One cable. Power and data together. Here is what you actually need to know.
You plugged in a PoE camera and it did not power on. Or you bought a PoE switch and a “PoE” device and they refused to talk to each other. Or someone handed you a spec sheet with “802.3af” on it and you nodded confidently while having no idea what it meant. This guide covers all of it, from first principles, in plain English.
What we cover
- What is PoE?
- Active PoE vs passive PoE – the most important distinction
- PoE standards: af, at, bt, and passive
- Cable requirements, run lengths, and the CCA problem
- PSE and PD: the two roles explained
- PoE budget: what it is and how to calculate it
- Real-world case studies
- Teltonika TSW and SWM switches
- PoE planning calculators
- Common mistakes and how to avoid them
- Pre-purchase checklist
- Frequently asked questions
- Test your knowledge
What is PoE?
A standard Ethernet cable carries data. That is its job. PoE – Power over Ethernet – adds something to that cable: DC electrical power. One cable, two jobs. The device at the far end receives both data and power through the same run of Cat5e or Cat6. No mains socket needed at the device end. No separate power cable. Just the one cable you were already planning to run.
That is the core concept. A camera on a building exterior, a WiFi access point on a ceiling, a 5G router on a pole outside a building – all of them can be powered and connected through a single Ethernet cable.
There are two roles in every PoE installation. The PSE (Power Sourcing Equipment) supplies the power. The PD (Powered Device) receives it. Usually the PSE is a PoE switch or a PoE injector. The PD is whatever device you are deploying. Both terms appear on every PoE datasheet.
Active PoE vs Passive PoE
This is the most important thing in this guide. Read it twice. Getting this wrong causes damaged equipment and wasted money.
Active PoE (IEEE 802.3af / at / bt)
Before power flows, the PSE and PD have a conversation. The switch sends a test signal. The device responds, identifies itself, and tells the switch what it needs. Only then does power flow, at exactly the right voltage. If the device does not respond correctly – because it is not a PoE device, or it does not support that standard – no power flows at all. Nothing gets damaged.
This negotiation process is defined by the IEEE standards. Active PoE runs at nominally 48V. It is safe, standards-based, and what you want in almost every modern installation.
Passive PoE
No conversation. No handshake. Power flows the moment the cable is plugged in. The voltage is fixed by the injector or switch – commonly 24V, sometimes 48V. If the device is rated for that voltage, it works. If it is not, the device may be destroyed immediately, with no warning. There is no protection layer.
Passive PoE is common in budget wireless equipment, some older outdoor enclosure setups, and specific low-cost hardware. It works well when everything is matched correctly. The problem is that “PoE” on a box does not tell you which type. Always check the datasheet.
PoE Standards: af, at, bt, and Passive
The IEEE standards are cumulative. A higher standard supports the lower ones. A PoE++ switch (bt) will negotiate down and power an 802.3af device perfectly well. This makes upgrades straightforward. What you cannot do is mix active and passive systems and expect them to work together.
The port power figure is what the switch delivers. The device power figure is what arrives at the far end of the cable after accounting for cable resistance losses. Plan your installation against the device power figure, not the port figure.
| Standard | Max Port Power | Max Device Power | Voltage | Cable Pairs Used | Negotiation? |
|---|---|---|---|---|---|
| IEEE 802.3af (PoE) | 15.4W | 12.95W | 44-57V | 2 pairs | Yes |
| IEEE 802.3at (PoE+) | 30W | 25.5W | 50-57V | 2 pairs | Yes |
| IEEE 802.3bt Type 3 (PoE++) | 60W | 51W | 50-57V | 4 pairs | Yes |
| IEEE 802.3bt Type 4 | 100W | 71.3W | 52-57V | 4 pairs | Yes |
| Passive 24V | Varies | Varies | 24V fixed | 2 pairs typically | No |
| Passive 48V | Varies | Varies | 48V fixed | 2 pairs typically | No |
Cable Requirements, Run Lengths, and the CCA Problem
PoE works over standard Ethernet cable. The rules are simple but non-negotiable.
- Minimum: Cat5e. Adequate for 802.3af and 802.3at on all run lengths up to 100m.
- Cat6 or Cat6a recommended for 802.3bt (PoE++), runs over 70m, or anything where voltage drop matters.
- Maximum run: 100 metres. The IEEE limit for Ethernet, PoE or not. Beyond 100m, use a PoE extender or an intermediate switch.
- Solid copper core, not stranded, for any in-wall or fixed installation.
- Round cable, not flat. Flat Ethernet cable and telephone cable are not rated for PoE.
PSE and PD: The Two Roles Explained
PSE – Power Sourcing Equipment
The PSE supplies power. It is always either a PoE switch or a PoE injector. A PoE switch has PSE capability built into its PoE ports. An injector sits inline – it takes a standard Ethernet connection from a non-PoE switch, adds 48V from a mains connection, and outputs PoE on the cable run to the device. Use a switch when building a new network. Use an injector when you have an existing non-PoE switch and need to add PoE to one or two ports without replacing the switch.
PD – Powered Device
The PD receives power. Camera, access point, router, sensor, phone, luminaire – anything at the far end of the cable. The PD’s datasheet will state which PoE standard it supports and its power consumption in watts. Those are the two figures you need for budget planning.
Midspan vs Endpoint PSE
An endpoint PSE is a PoE switch where PoE is built into the port hardware. A midspan (injector) sits between an existing switch and the cable run. Single-port injectors are inexpensive and useful for one-off additions. Rack-mount multi-port midspan units handle several ports at once and are common in camera installations where the switch is not PoE-capable.
06PoE Budget: What It Is and How to Calculate It
The PoE budget is the total watts a PoE switch can deliver across all its PoE ports simultaneously. It is a shared pool. Every powered device draws from it. When the pool is empty, no more devices can be powered.
Here is the calculation that trips people up constantly:
A switch has 8 PoE ports and a 120W budget. Eight cameras, each drawing 15.4W. That is 123.2W total. The switch has 120W. The switch cannot power all eight simultaneously. One camera will not power on. This catches people out every time.
The rule is simple: add up the actual draw (not the theoretical maximum) of every device you plan to connect. Then add 20% headroom for thermal derating. If that total exceeds the switch budget, you need a higher-budget switch.
Use the actual draw from the device datasheet, not the IEEE standard maximum. A camera that draws 8W on a PoE+ switch (30W standard) still only uses 8W of your budget. The standard defines the maximum available, not the actual consumption.
Real-World Case Studies
Three real installation types. The first is the one I see most often go wrong – and when it goes right, it is an elegant piece of engineering.
Outdoor 5G Router Powering an Underground Car Park
This is a scenario that comes up repeatedly in commercial and industrial settings. An underground car park, a basement server room, a tunnel entrance – any location with no usable cellular signal. The 5G signal exists outside the building. The equipment needing connectivity is inside it.
The OTD500 is an outdoor 5G Sub-6GHz router built for exactly this kind of deployment. It has an IP67-rated weatherproof enclosure, integrated directional antennas, and – critically for this application – an active IEEE 802.3af/at PoE input. That last detail is the key to the whole installation.
Here is how it works. Mount the OTD500 on the exterior wall or a pole at street level, positioned where there is a strong 5G or 4G signal. Run a single Cat6 cable from the OTD500 down through the building wall or conduit into the underground space. Connect that cable to a PoE switch or PoE injector inside the car park.
That single cable does two jobs simultaneously. The PoE switch sends 48V DC up the cable to power the OTD500. The OTD500 connects to the 5G network via its radio, and sends the data connection back down the same cable to the indoor switch. No mains supply is needed at the OTD500 mounting point. No electrician needs to run a separate power cable up the outside of the building. One cable in conduit, properly weatherproofed at the entry point, solves the whole problem.
The indoor switch then distributes the cellular WAN connection to any devices in the car park – access control systems, payment terminals, CCTV, parking sensors, WiFi access points. All of them get connectivity through the OTD500 above, delivered via PoE.
IP Camera Installation on a Commercial Building
Eight cameras around a commercial building. The cameras are a mix of exterior dome cameras drawing 8W each and interior PTZ cameras drawing 12W each. Five exterior at 8W = 40W. Three interior at 12W = 36W. Total: 76W. Add 20% headroom: 91W required from the switch budget.
A TSW202 with a 120W budget covers this comfortably, with 44W in reserve. Cable runs are all under 80m from the comms room to each camera position. Cat5e throughout. Every camera negotiates correctly with the switch on the 802.3af standard. The installation is clean, the cameras are powered from the comms room, and there is no mains supply required at any camera mounting point.
Warehouse WiFi Coverage Without Ceiling Power Sockets
A distribution warehouse needs full WiFi coverage for barcode scanners, forklifts, and inventory management tablets. The ceiling is 9 metres high. There are no power sockets at ceiling level, and fitting them would require a substantial electrical installation involving cherry pickers and certified electricians.
Six access points on PoE+ (802.3at). Each draws 18W. Total: 108W. With 20% headroom, 130W of budget is needed. A single PoE switch with a 150W+ budget, located in the ground-floor comms room, powers all six APs via Cat6 cable runs up the steel columns to the ceiling. Cable ties and conduit handle the routing. The APs negotiate at 802.3at. No electrical works above ground floor required.
Teltonika TSW and SWM Managed Switches
Teltonika produces a managed switch range under the TSW and SWM families. They are common in industrial IoT, remote monitoring, and cellular connectivity setups, primarily because they integrate well with Teltonika routers and the RMS (Remote Management System) platform. RMS gives you remote visibility and control of every device on the network.
| Model | PoE Ports | PoE Standard | PoE Budget | SFP Uplinks | Management |
|---|---|---|---|---|---|
| TSW212 | 8x GbE | 802.3af/at (PoE+) | 240W | 2x SFP | Web / CLI / SNMP / RMS |
| TSW202 | 8x GbE | 802.3af/at (PoE+) | 120W | 2x SFP | Web / CLI / SNMP / RMS |
| TSW142 | 4x GbE | 802.3af/at (PoE+) | 120W | 2x SFP | Web / CLI / SNMP / RMS |
| TSW114 | 4x GbE | 802.3af/at (PoE+) | 60W | 1x SFP | Web / CLI / RMS |
| SWM12-8P | 8x FE | 802.3af (PoE) | 65W | 2x GbE combo | Web / CLI / RMS |
Verify these figures against current Teltonika datasheets before specifying. Models are updated periodically.
09PoE Planning Calculators
Teltonika TSW / SWM PoE Budget Calculator
Select your switch. Enter the number and type of PoE devices you plan to connect. The calculator checks whether the switch budget covers the load, with 20% headroom flagged as a warning threshold.
General PoE Power Planning Calculator
Add every device you plan to connect. Enter your switch PoE budget. The calculator checks total draw, remaining headroom, and flags if you are below the recommended 20% reserve.
Common PoE Mistakes
These appear on real installations. Regularly. Most of them are discovered after the cable is already in the wall.
Wrong PoE type
Buying an active PoE switch for passive PoE devices. Or connecting a passive 24V device to a passive 48V injector. Check the datasheet of every device before ordering the PSE. This cannot be fixed by configuration – only by purchasing the correct equipment.
Ignoring the budget
Counting ports, not watts. A 16-port switch with a 150W budget cannot run 16 devices at 10W each (160W). Always add up actual device draw against the switch budget, plus 20% headroom.
CCA cable
It looks like copper. It costs less than copper. It is not copper. Aluminium core cable causes voltage drop that kills PoE performance on runs beyond 30-40m. Specify solid copper, demand certification.
Runs over 100m
100 metres is the IEEE limit for Ethernet. PoE does not extend this. Use a PoE extender, a fibre run with a media converter, or an intermediate switch if distances exceed 100m.
Daisy-chaining switches expecting power pass-through
PoE power does not pass from one switch through to another switch’s ports. Each PSE powers only the PD directly connected to it. Data daisy-chains. Power does not.
Assuming all ports deliver simultaneously
Budget switches may not deliver full power on all PoE ports at the same time. Check the datasheet for the total budget, not just the per-port figure. Eight ports at 30W each requires a 240W budget.
Wrong cable type entirely
Flat patch cables, telephone cables, and alarm cables are not suitable for PoE. Use round, twisted-pair, solid-core Ethernet cable only.
Before You Buy: The PoE Checklist
Two minutes spent on this list prevents hours of troubleshooting on site.
- What PoE type does your device need? Active (IEEE standard) or passive (fixed voltage)? The device datasheet must answer this. If it does not, contact the manufacturer before buying.
- If active: which standard? 802.3af (up to 12.95W), 802.3at (up to 25.5W), 802.3bt Type 3 (up to 51W), or 802.3bt Type 4 (up to 71.3W)?
- If passive: what voltage exactly? 24V or 48V? Does the injector you are buying match that exactly?
- How many devices in total? What is the actual draw of each device? (From the datasheet, not the standard maximum.)
- Does the switch PoE budget cover the total device draw, plus 20% headroom? Calculate it before ordering the switch.
- What cable is installed? Is it solid copper Cat5e or better? If you are not certain it is solid copper, assume it is CCA until proven otherwise.
- What is the longest cable run from switch to device? Has it been measured, not estimated? Is it under 100 metres?
Frequently Asked Questions
Test Your Knowledge
Ten questions drawn from a bank of twenty. You will get a different set each time. Pass mark is 7 out of 10.
PoE Knowledge Check
Ten questions. Different each time. Pass mark: 7 / 10.
Questions are drawn at random from a bank of 20. You will get a different set each attempt.