Is it safe to plug a regular ethernet cable into PoE?

It depends which category cable you have. CAT5E and higher will support PoE on all voltages. CAT5 will support lower voltages too. Anything lower is not recommended.

Given that CAT5E is a standard for a long time now, chances are high that you currently have CAT5E cables.

That said, PoE have 3 classes.

Class1 has 4 Watt available.
Class2 has 7 Watt available.
Class3 has 15 watt available.

There are devices that can output higher watts. I've seen up to 30 Watts. These are all supported on CAT5e though. For Cat5, I would not exceed Class1 to be on the safe side, even though I have a feeling Class 2 will probably work depending on the usecase and where the cable is placed. For example a Class2 CAT5 cable that runs close to a heat source is not recommended.

As for delivery of the power, before there is any power transmitted over the cable, a low (non-damaging) voltage is sent over the cable to initiate a handshake. If the handshake is successful, only then will a higher current being transferred. So you can use a CAT3 cable on a PoE port if you purely use that for data. In that case, the PoE is simply not used and it is being used as a normal Data port.

LPChip

Posted 2020-02-04T12:07:42.960

Reputation: 42 190

I actually think I might have used Cat6 or higher (not 7) but thansk for calrifying that – Mr. Boy – 2020-02-04T12:46:50.387

In this case I might put the adapter as the cable exits the hub, so I can have a PoE socket in the room which needs it to simplify – Mr. Boy – 2020-02-04T12:52:49.563

1I was actually planning to ask the exact same question earlier today, and I would find this answer mostly if not completely useless. My biggest concern would be that the non-PoE device's Ethernet port would get damaged long before the cable shows any sign of melting. (I've gathered that Ethernet ports use magnetic isolation, but does that help in this situation at all?) – user1686 – 2020-02-04T15:55:30.890

2(An important caveat is that quite a few manufacturers use so-called "passive PoE", which just sends approx. 24V with no negotiation at all -- which I'd guess is nonstandard but cheaper to implement.) – user1686 – 2020-02-04T15:57:01.827

@user1686 there is a handshake that starts PoE. If a non-PoE connects to the device (as stated in the last paragraph of the answer), it is treated as a normal data connection and no high current is transferred at all, so every cable can be used and no damage to any device is dealth with. – LPChip – 2020-02-04T15:57:25.943

@user1686 can you give me such device that does not adhire the handshake? I'm not familiar with PoE devices that don't negotiate a connection. – LPChip – 2020-02-04T15:58:33.643

Ubiquiti Wi-Fi hardware previously was exclusively passive-PoE, e.g. the power bricks that come with airMAX M series are dumb as a brick. And although the current generation "UniFi AC" access points support 802.3af in all variants, the previous generation only supported 802.3af in the Pro variant -- but not in the Regular/Lite variants. (I don't have a copy of the older datasheets, but the current one still has a few mentions of "Passive PoE".)

– user1686 – 2020-02-04T16:02:51.797

2It's important to realize that "PoE" can mean simply "Delivering power over Ethernet cables in some unspecified fashion", or it can mean "IEEE 802.3af". Only the latter has precisely defined semantics and guaratees that power will only delivered after the device explicitly requests it. – Jörg W Mittag – 2020-02-04T21:33:00.900

Is it safe to plug a regular ethernet cable into PoE?

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