Power over Ethernet (PoE) Explained

What is Power over Ethernet?

Imagine running a single cable to a security camera and having it carry both the network connection and the power to run the device. No power outlet needed at the camera location. No separate power cable. That is what Power over Ethernet (PoE) does, and it has fundamentally changed how IP cameras, wireless access points, VoIP phones, and access control systems are installed.

PoE transmits DC electrical power alongside data over standard Cat5e, Cat6, or Cat6a structured cabling. The technology is defined by the IEEE 802.3 standard and has evolved through four generations, each delivering more power to devices with higher requirements. Understanding which standard your equipment uses, what it means for cable selection, and where the real-world limits are is essential for reliable installations.

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How PoE Works

A PoE installation has two main components: the Power Sourcing Equipment (PSE) and the Powered Device (PD). The PSE is the device that supplies power, typically a PoE-capable switch or a standalone PoE injector. The PD is the device that receives power: a camera, access point, VoIP phone, or reader.

When a cable is connected between a PSE port and a device, the PSE does not immediately apply power. It first runs a detection sequence, applying a small test voltage to measure the resistance of the connected device. If the resistance falls within the IEEE 802.3 standard range (23 to 26.5 kilohms for a compliant PD), the PSE confirms a valid PoE device is connected. It then runs a classification sequence to determine how much power the device requires, and finally applies the full supply voltage.

This detection and classification process protects non-PoE devices. If you plug a standard laptop or a printer into a PoE switch port, the PSE measures the wrong resistance value, classifies it as a non-PoE device, and does not apply power. The data connection works normally.

How PoE works: detection, classification and power delivery from PSE to PD Flow diagram showing PoE switch on left connected by cable to IP camera on right, with detection, classification and power-on steps labelled PSE PoE switch 1. Detect 23-26.5kΩ 2. Classify Power class 3. Power on 48V DC PD Powered device Single Cat5e / Cat6 / Cat6a cable carries data + power

The PSE detects, classifies, then delivers 48V DC power alongside data over a single cable

The supply voltage used by IEEE-standard PoE is nominally 48V DC (typically 44V to 57V depending on load). This relatively high voltage minimises the current required to deliver a given power level, which in turn minimises resistive heating in the cable. The voltage is stepped down to whatever the device requires (typically 5V, 12V, or 24V) inside the PD itself.

PoE Standards: Type 1 to Type 4

The IEEE 802.3 PoE standards have evolved through four generations. Each generation is backward compatible with the previous ones: a Type 4 PoE switch can power Type 1, 2, 3, and 4 devices. A Type 1 switch can only power Type 1 devices.

PoE Standards Comparison: Type 1 to Type 4 Table comparing IEEE 802.3af, 802.3at, 802.3bt Type 3 and Type 4 with power levels, cable pairs, voltage and typical devices PoE (Type 1) IEEE 802.3af 2003 PoE+ (Type 2) IEEE 802.3at 2009 PoE++ (Type 3) IEEE 802.3bt 2018 PoE++ (Type 4) IEEE 802.3bt 2018 PSE PD Voltage Pairs 15.4W 30W 60W 90W 12.95W at device 25.5W at device 51W at device 71.3W at device 44-57V DC 50-57V DC 50-57V DC 52-57V DC 2 pairs 2 pairs 4 pairs 4 pairs Typical devices VoIP phone IP camera Access point 4K camera PTZ camera Laptop Display / TV Video conf. Network switch Video phone Wi-Fi 6 AP High-power AP

PoE standards comparison: power levels, cable pairs and typical devices

Note on power figures: The PSE power figure is what the switch delivers at its port. The PD power figure is what the device receives at the end of the cable. The difference is the power lost as heat in the cable due to resistance.

Note on naming: The PoE industry has inconsistent naming conventions. "PoE++" is used by different manufacturers to mean Type 3 or Type 4. Always check the IEEE standard reference (802.3af, 802.3at, or 802.3bt Type 3/4) rather than relying on marketing names.

Two-Pair vs Four-Pair PoE

A Cat5e or Cat6 cable contains four twisted pairs (eight conductors). IEEE 802.3af (Type 1) and 802.3at (Type 2) deliver power over two pairs only. IEEE 802.3bt (Type 3 and 4) uses all four pairs to deliver higher power levels without exceeding safe current limits per conductor.

Two-pair PoE versus four-pair PoE cable pair usage Left side shows Type 1 and 2 using two powered pairs and two data-only pairs. Right side shows Type 3 and 4 using all four pairs for power and data. Type 1 and 2 (802.3af / 802.3at) Type 3 and 4 (802.3bt) Pair 1 — Power Pair 2 — Power Pair 3 — Data only Pair 4 — Data only Pair 1 — Power + Data Pair 2 — Power + Data Pair 3 — Power + Data Pair 4 — Power + Data Carries power Data only

Type 1 and 2 power over two pairs. Type 3 and 4 use all four pairs to safely deliver 60W and 90W.

Using all four pairs for Type 3 and 4 allows more total power to be delivered while keeping the current per conductor within safe limits. The maximum current per conductor under IEEE 802.3bt is 600mA. This has a practical implication for cable selection: Type 3 and 4 PoE require cable with a 60V operating voltage rating. Access Communications Cat6 cable is rated at 60V and is suitable for all PoE types.

Voltage Drop and Cable Distance

The most common source of PoE installation failures is voltage drop. Every metre of cable has electrical resistance, and when current flows through that resistance, voltage is lost. The longer the cable run and the higher the current draw, the greater the voltage drop. If the voltage at the device falls below its minimum operating voltage, the device will reset, fail to boot, or operate intermittently.

The IEEE 802.3 standard sets the maximum cable length at 100 metres. But this limit is based on data transmission performance, not power delivery. For high-power devices, voltage drop becomes the limiting factor well before 100 metres.

PoE voltage drop over distance on Cat6 cable Three horizontal bars showing safe, marginal and high-risk operating distances for Type 1 15W, Type 2 30W and Type 3 60W PoE devices on Cat6 cable Safe operating distance on Cat6 cable Type 1 (15W) Safe to 100m 100m Type 2 (30W) Safe to ~75m Check drop 100m Type 3 (60W) Safe to ~50m Marginal Risk of failure 100m

Higher-power devices require shorter cable runs to maintain adequate voltage at the device.

Cable Run Voltage Drop Voltage at Device Result (60W Type 3)
30m ~4V ~44V Works reliably
60m ~8V ~40V Works reliably
80m ~11V ~37V Marginal, may fail under peak load
100m ~14V ~34V Likely to fail or reboot under load

CCA (Copper Clad Aluminium) cable makes voltage drop significantly worse. Aluminium has approximately 60% higher resistance than pure copper, meaning a 50m CCA run behaves like an 80m pure copper run. This is one of the primary reasons CCA cable is illegal for customer cabling in Australia under AS/CA S008. For a detailed breakdown of voltage drop across all cable gauges, see our Low Voltage Power Drop Charts.

Solutions for Long Cable Runs

  • Use Cat6a instead of Cat6. Lower resistance conductors extend reliable operating distance for high-power devices.
  • Install a PoE extender midway. Placed at around 80-90m, an extender regenerates the PoE signal and extends the run by another 100m.
  • Relocate the switch or injector. Moving the PSE closer to the device cluster reduces run lengths for all devices.
  • Use a separate power supply at the device. For devices over 90m from the switch, run Cat6 for data only and provide power locally.

Choosing the Right Cable for PoE

Application Cable Type Notes
Type 1 or 2 PoE, runs under 60m, indoor Cat6 UTP, 30V rated Standard specification for most commercial installations
Type 1 or 2 PoE, runs 60-90m, indoor Cat6a UTP Lower resistance conductors extend reliable range
Type 3 or 4 PoE (60-90W), any run length Cat6a, 60V rated 60V operating voltage essential for Type 3 and 4 compliance
Outdoor runs, any PoE type Cat6 or Cat6a with PE jacket UV-resistant jacket required. Standard PVC degrades outdoors within 6-12 months.
High EMI environments Cat6a F/UTP or S/FTP Shielded cable prevents interference from nearby high-voltage equipment
Plenum spaces, air ducts Cat6 or Cat6a LSZH Low smoke zero halogen jacket required in air-handling spaces

Always specify pure copper cable. CCA cable is illegal for customer cabling in Australia (AS/CA S008), creates a fire risk under high PoE current loads, and causes excessive voltage drop. All Access Communications Datamaster cable is 100% pure copper, Fluke tested, and fully compliant with Australian standards.

Common Deployment Scenarios

Three common PoE deployment scenarios Three diagrams showing a single injector, a PoE switch powering multiple devices, and a PoE extender for long runs 1. Single injector Switch PoE injector IP camera Best for adding PoE to a single device on an existing non-PoE network. 2. PoE switch PoE switch Camera Access point VoIP phone Powers multiple devices. Confirm total switch power budget covers all ports at expected load. 3. PoE extender
PoE extender deployment extending total reach to 180 metres PoE switch connected via 90m cable to extender, then 90m to remote IP camera PoE switch ~90m PoE Extender Regenerates signal ~90m Remote camera Total reach ~180m. Extender draws power from the first switch.

Three deployment configurations to match your installation requirements

Testing PoE Installations

A standard network cable tester confirms correct wiring but tells you nothing about PoE power delivery. If a PoE device is rebooting, failing to start, or performing intermittently, the cause is almost always insufficient voltage at the device, not a wiring fault. You need a PoE-specific tester to diagnose power delivery issues.

A professional PoE tester measures the voltage delivered at the device end, the current being drawn, the calculated power, and the PoE class negotiated between the PSE and PD. This tells you immediately whether the switch is delivering the expected power level and whether the device is receiving sufficient voltage.

Professional PoE Network Cable Auditor (colour LCD) — Verifies PoE power delivery, measures voltage and current at the device end, displays PoE class, and performs standard cable wire map testing. Suitable for daily installation verification and fault-finding on all PoE types including 802.3bt.

Goldtool TCT-207 PoE Ethernet Tester Kit with TDR — Adds Time Domain Reflectometry (TDR) for locating cable faults by distance, in addition to PoE measurement and wire map functions. Suited to installations where fault location by cable length is needed.

View our professional PoE testers

Frequently Asked Questions

Can I use Cat5e cable for PoE?

Yes, Cat5e supports all PoE types including 802.3bt Type 3 and 4, provided it is pure copper cable with a 60V operating voltage rating for Type 3 and 4. For new installations, Cat6 is the recommended minimum.

Will PoE damage my non-PoE devices?

No. The PSE runs a detection sequence before applying power. If it does not detect a compliant PoE signature from the connected device, it does not apply power. Non-PoE devices connect to PoE switch ports safely.

Why does my PoE camera keep rebooting?

The most common cause is insufficient voltage at the device due to voltage drop over the cable run. Check the cable length against the power requirements of the camera using the tables above. CCA cable significantly worsens voltage drop. Test with a PoE tester to measure the actual voltage at the device under load.

Does PoE affect network performance?

No. PoE uses the same pairs as the data signal at DC frequencies that do not interfere with the high-frequency data signals. A well-installed PoE system has no measurable impact on network performance.

How much does PoE heating affect cable performance?

PoE current causes resistive heating in the cable conductors. Elevated temperature increases conductor resistance, which increases voltage drop. For high-power PoE installations, avoid tight cable bundles and derate run length estimates by 10-15% for installations in hot environments such as roof spaces.

What is the difference between PoE and PoE+?

PoE refers to IEEE 802.3af (Type 1), which delivers up to 15.4W per port. PoE+ refers to IEEE 802.3at (Type 2), which delivers up to 30W per port. Both use two pairs of the cable. PoE++ refers to IEEE 802.3bt Type 3 (60W) and Type 4 (90W), which use all four pairs.

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