Ethernet PoE Power Calculator
Estimate IEEE Power over Ethernet power budget, powered-device available power, cable voltage drop, cable power loss, current, efficiency, and multi-port switch power requirements.
COM-008 is a first-pass PoE power calculator. It does not model Ethernet PHY behavior, cable certification, detection signatures, maintain power signatures, 802.3bt LLDP negotiation, controller registers, surge protection, or compliance testing.
Engineering tool
Ethernet PoE Power Calculator
Calculate IEEE PoE power budget, PD available power, cable power loss, current, multi-port switch budget, and PoE standard comparisons.
Powered-device or load power used for the selected estimate.
PoE power summary
25.5 W
Selected budget is near limit; verify startup, temperature and cable loss.
Result console
- PoE standard
- IEEE 802.3at
- PSE power
- 30W
- IEEE PD reference power
- 25.5W
- PD available power
- 25.5W
- Load power
- 25W
- Power margin
- 500mW
- Power margin
- 1.96%
- Budget status
- Near Limit
- Recommended standard
- IEEE 802.3at
Power margin is below 10%; verify worst-case load, cable temperature, startup demand and PD class behavior.
PSE voltage is below the common IEEE PoE operating input range used by many references.
Formula reference
PoE Power Formulas
The calculator uses basic power, current, cable loss, efficiency, and switch budget equations.
Power: P = V × ICurrent: I = P / VPD Available Power: PPD = PSE Power × EfficiencyCable Voltage Drop: Vdrop = I × RcableCable Power Loss: Pcable = I² × RcablePower Margin: Pmargin = Pavailable - PloadMulti-Port Budget: Ptotal = Pport × NportsRecommended Switch Budget: Precommended = Ptotal × (1 + Margin)Variable definitions
- PSE
- Power Sourcing Equipment, such as a PoE switch or injector
- PD
- Powered Device, such as a camera, access point, sensor, or phone
- V
- PSE voltage used for current and cable-loss reference
- I
- current delivered through the powered cable path
- Rcable
- effective powered loop resistance for the selected cable length
- Pavailable
- estimated or reference power available to the PD
- Pload
- expected powered-device load power
- Nports
- number of powered ports in a multi-port budget
IEEE PoE Reference
| Standard | Type | PSE Power | PD Reference Power | Notes |
|---|---|---|---|---|
| IEEE 802.3af | PoE Type 1 | 15.4 W | 12.95 W | Common legacy PoE reference for lower-power devices. |
| IEEE 802.3at | PoE+ Type 2 | 30 W | 25.5 W | Common PoE+ reference for cameras, radios and higher-power endpoints. |
| IEEE 802.3bt Type 3 | 4PPoE Type 3 | 60 W | 51 W | Four-pair PoE reference for higher-power powered devices. |
| IEEE 802.3bt Type 4 | 4PPoE Type 4 | 90 W | 71.3 W | High-power four-pair PoE reference. PD available power is commonly referenced around 71 W to 73 W. |
Worked Examples
802.3af reference
IEEE 802.3af is commonly referenced at 15.4 W from the PSE and about 12.95 W available at the PD.
802.3at reference
IEEE 802.3at is commonly referenced at 30 W from the PSE and about 25.5 W available at the PD.
802.3bt Type 3 reference
IEEE 802.3bt Type 3 is commonly referenced at 60 W from the PSE and about 51 W available at the PD.
802.3bt Type 4 reference
IEEE 802.3bt Type 4 is commonly referenced at 90 W from the PSE and roughly 71 W to 73 W available at the PD.
48 V and 25 W
For a 25 W load at 48 V, current is I = 25 / 48 ≈ 0.5208 A.
54 V and 90 W
For a 90 W load at 54 V, current is I = 90 / 54 ≈ 1.6667 A.
24-port switch
A 24-port switch at 15 W per port needs 360 W before adding design margin.
48-port switch
A 48-port switch at 15 W per port needs 720 W before adding design margin.
Efficiency 90%
A 90 W PSE budget at 90% efficiency gives an early PD available power estimate of 81 W.
Compare all standards
The comparison mode lists 802.3af, 802.3at, 802.3bt Type 3, and 802.3bt Type 4 with PSE power, PD reference power, and current at 48 V.
Engineering Notes
- PoE carries Ethernet data and DC power through the same cable assembly.
- PSE stands for Power Sourcing Equipment, such as a PoE switch or injector.
- PD stands for Powered Device, such as an IP camera, access point, sensor, or VoIP phone.
- Actual PD available power is lower than PSE output power because cable and conversion losses consume part of the budget.
- Longer cable increases conductor resistance, voltage drop, and power loss.
- A PoE switch has a total power budget; not every port can always deliver maximum class power simultaneously.
- PoE class references are useful for budgeting, but final behavior depends on the PSE, PD, cabling, negotiation and thermal limits.
- Cable loss should be reviewed with actual cable category, conductor gauge, pair count, bundle temperature, and installation conditions.
Common Mistakes
- Using PSE power as if it were guaranteed PD load power.
- Ignoring cable power loss and voltage drop.
- Ignoring DC/DC converter efficiency inside the powered device.
- Assuming every switch port can supply maximum power at the same time.
- Forgetting the total PoE switch budget.
- Ignoring startup and heater or motor peak loads.
- Mixing 802.3af, 802.3at and 802.3bt power classes without checking device support.
- Using a power budget without reserving margin.
- Assuming 100 m cable loss applies to every installation length.
- Treating this calculator as a PoE compliance or negotiation simulator.
Support reference
FAQ
What is PoE?
PoE, or Power over Ethernet, delivers DC power and Ethernet data over the same twisted-pair cable so a powered device can operate without a local power adapter.
What is PSE?
PSE means Power Sourcing Equipment. A PoE switch, injector, or midspan that supplies power onto the Ethernet cable is a PSE.
What is PD?
PD means Powered Device. Examples include IP cameras, wireless access points, VoIP phones, sensors, and embedded Ethernet devices that receive PoE power.
How much power does PoE provide?
Common IEEE references are 15.4 W PSE for 802.3af, 30 W for 802.3at, 60 W for 802.3bt Type 3, and 90 W for 802.3bt Type 4. Available PD power is lower.
What is 802.3bt?
IEEE 802.3bt is the higher-power PoE standard family. Type 3 is commonly referenced at 60 W PSE power, while Type 4 is commonly referenced at 90 W PSE power.
How do I calculate PoE current?
Use I = P / V. For example, a 25 W load at 48 V draws about 0.521 A before cable loss and converter efficiency are included.
What is cable power loss?
Cable power loss is the heat dissipated in the Ethernet cable conductors. A first-pass estimate is Pcable = I squared times the effective powered loop resistance.
How many PoE devices can one switch power?
Divide the switch PoE budget by the expected power per port, then reserve margin for startup, temperature, cable loss, and load variation.
What is PoE efficiency?
PoE efficiency describes how much source power remains available to the powered device load after conversion and delivery losses. It depends on cable, converter and load conditions.
Should I reserve power margin?
Yes. A PoE power budget should reserve margin for startup, temperature, cable resistance, PD class behavior, converter efficiency and future load changes.
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Future Engineering Guide Topics
What is PoE
In_DevelopmentIEEE 802.3af vs 802.3at vs 802.3bt
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In_DevelopmentPoE Cable Loss
In_DevelopmentPoE Switch Selection
In_DevelopmentEngineering Disclaimer
This calculator provides first-pass PoE power budget estimates. It does not guarantee IEEE compliance, cable certification, PD classification, LLDP negotiation, thermal safety, voltage tolerance, surge protection, isolation design, or actual switch behavior. Validate the final system with the selected PSE, PD, cable, length, temperature, installation and load profile.