About PoE Power Budgeting

A PoE (Power over Ethernet) Budgeter calculates whether a Powered Device (PD)—like an IP camera or Wireless Access Point—will receive enough voltage to operate after accounting for the electrical resistance of the Ethernet cable.

Why is this important?

• As cable length increases, voltage is lost as heat due to resistance
• PoE devices are constant-power loads, so they draw more current as voltage drops
• This can lead to a "death spiral" where devices fail to boot or power cycle repeatedly
• IEEE standards specify minimum voltage levels that must be maintained at the device

Key Considerations:

• 802.3af (PoE): 15.4W max, uses 2 pairs, minimum 37V at device
• 802.3at (PoE+): 30W max, uses 2 pairs, minimum 42.5V at device
• 802.3bt Type 3: 60W max, uses 4 pairs (half resistance!), minimum 42.5V
• 802.3bt Type 4: 90W max, uses 4 pairs, minimum 41.1V at device
• Cable gauge matters: 23 AWG has lower resistance than 24 AWG or 26 AWG

This calculator uses the quadratic formula derived from Ohm's Law and the Power Law to solve for the actual operating point where voltage, current, and power are balanced across the cable resistance.