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PoE power loss calculator
The switch says 30 W, the camera gets 24. It is not a fault — it is the copper. Enter the class, the device load and the run, and see the voltage drop, the watts left in the cable and the watts that reach the device.
Buying cable for a 4PPoE rollout? Type 3 and 4 run 600–960 mA through all four pairs. Tell us the run lengths and the device count — we quote the construction against the load, not just the category.
What 100 m costs at each type
23 AWG, four pairs where the type uses four, device at its standard maximum load, PSE at its minimum voltage. This is the worst case the standard is written around.
| Type | Pairs | Device | Volts at device | Lost in cable | Drop | Verdict |
|---|---|---|---|---|---|---|
| 802.3af — Type 1 | 2 | 12.95 W | 41.9 V | 0.6 W | 2.1 V | Inside the standard |
| 802.3at — Type 2 | 2 | 25.5 W | 46.3 V | 2.0 W | 3.7 V | Inside the standard |
| 802.3bt — Type 3 | 4 | 51 W | 46.3 V | 4.0 W | 3.7 V | Inside the standard |
| 802.3bt — Type 4 | 4 | 71.3 W | 46.9 V | 7.7 W | 5.1 V | Inside the standard |
Resistance per conductor at 20 °C: 26 AWG 0.137 Ω/m · 24 AWG 0.0842 · 23 AWG 0.0668 · 22 AWG 0.0529.
Where the power goes
A PoE link is a DC circuit: the switch pushes current down the pair, the device draws it, and the copper between them turns part of the power into heat. Two conductors per polarity sit in parallel, so the loop resistance is one conductor's resistance times the length — halve it again when all four pairs carry current.
The device behaves like a constant-power load, which is why the loss is worse than a simple V²/R estimate: as the voltage sags, the current rises to deliver the same watts, and the loss rises with the square of that current. That is also why 802.3bt moved to four pairs instead of pushing more current down two.
- 100 m is a cabling limit, not a PoE limit. The standard's power budget assumes 100 m; a shorter run simply leaves more volts at the device.
- Gauge beats category. 23 AWG has ~21% less resistance than 24 AWG, so the same run loses a fifth less power.
- Heat is cumulative. Bundled 4PPoE cables running near their current limit warm each other — derate in dense trays.
- Watch the class, not just the watts. A 30 W port feeding a 25.5 W device is at the edge; the 4.5 W difference is the cable budget.