What is a PoE Budget Calculator?
A PoE Budget Calculator is an essential engineering utility used to ensure that a network switch has sufficient electrical power capacity to drive all connected Power over Ethernet devices, such as wireless access points, IP cameras, and VoIP phones.
Every PoE-enabled network switch has a maximum PoE Budget (measured in Watts). When you connect a device, the switch allocates a specific amount of power to that port based on the IEEE standard requested by the device (e.g., Class 4 allocates 30W). If the total sum of these allocations exceeds the switch's overall budget, subsequent devices will not boot, or random ports will shut down to protect the hardware.
Use this calculator during the infrastructure planning phase to avoid under-speccing your access switches or over-loading your battery backups (UPS).
Example Usage
Scenario: Provisioning an IDF Access Node
You are installing a 48-port switch with a strict 370W PoE budget. You need to connect 8 Wi-Fi 6 access points (802.3at) and 12 IP Phones (802.3af).
- 1. Input Switch Budget: 370W
- 2. Add Device Group: 8x Class 4 (at) = 240W
- 3. Add Device Group: 12x Class 3 (af) = 184.8W
- Result: 424.8W Total (Over Budget). You must upgrade to a 740W switch model.
Frequently Asked Questions
What is the difference between PSE power and PD power?
PSE (Power Sourcing Equipment) is the power allocated at the switch port. PD (Powered Device) is the power actually available at the device. IEEE standards account for power loss over the physical copper cable. For example, 802.3at allocates 30W at the switch (PSE) but guarantees 25.5W at the device (PD). You must budget using the PSE numbers.
What happens if I exceed my PoE budget?
Switches handle power over-subscription differently. Usually, the switch will prioritize ports based on configuration or port number. Lower priority ports will be denied power (devices won't boot). If an active device suddenly draws more power causing an overload, the switch may shut down the port entirely, leading to network outages.
What is IEEE 802.3bt (Type 3 and Type 4)?
Commonly known as PoE++ or 4PPoE, 802.3bt utilizes all four pairs of wires in an Ethernet cable. Type 3 allocates up to 60W (for high-end PTZ cameras or building automation). Type 4 allocates up to 90W (often used for IT-managed LED lighting and displays).