Power supplies, PoE, input power

Posted on 2012-08-14
Last Modified: 2012-08-24
The question regards power input, and the affect on this on PoE capability. The following is from an HP datasheet regarding 1500W power supplies for several of their chassis switches:

"Standard PoE+ power supply for zl series switches. Supplies 300W for PoE+ power at 110-127 volts, 900W for PoE+ power at 200-240 volts, and 600 W for switch power. "

These are 1500W switches. According to this, they can/will supply chassis power at 600W, but the PoE budget will triple based on the input power (110 vs. 2x0).

Please explain why(if) this is true. I've always thought it was a simple formula, where voltage (x) input power (=) watts. If a power supply is rated at 1500W, then why would input power affect the amount of power the supplies could provide for PoE device support?

Thank you - as always, reference links/docs are appreciated.
Question by:cfan73
    LVL 17

    Expert Comment

    It is important to plan for peak PoE power
    needs so that sufficient power is available in
    the switch. When the peak power needs of the
    powered devices (PDs) connected to the switch
    exceed the PoE power available from the
    supplies, the PoE power priority in the switch
    is used to determine which ports lose PoE

    Ports that lose their PoE power will
    not be powered again to prevent them from
    turning on and off, unless the loss of power
    was due to a power supply failure.
    LVL 32

    Assisted Solution

    Have you asked HP?  It may be an issue with ampacity...but even at max output, the input on the hot wire is less than 10A.  At 120V, there are also issues with efficiency.  Same with heat generation.  IIRC, the heat (power loss via resistance) is the square of amps * ohms.

    Or, it could be a de-rating for the reliability/longevity of the components.

    120VAC: 1114W max, 600W for chassis, 300W for PoE
    amps = 900W / 120VAC
    @85% efficiency = 900 / .85 / 120
    amps = 8.82 A

    240VAC: 1768W max, 600W for chassis, 900W for PoE
    amps = 1500W / 240VAC
    @90% efficiency = 1500 / .90 / 240
    amps = 6.94A

    Author Comment

    @aleghart - HP was on a con-call to discuss all of this yes... oddly, they couldn't explain it either. I also had a UPS/power provisioning sales rep on the call and he had the same questions I did.

    It comes down to this - if the datasheet is correct, and 240 allows a power supply to provide 3x the available power for PoE devices, then we'd obviously want to run 240 in every closet. This is non-standard (based on my experience) for non-data center closets, which are typically always 110. Converting them to 240 is crazy expensive, so I'm needing to determine if this is really a true statement.

    I look at datasheets for Cisco equipment, and I've never seen anything like this.  A 3750-X switch w/ an 1100W power supply will provide 800W of PoE power, regardless of 110 or 240 input power.
    LVL 32

    Expert Comment

    Tell them for a fraction of the cost of new electrical drops, you can install NETGEAR PoE switches with RPS and UPS, and have money left for a vacation in Hawaii.

    They might track down the tech writer and engineer who wrote that spec.

    Accepted Solution

    Upon further analysis, the Cisco higher-watt power supplies indicate the same behavior (providing much higher voltage only when getting 2x0v input power over 110v).  The problem with these particular HP chassis is that they only supported smaller (1500w) internal power supplies, so at 110v, you'd only have 1200w of PoE fully populated with four. (HP does provide external shelves if you demand more and or unable to convert to 220 input.)  

    The bigger Cisco chassis support 6K (and higher) power supplies, so it's not as immediate an issue unless your PoE requirements are very high.

    Author Closing Comment

    Additional research beyond this thread answered my question fully.

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