PoE Explained: How Power over Ethernet Works and What You Need to Specify

Quick overview: This guide explains poe and the practical checks that help you understand and plan an installation. Use the contents below to go directly to the relevant section.

Power over Ethernet, commonly called PoE, allows compatible network equipment to receive data and electrical power through Ethernet cabling. It is widely used for wireless access points and other network devices and can be particularly useful where installing a separate power connection at every device would be inconvenient.

PoE can simplify network installations, but selecting the right equipment involves more than counting the number of PoE ports on a switch. The PoE standard, power required by each device, total switch power budget, cabling, distance and installation environment all matter.

This guide explains the practical fundamentals and the information purchasing and technical teams should confirm before ordering PoE equipment.

What Is Power over Ethernet?

PoE is a method of delivering electrical power together with Ethernet communications over compatible twisted-pair network cabling.

This allows a compatible device to use one network connection for both data and power.

Typical PoE applications include:

• Wireless access points
• VoIP phones
• Selected industrial wireless devices
• Network intercoms
• Other compatible Ethernet devices

PSE and PD: Two Important PoE Terms

PoE systems commonly distinguish between the equipment supplying power and the equipment receiving it.

Power Sourcing Equipment, or PSE, provides PoE power onto the Ethernet connection. A PoE network switch or PoE injector can be PSE equipment.

Powered Device, or PD, is the compatible device receiving power through the Ethernet connection.

Examples of powered devices include wireless access points and radios designed for PoE operation.

What Is a PoE Switch?

A PoE switch combines Ethernet switching and PoE power delivery in one device.

Instead of connecting a network device to a normal Ethernet switch and then providing separate local power, a compatible powered device can connect directly to a PoE-enabled switch port.

PoE switches are available in managed and unmanaged versions and in commercial, rack-mount and industrial DIN-rail formats.

What Is a PoE Injector?

A PoE injector adds power to an Ethernet connection where the existing network equipment does not provide PoE.

A simple application might use a normal Ethernet switch connected to the data input of an injector, with the injector's PoE output connected to the powered device.

Injectors can be useful when only one or a small number of PoE devices need to be added to an existing non-PoE network.

PoE Switch vs Injector

If many powered devices are required, a PoE switch can provide a cleaner and more manageable installation than using numerous separate injectors.

For one existing device, an injector may avoid replacing an otherwise suitable network switch.

The correct choice depends on device count, power requirements, network architecture, redundancy, management and installation environment.

PoE, PoE+ and Higher-Power PoE

Different generations of IEEE Power over Ethernet support different power capabilities.

Common terminology includes:

• PoE, commonly associated with IEEE 802.3af
• PoE+, commonly associated with IEEE 802.3at
• PoE++ or higher-power PoE, commonly associated with IEEE 802.3bt

IEEE 802.3bt includes multiple power types and uses additional conductor pairs to support higher power levels.

When selecting equipment, it is better to confirm the exact standard and required device power than to rely only on marketing terms such as PoE+ or PoE++.

Why the PoE Standard Matters

A powered device must receive enough power for its intended operation.

A device designed for a higher-power PoE standard may not operate correctly from a lower-power PSE.

Likewise, a switch supporting higher-power PoE on selected ports may still have limitations on how much total power it can supply across all ports simultaneously.

Check both the powered device requirement and the switch specification.

What Is a PoE Power Budget?

The PoE power budget is the total amount of power a switch can make available to connected powered devices, subject to its design and operating conditions.

This is one of the most important PoE specifications and one of the easiest to overlook.

A switch can have many PoE-capable ports without having enough total power to operate the maximum possible load on every port simultaneously.

A 24-Port PoE Switch Does Not Necessarily Power 24 Maximum-Power Devices

Suppose a switch provides 24 PoE ports. That tells you how many ports are capable of PoE, but it does not tell you the total power available to all connected devices.

If every connected device required a relatively high amount of power, their combined requirement could exceed the switch's total PoE budget even though there are enough physical ports.

Before ordering, add the maximum expected power requirements of the powered devices and compare the total with the switch's available PoE budget, while allowing an appropriate design margin.

Per-Port Power vs Total Power

Two different limits need to be considered:

• The maximum power supported by an individual PoE port
• The total PoE power available across the switch

A switch may support a particular PoE standard per port while still being unable to provide that maximum simultaneously on every port because of its total power budget.

Where Does a PoE Switch Get Its Power?

The PoE switch itself needs an adequate electrical supply.

Rack-mount commercial switches may have integrated AC power supplies. Industrial DIN-rail PoE switches commonly use an external DC supply.

When selecting an industrial PoE switch, confirm:

• Permitted input voltage
• Required input current or power
• Whether a power supply is included
• Whether redundant power inputs are supported
• PoE power budget at the intended input voltage

Some industrial PoE equipment can have different PoE capabilities depending on the supply voltage, so the manufacturer's specification must be checked.

Do All Ethernet Cables Support PoE?

PoE requires suitable balanced twisted-pair cabling and correctly installed connectors.

The cable category, conductor size, construction, temperature rating and installation conditions can become increasingly important as power levels and cable bundles increase.

Use cabling that is appropriate for the required Ethernet performance, PoE application and installation environment.

Solid Copper vs Copper-Clad Aluminium

For structured cabling and PoE applications, conductor material matters.

Copper-clad aluminium, often abbreviated CCA, has different electrical characteristics from solid copper conductors and may not meet the requirements expected of compliant category cabling.

For professional structured cabling and PoE infrastructure, specify compliant cable from a reputable manufacturer and verify conductor construction and relevant standards rather than purchasing purely on a category label.

Distance and PoE

PoE operates over Ethernet cabling subject to the applicable structured-cabling and Ethernet distance limitations.

For many conventional twisted-pair Ethernet installations, a 100 metre channel is a familiar maximum, but the exact requirement depends on the network standard and installation.

Power delivery also experiences losses in the cable conductors. Cable construction, conductor resistance, current and temperature therefore matter.

Voltage Drop and Cable Losses

Current flowing through a cable produces voltage drop and power loss because the conductors have electrical resistance.

PoE standards and compliant equipment are designed with these effects in mind, but poor-quality cable, excessive resistance or unsuitable installation can reduce available power and increase heating.

This becomes particularly important in higher-power PoE applications.

PoE and Cable Bundles

When many PoE cables are grouped together, conductor heating can become a design consideration, particularly for higher-power applications.

Cable category, conductor size, bundle size, ambient temperature and installation method can all influence thermal performance.

Structured cabling should be designed and installed according to applicable cabling standards, manufacturer guidance and project requirements.

PoE and Fibre Optics

Standard fibre optic cable carries optical signals and does not deliver PoE electrical power.

This does not mean fibre and PoE cannot be used in the same network.

A common architecture is to use fibre for the long-distance backbone to a remote industrial switch and then use copper PoE ports from that switch to power nearby wireless or other PoE devices.

The remote switch requires its own suitable electrical power source.

Fibre-to-PoE Applications

Consider a wireless access point installed hundreds of metres from the main control room.

Instead of attempting to extend copper Ethernet beyond its normal design limits, fibre can connect the control room to a remote PoE switch. The switch then provides short copper PoE connections to local devices.

This combines fibre's distance and electrical-separation advantages with PoE's ability to power local devices over Ethernet cabling.

What Is a PoE Extender?

PoE extenders and other specialised equipment can be used in certain applications to extend network and power reach beyond a conventional single copper Ethernet segment.

However, extended-distance designs need careful consideration of equipment compatibility, available power, data rate, environmental conditions and maintainability.

For permanent infrastructure, fibre with appropriately located powered network equipment may be a more suitable architecture in many long-distance applications.

Managed vs Unmanaged PoE Switches

PoE capability and network management are separate characteristics.

An unmanaged PoE switch can provide straightforward Ethernet connectivity and power for simple installations.

A managed PoE switch can add features such as:

• VLANs
• SNMP monitoring
• Port statistics
• Port mirroring
• Network redundancy
• Multicast management
• Security features
• Remote PoE port control on supported models

The right choice depends on the network rather than on PoE alone.

Remote PoE Port Control

Some managed PoE switches allow administrators to disable and re-enable power on individual ports.

This can be useful for remotely power-cycling a compatible device such as an access point.

Capabilities vary by switch and should be confirmed if this feature is important to the application.

PoE Watchdog Functions

Some industrial PoE switches offer watchdog or device-checking functions that can monitor connectivity to a powered device and cycle PoE power when configured conditions indicate that the device is unresponsive.

This can be useful at remote installations, but implementation varies between manufacturers.

Do not assume that every managed PoE switch includes this capability.

Industrial PoE Switches

Industrial PoE switches can be designed for installations such as control panels, outdoor cabinets, factories, mines and infrastructure sites.

Depending on the model, industrial features can include:

• DIN-rail mounting
• DC power input
• Redundant power inputs
• Extended operating temperature
• Alarm relay outputs
• Fibre SFP uplinks
• Managed network features
• Industrial redundancy functions
• Robust mechanical construction

The actual installation requirements should determine which features are necessary.

Outdoor PoE Installations

Outdoor PoE installations require more than an outdoor-rated device.

The complete system can require consideration of:

• Outdoor-rated cabling
• UV resistance
• Waterproof cable entry
• Enclosure IP rating
• Temperature
• Condensation
• Surge protection
• Lightning exposure
• Earthing and bonding
• Power quality
• Fibre versus copper backbone design

Equipment must be selected for the actual environmental and electrical conditions.

Selecting a PoE Switch for Powered Devices

When selecting a switch for PoE devices, do not simply count devices and buy a switch with the same number of PoE ports.

Also consider:

• Maximum power requirement of each powered device
• Higher power required by PTZ, heaters or infrared functions where applicable
• Total PoE budget
• Network bandwidth
• Required uplink speed
• Fibre uplinks
• Network management
• Operating environment

PoE and Wireless Access Points

Wireless access points are another common PoE load.

Modern access points can have higher power requirements than older devices, particularly models with multiple radios or advanced features.

Confirm the access point's required PoE standard rather than assuming that any PoE port will be sufficient.

Passive PoE vs Standards-Based PoE

The term passive PoE is used for systems that place a specified voltage onto cable conductors without the same negotiation and classification mechanisms used by standards-based IEEE PoE.

Passive PoE equipment requires particular care because voltage and pinout compatibility must be confirmed.

Connecting incompatible passive PoE equipment can damage devices.

Do not assume that equipment labelled 'PoE' is interchangeable. Confirm whether it uses an IEEE standard or a manufacturer-specific passive power arrangement.

Can a Non-PoE Device Be Connected to a PoE Switch?

With standards-compliant PoE equipment, the PSE detects compatible powered devices before applying normal PoE power according to the relevant standard.

This allows ordinary non-PoE Ethernet devices to be connected to many standards-compliant PoE switch ports without receiving PoE power.

However, this protection should not be assumed for non-standard passive PoE systems.

PoE Splitters

A PoE splitter can receive PoE and separate it into an Ethernet data connection and a separate DC power output for a compatible non-PoE device.

This can be useful in specialised installations, but voltage, current, connector and device requirements must all match.

A splitter should not be treated as a universal adapter for any non-PoE device.

Common PoE Problems

When a PoE device does not power up or operates intermittently, possible causes include:

• Insufficient switch power budget
• Incorrect PoE standard
• Powered device requiring more power than the port can provide
• Unsuitable or damaged cabling
• Excessive cable resistance
• Incorrect industrial switch input voltage
• Power supply too small for the switch and PoE load
• Port configuration on a managed switch
• Connector faults
• Environmental or temperature conditions
• Incompatible passive PoE equipment

Common Selection Mistakes

Common mistakes when buying PoE equipment include:

• Ordering only by number of ports
• Ignoring the total PoE power budget
• Not checking the powered device's required PoE standard
• Assuming all PoE ports provide the same maximum power
• Ignoring the industrial switch input voltage
• Not sizing the external DC power supply for the PoE load
• Using unsuitable network cable
• Confusing passive PoE with standards-based PoE
• Forgetting fibre uplinks
• Not considering operating temperature
• Assuming the power supply is included
• Ignoring network management and redundancy requirements

Replacing an Existing PoE Switch

When replacing a PoE switch, record more than the manufacturer and port count.

Useful information includes:

• Exact existing model number
• Number of active PoE devices
• Device models and maximum power requirements
• Existing PoE standard
• Total required power budget
• Copper port speeds
• Fibre or SFP uplinks
• Input power arrangement
• Managed configuration
• VLANs and redundancy where used
• Mounting and environmental requirements

If the existing switch is managed, preserve or document its configuration before replacement where possible.

A Practical Planning Checklist

When requesting a PoE switch quotation, provide as much of the following as possible:

• Application: ___
• Managed or unmanaged: ___
• Total Ethernet ports: ___
• PoE ports required: ___
• Required PoE standard: ___
• Powered device models: ___
• Maximum power per device: ___
• Required total PoE budget: ___
• Ethernet speed: ___
• Fibre or SFP uplinks: ___
• Input voltage: ___
• Power supply required: yes or no
• Redundant power required: yes or no
• Network redundancy required: ___
• Mounting: DIN rail, rack or other
• Operating environment: ___
• Operating temperature: ___
• Existing model if replacing: ___
• Quantity: ___

If the powered device specifications are unknown, provide their exact manufacturer and model numbers so the relevant power requirements can be checked.

For PoE enquiries, providing the models and quantities of the powered devices is particularly useful because it allows the required PoE standard and total power budget to be considered as part of equipment selection.

Technical Note

This article provides general networking information. PoE power calculations, structured cabling, surge protection, earthing, environmental suitability, network architecture and equipment compatibility must be assessed for the specific installation. Manufacturer documentation, applicable standards and project requirements take precedence, and critical network or electrical design should be undertaken or verified by appropriately competent personnel.

Related catalogue examples: PoE & Power Solutions. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.