PoE Power Budget Calculator — Switch Planning | WireStandard

Enter the switch's total PoE power budget (W), the IEEE 802.3 class of each connected device, device count, and an ambient-temperature thermal derating factor. The calculator outputs total allocated power, remaining headroom, and a pass/fail verdict against the budget. Used by network designers planning wireless AP, IP camera, and IoT deployments.

Formula

Total_required = Σ (PSE_class_max × device_count) / thermal_derate. IEEE 802.3 PSE class limits: af Class 0/3 = 15.4 W; at = 30 W; bt Type 3 = 60 W; bt Type 4 = 90 W (some vendors rate 100 W). Thermal derate at 50 °C ambient is typically 0.70–0.85× the nameplate budget per vendor spec. Headroom = Budget × derate − Total_required.

Worked Example

Switch with 370 W budget: 8 × 802.3at APs (30 W PSE) + 4 × 802.3bt Type 3 cameras (60 W PSE). Total = 240 + 240 = 480 W — exceeds 370 W. Reduce to 4 APs + 4 cameras = 120 + 240 = 360 W; fits with 10 W headroom. Apply 0.85× derate at 50 °C → effective budget 314.5 W; 360 W still exceeds it, so redesign is required.

Frequently Asked Questions

Does a switch deliver its full nameplate PoE budget?

Rarely. Vendors specify maximum budget at a reference ambient (typically 30–40 °C). Above that temperature, internal thermal limits reduce available power. Always size for the worst-case ambient temperature of the installation environment.

Can I mix PoE classes on the same switch?

Yes. Switches negotiate class per port via LLDP or IEEE 802.3 Clause 33 Layer-1 classification. Each port draws only its negotiated class power. Verify that the sum of all port allocations stays within the derated budget.

What is the difference between PSE and PD power numbers?

PSE (Power Sourcing Equipment, i.e., the switch port) must supply more than the device receives because cable resistance dissipates roughly 15–17% as heat. PD (Powered Device) is the wattage reaching the endpoint. Power budget calculations always use PSE figures since that is what the switch must source.