Check whether one switch has enough total PoE capacity and ports, whether every port supports its device,
how much power cable loss adds, and what voltage reaches each powered device. Plan class or LLDP allocation,
simultaneous peak loads, and long cable runs in one place.
Calculation method v2.0 ยท Last technically reviewed 17 July 2026 ยท Prepared and reviewed by Starlight Robotics.
Calculator
Switch + device inputs
A 15โ20% reserve is a planning starting point; increase it for uncertain peaks or future growth.
Actual draw estimates electrical load. Class mode reserves the negotiated PSE maximum. LLDP mode uses the editable per-port allocation.
Device group 1
Included by default
Device group 2
Set quantity to 0 to ignore
Device group 3
Optional expansion
Budget results
Enter a design to check one-switch capacityThe verdict checks ports, allocation, reserve, and every link.
Utilization-
Devices / ports-
Actual PSE load-
Allocated PSE load-
Usable budget after reserve-
Remaining watts-
Minimum switch budget-
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.
Voltage drop, range, and cable comparison
The electrical estimate can model non-standard distances, but normal copper Ethernet channel planning remains 100 m (328 ft).
Temperature uses copper's approximate 0.393%/ยฐC resistance coefficient; bundle derating is an extra user-selected resistance allowance.
CCA cable has substantially higher resistance and is not compliant with structured-cabling standards for PoE. Use a separate verified datasheet value only for diagnosis, not as a design recommendation.
Link decision
Enter a link to testChecks voltage, current, per-port power, and distance.
Source watts-
PD voltage-
Voltage drop-
Cable loss-
Current-
Effective loop resistance-
Theoretical max PD power-
Port limit check-
Voltage drop-
Maximum acceptable cable-
Minimum required PSE voltage-
The single-link estimator solves a constant-power cable-drop model, which is more realistic than simply dividing watts by source voltage.
How to calculate a PoE switch power budget
Find the switch budget and port count. Enter the switch's total PoE watt budget and number of available PoE ports.
Use maximum device demand. Choose a preset or enter each device's maximum PD draw and any simultaneous startup, radio, motion, IR, or heater allowance.
Choose class and allocation policy. Select the required IEEE PoE type and whether the switch budgets actual draw, the class maximum, or a documented LLDP or manufacturer value.
Enter cable conditions. Enter link length, cable construction, connectors, temperature, and minimum PD input voltage.
Set reserve. Keep appropriate power headroom for simultaneous peaks, environmental conditions, and future changes.
Review both result levels. Confirm the one-switch verdict, ports and total allocation, then resolve every per-port class, current, voltage, and distance warning.
When to use this calculator
Use it when procuring a PoE switch, adding cameras or access-control devices, rolling out Wi-Fi access points, checking intercom or speaker loads, or troubleshooting a device that resets on a long cable. It is most useful before purchase and whenever night mode, PTZ movement, radios, heaters, or cold starts may overlap.
Worked PoE budget and voltage-drop examples
Values below use the same constant-power equations as the calculator and rounded results. They are transparent checks, not product ratings.
Eight security cameras
Inputs: 8 ร 12 W maximum PD, Type 2, 80 m Cat6 (76 ฮฉ/km), 50 V PSE, 15% reserve.
Per link: Reff = 76 ร 0.080 = 6.08 ฮฉ; I = 0.248 A; loss = 0.37 W; PSE = 12.37 W; PD voltage = 48.49 V.
Decision: actual PSE load โ 99.0 W; minimum 15%-reserve switch budget โ 116.5 W. A 120 W, 8-port switch is electrically close and leaves no spare port; a larger model is prudent.
Twelve Wi-Fi access points
Inputs: 12 ร 22 W maximum PD, Type 2, 60 m Cat6A (72 ฮฉ/km), 52 V PSE, 20% reserve.
Per link: Reff = 72 ร 0.060 = 4.32 ฮฉ; I โ 0.439 A; loss โ 0.83 W; PSE โ 22.83 W; PD voltage โ 50.10 V.
Decision: actual PSE load โ 274.0 W; required budget after reserve โ 342.5 W. Class-maximum accounting would reserve 360 W before reserve, so the switch policy changes the purchase decision.
Long-run heated PTZ camera
Inputs: 55 W maximum plus 8 W heater allowance, Type 4, 150 ft (45.72 m) Cat6, 54 V PSE, 0.20 ฮฉ connectors.
Per link: Reff = (76 ร 0.04572) รท 2 + 0.20 = 1.94 ฮฉ; I โ 1.22 A; loss โ 2.89 W; PSE โ 65.89 W; PD voltage โ 51.64 V.
Decision: within a 90 W Type 4 port, but simultaneously test heater, PTZ motion, IR, and cold-start demand. A Type 2 port cannot support it.
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. The constant-power model includes conductor resistance, pair sharing, user-selected connector
resistance, copper temperature adjustment, construction factor, and optional bundle derating. It excludes
pair-to-pair resistance unbalance, converter efficiency changes, transient waveforms, contact aging, and data-signal limits.
Effective loop resistance
For one conductor, R_conductor (ฮฉ) = r (ฮฉ/km) ร L (km). In two-pair PoE, two conductors in parallel carry positive current and two return it: R_conductor/2 + R_conductor/2 = R_conductor, so R_eff (ฮฉ) = r ร L.
Four-pair PoE doubles the parallel conductors in each direction: R_conductor/4 + R_conductor/4 = R_conductor/2, so R_eff (ฮฉ) = (r ร L) / 2. Connector allowance is then added in ohms.
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.
Validation checks: the implementation also rejects PD voltage below the entered minimum, total current above the type limit, per-port PSE or PD power above the selected type, and impossible quadratic operating points. At 100 m and the conservative Cat5e value of 93.8 ฮฉ/km, the two-pair loop is 9.38 ฮฉ; with four-pair delivery it is 4.69 ฮฉ before connectors and derating. These boundary resistances are the intended cross-check.
IEEE PoE types and power limits
IEEE type
Common name
Classes
Powered pairs
Maximum PSE power
Maximum power available at PD
Planning voltage range
Common applications
802.3af Type 1
PoE
0โ3
2
15.4 W
12.95 W max at PD
PSE 44โ57 V; PD 37โ57 V
VoIP phones, basic cameras, sensors
802.3at Type 2
PoE+
4
2
30 W
25.5 W max at PD
PSE 50โ57 V; PD 42.5โ57 V
Access points, intercoms, PTZ cameras
802.3bt Type 3
PoE++ / 4PPoE
5โ6 (also lower classes)
4
60 W
51 W max at PD
PSE 50โ57 V; PD 42.5โ57 V
Multi-radio APs, displays, building controls
802.3bt Type 4
Higher-power PoE++ / 4PPoE
7โ8
4
90 W
71.3 W max at PD
PSE 52โ57 V; PD 41.1โ57 V
Heated cameras, lighting, speakers, terminals
PSE is the power sourcing equipmentโthe switch or injector. PD is the powered device. PoE+ is Type 2; PoE++ is commonly used for Types 3 and 4; 4PPoE means power over all four pairs. Physical-layer class allocation lets the PSE reserve a defined maximum. LLDP is a data-link negotiation that compatible equipment may use to request and allocate a more specific value.
PoE budget FAQs
How do I calculate a PoE power budget?
Add the maximum powered-device demand for every port, include cable loss, compare the switch's actual or allocated PSE load with its usable budget after reserve, and confirm that enough PoE ports are available.
Does the PoE budget include cable loss?
Yes. Actual-draw mode solves a constant-power link model and adds estimated resistive cable and connector loss to each device's PD demand. Class allocation remains a separate switch-accounting value.
How much PoE headroom should I keep?
A 15 to 20 percent reserve is a practical planning starting point, but use the switch vendor's guidance and increase it for uncertain loads, cold starts, heaters, future additions, or shared power supplies.
What happens when the switch PoE budget is exceeded?
Behavior is vendor- and configuration-specific: the switch may deny power to a new port, remove power from a lower-priority port, or report an allocation fault. Do not rely on overload behavior as capacity planning.
What is PSE power versus PD power?
PD watts are available at the device input. PSE watts leave the switch or injector and therefore include cable loss; PSE power is always at least the PD power in this model.
What is actual draw versus class allocation?
Actual draw estimates what the PSE electrically supplies. Class allocation is the maximum power the switch may reserve after classification; LLDP or a manufacturer value can let a compatible switch reserve a more specific amount.
Can a PoE+ switch power a PoE++ device?
Only if the device supports a lower-power fallback mode within the Type 2 limit. A device that requires Type 3 or Type 4 power will not gain that power from a PoE+ port.
Can a midspan injector solve a failed design?
A standards-compliant injector can provide the required type on selected runs and remove that load from the switch's PoE budget. It does not add switch data ports, and cable voltage and distance checks still apply.
Does port prioritization increase the PoE budget?
No. It controls which ports keep power when allocation is scarce. Size the switch for all loads that must operate simultaneously.
Why include startup, IR, PTZ, radio, and heater loads?
Those loads can overlapโfor example a cold camera can start its heater while IR is on and PTZ motors move. Use manufacturer maximums and add allowances only for loads not already included.
What is the maximum Ethernet cable distance?
A normal balanced-copper Ethernet channel is planned to 100 m (328 ft). The range result answers an electrical PoE question only; a longer electrical result does not extend Ethernet's data-channel limit.
When does passive PoE need a separate calculation?
Always treat passive PoE separately unless its exact voltage, polarity, conductor use, current limit, and device input range match your custom model. Passive systems do not provide IEEE detection and classification protection.
Method: v2.0Last reviewed: 17 July 2026Technical reviewer: Starlight Robotics
Validation note: checked against 100 m loop-resistance boundaries, the published IEEE-style PSE/PD limits above, zero-length conservation, and representative two-pair and four-pair cases. Presets are conservative planning examples, not a substitute for the installed cable datasheet or field certification.
Known limitations: this is a steady-state DC planning model, not an IEEE compliance test or safety sign-off. It does not simulate transient waveforms, pair imbalance, conversion-efficiency curves, cable aging, data insertion loss, or proprietary passive PoE. Confirm switch allocation policy, power-supply sharing, port priority, cable temperature rating, local code, PD minimum voltage, and all manufacturer maximums.
