 10-30a and the 20 amp model PM 10-20a. Based on what power is desired or, in this case, what power is available I could choose the best model to suit our needs.  At the beginning of this project I did not know what power was available so I decided to calculate how many PDUs would be necessary for two different scenarios.  Scenario one is assumes that there are enough 20 AMP 5-20R receptacles available and scenario 2 assumes that there are enough 30 amp L5-30R receptacles available.

I found out that the remote site had QTY 1 Netapp FAS960 controller and QTY 12 DS14 disk shelves. The FAS960 has redundant power supplies as do each of the DS14 shelves. After searching the internet for a while I was able to find where Netapp recommended putting up to five of such devices (heads and shelves) on a single 20AMP rack-based PDU. Since I had a non-production FAS940 and DS14 MK2 FC shelves available I decided to test this information that I found on the NOW site prior to determining how many PDUs to purchase. This is what I found out:

The FAS940 controller and the DS14 shelf each use a total of 2.5 AMPs with 1 power supply or both power supplies in use. When both power supplies are in use the 2.5 AMPs is split between the two power supplies each using 1.25 AMPs.  If power is lost to the redundant power supply then the primary power supply uses all 2.5 Amps.

When determining how much load to place on a rack-based PDU use 80% of the total amperage. For example, use 16 amps for a 20 amp PDU and 24 amps for a 30 amp PDU.

Note: Please make sure not to overload a PDU. For systems that use redundant power supplies, calculate the total amps of power that a single power supply may have to draw if the other power supply fails. This is the Max amps.

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