Total budget is not enough
A switch can have spare global wattage and still fail a single long run because the port cannot deliver enough launch power.
Per-port limit
PoE Budget + Voltage Drop Calculator
Estimate how much power a PoE switch or injector really has to source after cable losses, and check whether long links still leave enough voltage at the powered device. The budget planner handles multiple device groups; the link estimator gives a single-run view for one cable.
Calculator
Switch + device inputs
Default conductor presets are typical solid-copper DC resistances at about 20 C: 24 AWG ≈ 84.2 ohm/km, 23 AWG ≈ 66.8 ohm/km, 22 AWG ≈ 53.0 ohm/km.
Device group 1
Included by defaultDevice group 2
Set quantity to 0 to ignoreDevice group 3
Optional expansionBudget results
Required source power-
Usable budget after reserve-
Spare or shortfall-
Cable loss total-
Enter one or more device groups to estimate total source power and spare budget.
| Group | Qty | Per-port source W | PD voltage | Voltage drop | Loss per port | Total source W | Status |
|---|---|---|---|---|---|---|---|
| No calculation yet. | |||||||
Single-link inputs
Single-link results
Source watts-
PD voltage-
Voltage drop-
Cable loss-
Current-
Effective loop resistance-
Theoretical max PD power-
Port limit check-
The single-link estimator solves a constant-power cable-drop model, which is more realistic than simply dividing watts by source voltage.
Assumptions and formulas
PoE planning has two separate constraints: the switch must have enough total budget across all powered ports, and each individual link must still leave enough voltage at the powered device after cable losses. This tool estimates both. It assumes balanced pairs, solid-copper conductors, and a powered device that behaves roughly like a constant-power load over its allowed input-voltage range.
Effective loop resistance
R_eff = r x L for two-pair PoE, where r is conductor resistance in ohm/km and L is length in km.
R_eff = (r x L) / 2 for four-pair PoE, because current is shared across twice as many conductors.
Constant-power cable model
P_PD = V_PD x I
V_PD = V_source - I x R_eff
R_eff x I^2 - V_source x I + P_PD = 0
I = (V_source - sqrt(V_source^2 - 4 x R_eff x P_PD)) / (2 x R_eff)
P_source = V_source x I and P_loss = I^2 x R_eff
If the quadratic has no real solution, the requested powered-device wattage is not feasible for that cable resistance, length, and source voltage. In that case you need a shorter run, lower-loss cable, lower device power, or a different power architecture.
PoE reference limits used here
This page uses common IEEE-style launch-power planning limits: 802.3af Type 1 up to 15.4 W from the PSE and about 12.95 W at the device, 802.3at Type 2 up to 30 W and about 25.5 W at the device, 802.3bt Type 3 up to 60 W and about 51 W at the device, and 802.3bt Type 4 up to 90 W and about 71.3 W at the device. Four-pair modes reduce effective loop resistance compared with two-pair modes, which is why they can support higher delivered power over the same cable length.
Cable resistance matters just as much as the PoE class. A long run on thin conductors can stay under the switch's total watt budget and still fail at the device because the cable consumes too much power as heat. That is why the planner reports both total source watts and per-link voltage drop.
Frequently asked questions
Should I enter the switch port budget or the device watts?
Enter the device watts at the powered device whenever you know them. The calculator then estimates how many watts the switch must source after cable loss.
Why does a 100 m cable look worse on 802.3af or 802.3at?
Those standards use two-pair power delivery, so the effective loop resistance is higher than with four-pair 802.3bt links.
Does this include connector resistance, temperature rise, or bundled-cable derating?
No. Those effects can matter in dense or warm installations. Treat this as a planning estimate, not a compliance test.
What if my cable datasheet provides a resistance value?
Use the custom ohm/km field. That is preferable to any generic preset because actual cable constructions differ.
Can I use this for passive PoE?
Not reliably. The presets are for IEEE-style PoE classes and pair-sharing behavior. Passive systems vary too much by vendor and voltage.
Practical planning notes
Four pairs reduce loss
802.3bt shares current over more copper, which lowers effective resistance and usually improves voltage at the device.
Lower drop
Device watts are what matter
A powered device rated at 25.5 W does not mean the switch only spends 25.5 W. Cable loss pushes source watts higher.
Budget reality
Conductor size changes outcomes
Going from 24 AWG to 23 AWG or 22 AWG can materially improve long-run feasibility for high-power PoE devices.
Cable selection
Bundle heat can steal margin
Dense cable bundles and warm ceiling spaces raise conductor resistance, so a link that looks safe on paper can lose extra voltage in the field.
Temperature
Disclaimer
This calculator is for engineering planning and comparison, not final electrical or safety sign-off. Confirm the exact switch power budget, port-class policy, cable datasheet resistance, bundle temperature limits, and powered-device minimum input voltage before deployment.