How to Choose a PoE switch – Actual Power Budget

The PoE switch sends not only the data, but also the power to the remote IP device, so the AC outlet closed to devices can be eliminated in the field. As we all known, the electric equipments only take the power it consumes. It has no relationship with the power budget from the power source. For example, the access point only draws 5W even 100W power adapter is being used. The below is power reference consumption of convenient network device.

Access point – 5W
IP camera without infrared – 6W
IP camera with infrared – 15W
Infrared PTZ camera – 20W
Infrared camera with heater -50W
Access control with door lock strike – 60W
Point of sales (POS) – 65W

These are the general estimated power. You may find the accurate data of your device from the label.

How to pick the PoE switch to meet the power demanding? There is IEEE 802.3 standard you can refer to. The typical two are IEEE 802.3af (also name PoE) and IEEE802.3at (also name PoE+). The PoE output 15.4W and the PoE+ output 30W. However this is not the actual power budget. It is only the maximum power that PoE switch injects to the Ethernet cable. The remained power budget is 12W for PoE and 25.5W for PoE+. Why there is difference between the output from PoE switch? That is because it always has power loss inline when the power is transported on the conduct. Then the power loss is converted to the heat eventually. The new IEEE 802.3bt standard is ready now. This new standard is capable of injecting 95W high power and the remained budget is kept to 72W at the front devices. It is going to benefit the application such as access control, POS, interactive display, cellular base stations, etc.

Can we have larger power budget if the distance is totally shorter than 100 meter, like only 50 meters? The answer is negative. The power budget has been predefined in IEEE 802.3 specification. The remained power is set to 72W even the distance is 1 meter. There are IC on both PoE switch and powered devices to control the power budget. However the low quality Ethernet cable could compromise the remain power. If the power loss is larger than it expects, it will create seriously voltage drop which could make the powered device(PD) fail to start up.

Do I actually have 30W on each ports if the PoE switch support PoE+? That may not be true. The PoE switch uses the shared power source. You may read the specification – 16 port PoE+ switch with 180W power. If all 16 ports supply 30W, it should be 480W power budget in total. As the electrical device only will take the power it consumes, some of the devices may only need 5W, like the access point. The 180W still could be enough for the whole system. It doesn’t have to be 480W for 16 ports outputting 30W.

Except for considering the maximum output of each ports, the total power demanding of the whole system needs to be calculated while choosing PoE switch. The IEEE 802.3 af/at/bt predefine the power capacity of single port. The real output also depends on the total power budget in the PoE switch.

There is one more thing about the power budget. It is better to reserve margin when you design the system. If the devices needs 12W, choosing PoE+ switch(FC 16 port switch) which can supply 25.5W is better because the system will not fully load all the time. The maximum cable distance is 100 meters between the PoE switch and powered device. Click choose the PoE extender can extend the network beyond 100 meters to learn about information.

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