Question power budget math: PoE total power budget vs number of ports connected

[hot_cocoa]

If my 8-port PoE switch has a total power budget of 60W, and I only use it to provide power to 4 ports, can I assume that I've got 15W per port available?

 

[kanewolf]

If my 8-port PoE switch has a total power budget of 60W, and I only use it to provide power to 4 ports, can I assume that I've got 15W per port available?

Generally that is the case. The specs for the switch should provide definitive guidance. It may be obscure, like saying 802.3af -- https://en.wikipedia.org/wiki/Power_over_Ethernet#Standards_development

 

[bill001g]

It unfortunately depends on the switch and how smart it is.

Without looking this up I am unsure and my knowledge likely outdated. What cisco switches did was have the ability to request 4 or 7 or 15 watts. If all the devices request only 4 watts you could run more ports.

What I don't know is if this is something cisco proprietary or if it is part of the poe standard and if consumer switches implement it. I know even consumer switches can tell the difference between devices that need poe+ wattages compare to normal poe.

 

[Ralston18]

And I am compelled to ask about the reason(s) that power management is a concern.

Certainly, saving energy and any corresponding costs is important.

However, a few watts here and there, could be offset by simply turning off something else - a light switch perhaps.

 

[bill001g]

And I am compelled to ask about the reason(s) that power management is a concern.

Certainly, saving energy and any corresponding costs is important.

However, a few watts here and there, could be offset by simply turning off something else - a light switch perhaps.

When you talk power budget with poe it is not so much a money thing or saving thing. It is how is power being divided between the devices on the switch and how many devices you can connect. Not sure why they use the word "budget" rather than say "capacity" but it has always been called that since I heard it years ago.

 

[hot_cocoa]

@Ralston18 - fleshing out my situation a little more could be instructive. I have 4 access points. Each of these has a max power consumption of 12.4 watts.

Does my 8 port PoE switch divide its total power budget of 60w by 8 ports leaving me with only 7.5w per port? If this was the correct interpretation, I would short my devices on power - they might not even work.

On the other hand, if I only use 4 ports, will my power situation give me 60/4, or 15w per port? This would give all 4 devices all the power they needed.

 

[kanewolf]

@Ralston18 - fleshing out my situation a little more could be instructive. I have 4 access points. Each of these has a max power consumption of 12.4 watts.

Does my 8 port PoE switch divide its total power budget of 60w by 8 ports leaving me with only 7.5w per port? If this was the correct interpretation, I would short my devices on power - they might not even work.

On the other hand, if I only use 4 ports, will my power situation give me 60/4, or 15w per port? This would give all 4 devices all the power they needed.

We don't know, because you haven't identified the make and model of your switch.
For example, Ubiquiti has an 8 port 60W switch. Only 4 of the ports are POE enabled. Four ports are not.

 

[bill001g]

This is read the documentation question. What tends to be common will be the switch will allocate power to the first devices plugged in. If each device wants 15 watts then only 4 ports will work on a 60 watt power budget. The rest of the ports will not be able to request power. If you had 4 ports asking for 12 watts then a 5th port could also request 12 watts. There are too many combinations because some device might ask for only 7 watts.

But as kanewolf indicates some switches will not provide POE ability on all the ports. In some ways this makes it less confusing when all the poe ports can provide maximum 15 watt power and are not affected by how much is being used by other ports. It tends to be more flexible when the user has to understand his power usage to know how many ports can provide power based on their various levels of usage.

In the end it comes down to reading the fine print on the switch to know for sure. In general they do not just divide the power equally to the ports. A POE (ie not plus) port is required to be able to provide up to 15 watts.