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PoE budget calculator

List cameras, access points and phones with their class or power to see whether the switch can carry them, how much is lost in the cable and how big a UPS the setup needs.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Check
Switch (PSE)

Use the minimum voltage guaranteed by the standard (44 V for 802.3af, 50 V for 802.3at and bt) for the worst case; most switches deliver 52 to 56 V.

Cabling

Powered pairs: 2 pairs up to class 4 and 4 pairs for classes 5 to 8, or 4 pairs on all.

Powered devices (PD)
See what was read

Enter the class, the power or both. Without a class it is derived from power; without power, the class PD maximum is used. Length in page units; blank uses the default length.

UPS
—PoE power required from the switch
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Power by device type

Sum of all ports of each type: power delivered to the PD, cable loss and class reserve the switch locks without using.

Tap the chart to see the values.

Power available at the PD versus length

Port delivering the class limit at the voltage entered, with the chosen cable. The gap to port power is the cable loss.

Tap the chart to see the values.

Detail by device type

Values per port except the last column. Current per pair set.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Class → minimum PSE power / maximum PD power: 0 and 3 = 15.4 / 12.95 W; 4 = 30 / 25.5 W; 6 = 60 / 51 W; 8 = 90 / 71.3 W
Loop resistance: R = ρ/A · L · [1 + 0.00393 (T − 20)]; with 4 pairs, R ÷ 2. AWG 24 = 84.2 Ω/km; AWG 23 = 66.8 Ω/km; AWG 22 = 53.0 Ω/km
Current: V · I − I² · R = P_PD → I = [V − √(V² − 4 R P_PD)] ÷ (2R); loss = I² R; P_port = P_PD + loss; V_PD = V − I R
Required budget = Σ (allocated per port × quantity) × (1 + margin); allocated = max(class PSE power, P_port) or P_port (LLDP)
UPS: P = Σ P_port ÷ η_supply + switch consumption; VA = P ÷ PF × 1.25; energy = P × runtime ÷ 0.92

Assumptions and limits

  • Solid copper conductors at 0.017241 Ω·mm²/m at 20 °C; patch cords and connectors not added (use “standard worst case” to include them).
  • Port voltage is taken as constant; pair-to-pair resistance unbalance and dual-signature classes (802.3bt) are not modeled.
  • The 92% UPS inverter efficiency and 25% headroom are typical values; check runtime on the manufacturer’s curve.

Standards and technical documents

  • IEEE 802.3-2022, Clause 33: Power over Ethernet (802.3af/at) and Clause 145: Power over Ethernet over 4 pairs (802.3bt)
  • ANSI/TIA-568.2-D: Balanced Twisted-Pair Telecommunications Cabling and Components Standard
  • TIA TSB-184-A: Guidelines for Supporting Power Delivery Over Balanced Twisted-Pair Cabling
  • ISO/IEC TR 29125: Telecommunications cabling requirements for remote powering of terminal equipment
  • ASTM B258: Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires

U.S. regulations

  • NFPA 70 (NEC) Article 725.144: transmission of power and data over Class 2 and Class 3 cables (bundle ampacity)
  • NFPA 70 (NEC) Article 800: general requirements for communications systems
  • OSHA 29 CFR 1910 Subpart S: electrical safety

Frequently asked questions

How much power does a PoE port deliver?

802.3af gives up to 15.4 W at the port and 12.95 W at the device; 802.3at (PoE+) 30 W and 25.5 W; 802.3bt Type 3 60 W and 51 W; Type 4 90 W and 71.3 W. The gap is the loss allowed over 100 m (328 ft) of cable.

How much power is lost in the cable?

It depends on current and length. A 25.5 W PoE+ port at 50 V over 328 ft of Cat6 (23 AWG) at 104 °F loses about 2 W. In the standard worst case (12.5 Ω per loop) the loss reaches 4.5 W.

Does the switch reserve class power or actual draw?

It depends on configuration. With class-based allocation, a class 3 camera drawing 6.5 W locks 15.4 W of budget. With LLDP allocation the switch reserves only what the device requests, freeing power for more ports.

How do I size a UPS for a PoE switch?

Add the real power delivered at the ports, divide by power supply efficiency (about 90%) and add the switch’s own draw. With 330 W of PoE and 60 W for the switch that is about 430 W, or a 600 to 1,000 VA UPS depending on runtime.