Camera Power Consumption and PoE
Updated: Apr 9, 2019
February 18, 2019
Power over Ethernet, or PoE, is probably now the most common method for supplying power to an IP camera. Using standard Cat5/5e/6 data cable for both data and power, PoE is a cost effective method for getting power to camera devices on a network without the need to install mains power at each camera location. This significantly reduces installation and maintenance cost and makes camera relocation comparatively easy.
So, how does PoE work…
Typical data cables, used by computing devices, including IP cameras, consist of 4 twisted pairs of wire. The actual data component only uses 2 of these pairs (4 wires) leaving 2 pairs spare.
There are two variations to how PoE is carried on a Cat5/5e/6 cable. These are Alternative A and Alternative B. Alternative A injects power along the wires used for data while Alternative B uses the 4 spare wires. DC power is pushed across 2 of the spare wires while the remaining two are used to complete the circuit back to the power source (see diagram).
It is more common for IP cameras to use the B standard although most cameras will identify and adjust to whichever option is in use. You should check your camera’s technical details before assuming compatibility with your PoE power source.
Ethernet cable connections also use one of two cabling standards, 568A and 568B. These standards only affect pins 1,2,3 and 6 which are the data only pins in PoE Alternative B. As PoE device standards call for polarity insensitivity, using either cabling standard with Alternative A will not affect the function of the camera.
To further complicate matters, PoE also has a number of standard implementations referred to as types. The most common type used for IP cameras is 802.3af and, within this, there are a number of classes. These classes define the operational parameters for PoE, the most important to us being the amount of power, in Watts, that is supplied to the device.
The most common specification used for IP cameras is 802.3af with no further class nominated. This specification means that between 0.44W and 15.4W of power is supplied at the source. Some IP cameras specify type 802.3af with class 3. This is a more stringent class and requires a minimum of 6.49W to be available to the device with an upper limit of 15.4W.
It is important to check the power requirements of the camera device when selecting a power source. Many cameras do not specify a type and class so 802.3af with no class is assumed.
So far we’ve looked at the PoE standards from the viewpoint of the camera requirements. Now we need to understand the source of that power, typically a separate PoE switch, an inline injector or a NVR integrated PoE switch.
It is important to understand the total wattage that may be drawn by all cameras when selecting a power source, typically a switch, and sizing that device accordingly.
Many PoE switches do not support PoE on all ports so the first step is to ensure that you select a switch with sufficient PoE enabled ports.
You will need to identify the total power consumption for all of your cameras, to be connected to the switch, under peak load. Many IP cameras have built in infra red LEDs for night vision and these consume power on top of the normal daytime operational load. It is important to calculate the total load including IR. For example, our 1.3MP bullet camera consumes 5 watts during daylight but this rises to 8 watts when the night vision IR is active.
If the power consumption of the camera cannot be determined, an inexpensive power measuring device can be used. I use a unit supplied by Watts Clever. These can be obtained for around $30 online or in many electrical and hardware stores.
The next critical step is to check the Watt output rating on the switch to ensure that it will support all of your cameras. Many switches only supply half of the power that you might expect. Often, the total power output averages 7.5W per port. If you consider our 1.3MP camera from above, the total power consumption for 8 cameras would be 64W under IR conditions. A switch outputting 7.5W would supply 60W across 8 ports.
So what happens when power consumption outstrips supply? Well, many switches will progressively shut down the port power, starting with the highest port number. Some cheaper switches will simply randomly black out a port. This may not immediately appear to be a power shortfall, leading to assumptions of camera faults and wasted problem assessment time.
As an example of the above, the D-Link DGS-1100-08P is an 8 port PoE switch with the following specification taken from the D-Link web site:
DGS-1100-08P complies with the 802.3af standard for PoE and supports a 64 Watt power budget.
Now, I like the D-Link for small installations and this description correctly states that the switch is 802.3af compliant but it does not state to which class it adheres. As such, it does not have to guarantee 15.4W per port. In this case, the switch will average 8W per port under full load. If our peak camera load exceeded 64W, the switch will progressively shut down devices until it falls back within tolerance.
In this article, I have identified the benefits of using PoE for powering camera devices and reviewed some of the precautions that have to be taken when implementing and powering these devices. In a future article I will examine the relative merits of the different means for supplying power to cameras using PoE.