What is the limit in terms of power, for PoE?

u587162
u587162 used Ask the Experts™
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I'm fairly new to the concept of PoE, but I like the idea a lot since I want to try and install as many devices in my new house build that can work over this technology as part of a home automation system.

What is the maximum power (voltage, watts) that PoE can deliver?  I know its not high powered, so I am guessing you can never get an electric shock from it if you accidentally cut the cable?

Apart from security cameras, what other devices can now be powered by this route?

When did PoE start to become more prominent and what are the advantages / disadvantages of it?

Thx.
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We use PoE switches to power our Cisco VoIP phones, Cisco Wireless Access Points, as well as out IP Cameras.

I beleive our phones draw 7w and waps about 15w.
The advantages are that you can place devices in areas where you don't have power outlets (or don't have sufficient power outlets), and you don't have to worry about devices getting unplugged because someone "needs the power outlet for something else," and you don't have to worry about wall warts failing, and you have fewer wires running around.

As well, A/C adapters are a frequent point of failure...eliminating them can increase reliability, assuming your POE switch or injector is relatively reliable.
50 Volts is enough to shock you, depending on the exact circumstances.  36 Watts is sufficient to cause burns.  You're supposed to treat it as dangerous enough to hurt you.

From https://en.wikipedia.org/wiki/Power_over_Ethernet

The original IEEE 802.3af-2003[2] PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA[3][4]) to each device.[5] Only 12.95 W is assured to be available at the powered device as some power dissipates in the cable.[6]

The updated IEEE 802.3at-2009[7] PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power.[8] The 2009 standard prohibits a powered device from using all four pairs for power.[9]

Both of these amendments have since been incorporated into the IEEE 802.3-2012 publication.[10]
Power sourcing equipment (PSE) is a device such as a switch that provides (or sources) power on the Ethernet cable. The maximum allowed continuous output power per cable in IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at, offers 25.50 W.


Uses:
https://en.wikipedia.org/wiki/Power_over_Ethernet#Uses
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Distinguished Expert 2017
Commented:
POE is limited to the power that can be carried over the CAT5
Usually the range is a DC 5-10 V. usually the power is caried on the last two pins 7,8 (brown/white brown)
Either the device you connect supports the POE functionality internally (RJ45 plug and internally the power is separated from being passed to the network controller) or on the other side you have an adapter that takes the DC portion out to a separate DC plug that goes into the device, while the RJ45 connection from the device connects .... (a Y type of connection RJ45 in -dc power and rj45 out)

IMHO, you should define what it is you want to power devices, components, etc., and then see whether POE is an option.  There are different approached usually this deals with a limited option on running wires, so one tries to see whether there is a way to run multiple things over a single wire.
POE is one, Ethernet over powerline, power over coax,
Many signals can be sent over different media. one merely needs to have the correct adapters on each end.

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Commented:
As Im dealing with a total house rebuild, the more I look into the concept of PoE, the more I like and the more equipment I would like for it to use this, i.e. my CCTV cameras, speakers, phones....I'm not sure what other practical equipment there is.

Am I right in thinking that if the cable was accidentally cut and I touched it, I woudn't get a nasty shock?

Is it possible to send TV signals through PoE from a coax and then convert back to coax later?
Distinguished Expert 2017
Commented:
Speakers (powered?), are more difficult as they could vary, though most speakers are driven through the output presumably coming from an amplifier/media center. The only speaker that usually has its own power is the subwofer, but that most commonly takes AC power and can not be fed.......
Since you are in a total, rebuild, you can use/setup conduits and wires from the get go.


CCTV, can either be power over coax, or power over ethernet depending on which you have. power along the RCA type of connections... Many CCTV camera cables now are built as a single cable, power+coax/rca pair.. all you need at the source is connect the power to the DC plug.

No, usually you will not get a nasty shock, the CCTV power over coax, and POE you will not, as serialband post covers, the power from those is usually <100 W meaning low amperage and low voltage
You might get a spark from it when you cut it with a metal cutter, but it shouldn't zap you unless you grab the bare metal wires tightly and complete the circuit.  50 Volts is rather low, but it's just high enough to penetrate skin, so you can get a shock.  It's a much "safer" voltage than 120 Volts or 240 Volts.

The maximum power of the PoE standard is 15 Watts and PoE+ is 36 Watts.  You will not see 36 watts going to the vast majority of devices.  They're usually going to be under 12.5 Watts for PoE.  You will likely not experience anywhere close to those power levels as the human body has a much higher resistance or impedance and you'd only get microwatts going through you.

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