PoE power budget calculator
Power loss, voltage drop and switch budget for PoE devices by cable type and distance.
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Estimate at 20 °C using DC resistance per conductor. Warm cables and tight bundles have higher resistance, so leave a margin. The port voltage defaults to the standard minimum for the chosen type.
Why cable length matters for PoE
Power over Ethernet sends DC power down the same twisted pairs as data. The copper has resistance, so some power turns into heat in the cable and the voltage at the device drops. Longer and thinner cable means more loss, and the loss grows with the square of the current.
| Standard | Port (PSE) max | Device (PD) max | Pairs |
|---|---|---|---|
| 802.3af (PoE) | 15.4 W | 12.95 W | 2 |
| 802.3at (PoE+) | 30 W | 25.5 W | 2 |
| 802.3bt Type 3 | 60 W | 51 W | 4 |
| 802.3bt Type 4 | 90 W | 71.3 W | 4 |
Planning tips
- Check the switch's total PoE budget, not just the per-port figure: a 24-port PoE+ switch rarely powers 24 devices at 30 W.
- Use solid copper cable. Copper-clad aluminium has much higher resistance and can overheat with PoE.
- Keep within 100 m per channel and allow extra margin for warm ceilings and large cable bundles.