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So I was wondering about pulling more power down PoE, the default spec for most 802.3at switches is 25.5 watts per cable.

I was wondering would it be possible to join two PoE cables in parallel to double the theoretically maximum.

See diagram below 1, for example you have 2 ports with PoE+ enabled going two splitters, then you join the 12v rails together in parallel (using a simple DC Jack splitter 2). Would this theoretically work or would this damage the switch or just not work at all?

PoE Splitters in parallel DC Splitter

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    While the current PoE 802.3at maxes out at 25.5 W, the upcoming 4PPoE standard 802.3bt will be able to supply 50 W and more over Cat 5.
    – Zac67 ♦
    Jan 15, 2018 at 7:35
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    Most likely, you'd damage the splitter(s) by backfeeding power, but that's easily fixed with a diode, if the splitter doesn't have one built in. The other issue is how the PD handles only getting half the power it needs because one of the splitters isn't being powered for whatever reason.
    – Ricky
    Jan 15, 2018 at 9:00

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No. This isn't possible. The switch itself negotiates the amount of power to be provided when it comes up, and beyond the obvious non-starter of two data lines being paralleled to each other (two transmits connected to each other?) there is no accommodation in the various PoE specs to be splitting power. Indeed, you'd potentially end up in a situation where one port was pushing power into another one, which would be bad.

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  • As per the diagram only one would be using data the other would just be using the power out of a splitter, but yes good point about pushing power into the other port.
    – Chris.Jenkins
    Jan 15, 2018 at 0:21
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    The port has to be up to negotiate both the need for power and the level of power provided. As such, both ports would be up and trying to do their thing - definitely a no-go, unfortunately. You might be able to cobble something at 100M with in-line injectors (old-style injectors used the spare 2 pairs in the sheath), but even this falls into the realm of the dangerous/hacky and, of course, nobody really wants to start the game at 100M nowadays. I'd also be hesitant to push too much wattage down a 26 AWG wire (+/-) - especially at length, but that's a whole other issue.
    – rnxrx
    Jan 15, 2018 at 0:56
  • The diagram is somewhat misleading - Chris wants to use two PoE switch ports.
    – Zac67 ♦
    Jan 15, 2018 at 12:37
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Using two PoE+ ports on the switch and two splitters to 12 V you'd need to make sure that the splitters don't feed back to each other.

As @RickyBeam has pointed out, a diode would help but it would have the required amperage rating. It's probably easier to find splitters that have feedback protection.

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