PoE in the enterprise network: switch power budget

One cable instead of two is the greatest advantage of powering over Ethernet and at the same time the source of the most common mistake when shopping: assuming that all PoE is the same. This article explains how the standards differ, how to calculate the PoE power budget for a switch and how to know when it has run out.

Why this is a topic for a separate article

A power budget error rarely manifests itself directly because it looks like a failure that no one associates with a prior purchase.

In a typical scenario, the installation works after receiving, after a few weeks the camera arrives, and then the access point starts rebooting during peak hours. No one connects these events to a switch operating at the limit of its power capacity, because everything can appear normal on the panel.

Calculating the budget before purchasing takes fifteen minutes, while diagnosing the problem after the fact can take days.

Three things to distinguish

Standard that the device requires

Power over Ethernet is described by the IEEE standard family: 802.3af, 802.3at i 802.3bt, with subsequent versions providing higher power levels.

The rule of thumb is that a device requiring a higher standard will not work properly on a source that only supports a lower standard, so it may not start or it may start and work unstable, restarting under load.

The reverse configuration is safe because a source supporting the higher standard will power a device requiring a lower standard because the power level is negotiated between the switch and the receiver.

Passive power is not the same

This is a trap that can damage equipment because passive power supplies a constant voltage to the cable without negotiation, without checking what is on the other side, and without expecting a response from the device. This solution was used by older devices and some equipment operating outside the standard.

Consequently passive and standard power supply are not interchangeable, because connecting a standard device to a passive source with the wrong voltage may damage it, and equipment requiring passive power will not start from a standard switch that will not apply voltage without response.

If there is older equipment in the installation, it is worth checking its requirements before adding anything.

Port power and the budget of the entire switch

This is where the most common purchasing misunderstanding occurs, because the switch has two different parameters, i.e. maximum power on a single port and total power budget for all ports together, with the latter limitation determining the entire installation.

A switch with eight power ports does not have to power eight devices with maximum consumption, but divides its total budget among them. If the budget is exhausted, it will begin to deny power to additional devices or turn off those with lower priority.

This is the moment when the phrase "everything worked until we added one camera" appears.

How to calculate the PoE power budget

You can go through the following method yourself before purchasing, using the data from the catalogue cards:

1. List all cable-powered devices: access points, cameras, phones, access control, sensors.
2. For each, check the data sheet required standard and declared power consumption. Take the maximum value, not the typical value.
3. Sum up consumption.
4. Add expansion room. Cameras and access points are available almost always.
5. Include cable losses. Some power dissipates along the way, so the device at the end of the long path draws more from the port than it consumes.
6. Compare to total switch budget, not with the number of ports.
7. Check if the most demanding device is within the single port limit.

Points 4 and 5 are most often overlooked, although they determine whether the installation will survive two years without any surprises.

How to know when the budget is gone

Symptoms can be misleading because they look like a typical network problem:

• the device does not start after connecting, although it works on another port,
• access point or camera restarts periodically, usually under load,
• the device operates in limited mode: camera without illumination, access point with one band turned off,
• the symptom appears after adding another device, and it concerns a completely different one,
• the problem worsens in hot weather, because higher temperatures worsen working conditions.

The third point is particularly insidious, because some devices do not turn off when there is insufficient power, but limit their functions, as a result of which no one reports a failure, even though everything formally works.

The common feature is that none of these symptoms can be diagnosed without reading the status of the ports on the switch side, which is another reason why unmanaged hardware can be a false economy. We develop the issue in the article about router from the operator and managed corporate network.

The cable also matters

When planning installation, it is worth remembering three properties of the cable:

• the cross-section of the conductors affects the losses. Thinner wires, found in cheap patch cables, lose more power over the same distance,
• current flow heats the cable, and heating worsens the transmission parameters. The phenomenon is intensified in dense beams, where heat has no way to escape,
• non-copper cables are not suitable for power supply. Cables with an aluminum core covered with copper tend to be cheaper and look identical, but behave completely differently when powered.

The last point deserves separate attention when shopping, because the difference is not visible to the naked eye or in the general description of the store.

The influence of power supply on the selection of cabling categories is described in the article about Cat6 and Cat6A in the company, combining transmission requirements with cable routing conditions.

Emergency power supply

Backup power for PoE is sometimes overlooked, even though it has direct consequences for business continuity.

If cameras and access points are powered by a switch, turning it off means a simultaneous failure of the entire infrastructure, while in an installation with separate power supplies, the failure affected individual devices.

This is not an argument against PoE, but a hint that if monitoring or access control is to operate in the event of a power outage, an emergency power supply must be provided for the switch, calculating the backup time taking into account all connected devices.

Common errors

• switch selection based on the number of ports, without checking the total budget,
• no stock for devices that will arrive,
• assumes that all PoE ports support the same standard. Some models support various
• mixing passive power supply with standard power supply without checking,
• cheap patch cables in power supply paths,
• omission of losses on long sections,
• no emergency power supply for the monitoring power switch.

How we do it

We calculate the power budget at the design stage, compiling a list of devices and reserves for expansion, and not only after purchasing the switch, because in one installation there may be three different standards that require separate checking.

The as-built documentation includes a list of powered devices, their consumption and the use of the switch budget, so when adding another camera it is known whether it fits in the spare, without disassembling the installation.

With constant care, PoE budget utilization is one of the monitored parameters, because a switch approaching the limit can be noticed in the data earlier than the problem manifests itself by restarting the access point.

In corporate networks, we are a Zyxel Networks Partner, also implementing Ubiquiti UniFi technology depending on the needs of the environment.

Scope includes building and maintaining corporate networks and constant IT support, where budget utilization can be monitored.

Book a free IT review and let's figure it out before you buy the switch.