PoE Power Budget Calculator

Frequently Asked Questions

What is the difference between 802.3af, 802.3at, and 802.3bt?

802.3af delivers up to 15.4W per port. 802.3at (PoE+) delivers up to 30W. 802.3bt (PoE++) adds Type 3 (60W) and Type 4 (90W) using all four cable pairs.

Why is the power at the PD less than the port reservation?

Cable resistance dissipates power as heat. The switch must reserve the maximum port value. 802.3af guarantees 12.95W at the device from a 15.4W port reservation.

What happens if I exceed the PoE budget?

The switch may shut down existing ports, refuse to power new devices, or draw beyond PSU capacity causing a fault. Exact behavior is vendor-specific; always stay within budget.

Can I mix PoE and non-PoE devices on the same switch?

Yes. Non-PoE ports consume zero PoE budget. PoE activates only when the switch detects a powered device through the IEEE detection handshake.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How It Works

A PoE switch advertises a total power budget in watts shared across all ports. Each powered device (PD) causes the switch to reserve power by its 802.3 class: 802.3af (Type 1) reserves 15.4W at the port, 802.3at (PoE+, Type 2) reserves 30W, 802.3bt Type 3 up to 60W, and Type 4 up to 90W.

This calculator multiplies each device count by its class reservation, applies a cable-length derating (longer Cat5e/6 runs lose power as heat in the copper conductors, so the switch must reserve more per port), sums the total draw, and subtracts from the budget. It warns when the configuration exceeds the budget and shows how many additional PoE+ devices could still be added.

Use Cases

Network installers and IT managers use this calculator to:

Size a PoE switch before deploying a floor of IP cameras, wireless access points, VoIP phones, and door controllers.

Verify that a new wave of 802.3bt devices (PoE++ pan-tilt-zoom cameras, Wi-Fi 6E APs) can be added to an existing switch without exceeding its budget.

Determine whether a single switch suffices for a floor or whether devices must be split across two switches.

Identify when a switch requires a higher-capacity power supply or a second PSU for redundancy.

Plan cabling distance to keep per-port derating low on high-wattage 802.3bt deployments.

Tips

Size to port reservation, not device draw - the switch reserves the full class budget per port even if the device only draws half; a Type 2 port running a 10W phone still consumes 30W of switch budget.

Keep cable runs under 50 m for high-wattage devices - 802.3bt Type 4 at 90W on 100 m Cat5e can lose several watts to conductor resistance; use Cat6A or shorter runs.

Add 20% headroom to the budget - plan for future device additions; PoE budgets are not easily expanded without replacing the switch.

A 48-port switch at full 802.3at would need 1,440W - most switches top out at 370-740W; read the actual PoE budget from the datasheet, not just the port count.

LLDP-MED negotiation can reduce reservations - some switches use LLDP to negotiate actual power and reduce over-reservation, but always plan conservatively using class budgets.

FAQ

What is the difference between 802.3af, 802.3at, and 802.3bt?

802.3af (2003) delivers up to 15.4W per port at the PSE. 802.3at (2009, PoE+) delivers up to 30W. 802.3bt (2018, PoE++) adds Type 3 (60W) and Type 4 (90W), using all four pairs in Cat5e/6 rather than just two. A Type 1 switch cannot deliver Type 3 or 4 power regardless of remaining budget.

Why is the power at the PD less than the port reservation?

Cable resistance dissipates power as heat. 802.3af guarantees 12.95W at the PD (the remainder is cable loss). 802.3at guarantees 25.5W. 802.3bt Type 4 guarantees up to 71.3W at the PD from a 90W port. The switch must reserve the maximum port value, not the minimum PD delivery.

What happens if I exceed the PoE budget?

The switch can shut down existing ports to bring total draw within budget, refuse to power new devices connecting after the budget is full, or in some designs draw total watts exceeding what the PSU can sustain - potentially causing a PSU fault. The exact behavior is vendor-specific; always stay within budget.

Can I mix PoE and non-PoE devices on the same switch?

Yes. Ports connected to non-PoE devices do not consume any PoE budget. PoE is only active on ports where the switch detects a powered device through the IEEE-standard detection handshake. Non-PoE devices are not harmed.

PoE Power Budget Calculator

Frequently Asked Questions

Related Networking & IT Calculators