Detection
The port applies a low test voltage and looks for the 25 kΩ signature resistance every PD presents. No signature, no power, so a printer or a laptop on that port sees data and nothing else.
← All knowledge guidesNetworking
Power over Ethernet: one cable for data and power
A camera, an access point or a door station needs a network connection and a supply. PoE carries both over the same Cat5e or Cat6 cable, so the device position needs no socket. This guide covers the standards, the pairs, the cable and the power budget.
PoE feeds a device through the same eight conductors that carry its network traffic.
- Data
- Power
- Data and power
- Pair not used
Power over Ethernet is a family of IEEE standards that put a direct voltage of roughly 48 V on the twisted pairs of an ordinary network cable. The supplying end is the PSE, power sourcing equipment: a PoE switch, or an injector inserted into the link. The fed end is the PD, powered device: a camera, an access point, an IP intercom, a touch panel or a KNX IP interface.
Nothing about the cable changes. The same Cat5e or Cat6 that already runs to the device carries the supply, so that position needs no socket, no adapter and no second low voltage cable, and every device on the switch can ride through a power cut behind one UPS in the rack.
How the supply is switched on
A PoE port carries no voltage until it has proof that a PoE device is on the other end, which is what makes it safe to plug a laptop into it.
Classification
The device declares how much it will draw, as a class from 0 to 8. The switch reserves that much of its budget instead of guessing, and refuses the port when the budget is already spent.
Power up and monitoring
The port raises the full voltage and keeps watching. Unplug the device and the port falls back to the test voltage within a few hundred milliseconds. Type 2 and above can also renegotiate the power over LLDP once the link is up.
The three generations of PoE
Every standard is backward compatible: a Type 1 device works on a Type 4 port and draws only what it needs. The power is quoted twice because the cable itself consumes part of it.
PoE, Type 1
The original standard, still the right size for small devices.
- At the port15.4 W
- At the device12.95 W
- Voltage44 to 57 V
- Pairs under load2 pairs
- Typical devicesIP phones, sensors, simple cameras
PoE+, Type 2
The volume standard today. Most cameras and access points assume it.
- At the port30 W
- At the device25.5 W
- Voltage50 to 57 V
- Pairs under load2 pairs
- Typical devicesPTZ cameras, access points, door stations
PoE++, Type 3
Four pairs under load, which is what lifts the ceiling past 30 W.
- At the port60 W
- At the device51 W
- Voltage50 to 57 V
- Pairs under load4 pairs
- Typical devicesVideo door stations, touch panels, small displays
PoE++, Type 4
The top of the range, mainly for screens and PoE lighting.
- At the port90 W
- At the device71.3 W
- Voltage52 to 57 V
- Pairs under load4 pairs
- Typical devicesDisplays, thin clients, PoE luminaires
Power classes
| Class | Reserved at the port | Available at the device |
|---|---|---|
| 0, unclassified | 15.4 W | 12.95 W |
| 1 | 4.0 W | 3.84 W |
| 2 | 7.0 W | 6.49 W |
| 3 | 15.4 W | 12.95 W |
| 4 | 30 W | 25.5 W |
| 5 | 45 W | 40 W |
| 6 | 60 W | 51 W |
| 7 | 75 W | 62 W |
| 8 | 90 W | 71.3 W |
A device that does not classify is treated as class 0, so the port reserves the full 15.4 W for it. On a switch with a tight budget that is the difference between eight working ports and four.
Which pairs carry the power
A network cable holds four twisted pairs. Which of them carry the supply depends on the standard, and every device has to accept either arrangement.
Alternative A
The supply rides on the data pairs 1/2 and 3/6, superimposed on the signal. On a 10/100 link the other two pairs stay idle.
Used by most PoE switches.
Alternative B
The supply uses pairs 4/5 and 7/8, which on a 10/100 link carry nothing else, so power and data stay physically apart.
Typical of midspan injectors.
Four pairs, 802.3bt
All four pairs carry the supply at once. The current per conductor halves and the losses fall, and that is what makes 60 W and 90 W possible.
Required for Type 3 and Type 4.
- Data
- Power
- Data and power
- Pair not used
On a gigabit link all four pairs carry data in any case and the supply is simply superimposed on them. The drawings show the 10/100 case, where the difference between the two alternatives is visible.
Cat5e, Cat6 or Cat6a
PoE is a direct current on the same copper, so what decides the result is the cross section and the heat, not the bandwidth alone.
Cat5e
Enough for Type 1 and Type 2, provided the conductor is solid copper.
- Bandwidth100 MHz
- ConductorAWG 24 solid
- Resistance≈ 9.4 Ω per 100 m
- Suitable forType 1, Type 2
- Max channel100 m
Cat6
The thicker AWG 23 conductor runs cooler and drops less voltage, which is the reason to prefer it over Cat5e for PoE.
- Bandwidth250 MHz
- ConductorAWG 23 solid
- Resistance≈ 7.4 Ω per 100 m
- Suitable forType 1 to Type 3
- Max channel100 m
Cat6a
Better behaved in a full bundle, and the safe choice where Type 4 runs near the length limit.
- Bandwidth500 MHz
- ConductorAWG 23 solid
- Resistance≈ 7.4 Ω per 100 m
- Suitable forType 1 to Type 4
- Max channel100 m
What breaks a PoE link
- Copper clad aluminium (CCA) cable. Its resistance is up to half again as high, it heats under load and the device browns out. PoE needs solid copper.
- More than 100 m from port to device, patch cords included. The standard allows 90 m of fixed cable plus 10 m of patching, and there is no margin above that.
- Passive 12 V or 24 V injectors on an 802.3af or at device. They perform no detection, so the voltage is present the moment the plug goes in.
- Large bundles at full load with no derating. A hundred cables in one duct heat each other, and copper resistance rises with temperature.
- Network cable pulled in the same duct as 230 V without a divider, which is an installation rule rather than a PoE one.
What keeps it stable
- Solid copper Cat6 as the default for new work, Cat6a where Type 3 or Type 4 runs near the length limit.
- Twenty to thirty percent of the switch budget left free, so one added camera does not shut a port down.
- The finished channel certified, resistance unbalance included, not only the wire map.
- The PoE switch behind the UPS, so every device on it rides through a power cut.
- Surge protection on any run that leaves the building, an outdoor camera above all.
Switch, injector or splitter
Two ways to put power onto the link, and one way to take it back off for a device that has no PoE input.
PoE switch, endspan
The supply is built into the port. One device in the rack, one budget to manage, and per port control from the switch interface.
Use whenAnything above two powered devices.
Injector, midspan
Sits between an ordinary switch and the device and adds the supply from a mains socket. Cheap, and the quick answer when one camera has to join an existing switch.
Use whenOne or two devices, or a switch that cannot be replaced.
Splitter
The inverse of an injector: it takes the PoE link and hands the device an ordinary network port plus a 12 V or 5 V output.
Use whenA device with no PoE input sits where there is no socket.
Sizing the power budget
A switch has a total budget as well as a per port limit, and it is the total that runs out first. A worked example for a house with cameras and access points:
| Device | Qty | Per device | Total |
|---|---|---|---|
| Outdoor cameras with heater, Type 2 | 6 | 25 W | 150 W |
| Indoor cameras, Type 1 | 8 | 8 W | 64 W |
| Wi-Fi access points, Type 2 | 4 | 20 W | 80 W |
| Video door station, Type 3 | 1 | 45 W | 45 W |
| KNX IP interface and IP intercom, Type 1 | 2 | 6 W | 12 W |
| Connected load | 351 W | ||
Then add the headroom
351 W of load wants a switch of about 450 to 500 W, not 370 W. The reserve covers the losses in the cable, the camera heaters that only draw in winter, and the device someone adds two years later. Check the per port limit as well: a 500 W switch that gives 30 W per port still cannot feed a 60 W panel.
Planning the network for a smart home?
We size the cabling, the switch and the power budget together with the KNX and lighting side, so the rack is planned once and not three times.
Tools for your project
Budget estimatorApproximate KNX system cost for residential and commercial projects.Experience KNXA simulated home where lighting, climate, shading and scenes work together.Wiring diagramAn interactive diagram: which cable carries power, which carries commands.Knowledge baseEngineering notes on KNX, ETS, cable installation, and commissioning.