A PoE voltage drop calculator determines power loss and voltage delivered at the end of copper Ethernet cable runs for IP cameras, wireless access points, and VoIP phones. Enter cable length, gauge, and device power draw to get a Pass/Fail verdict against IEEE 802.3af/at/bt standards.
Cable Resistance & Loss Breakdown
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Pro Networking Gear
You cannot cheat physics. If you are pushing high-wattage PoE++ (802.3bt) over long distances to PTZ cameras or Wi-Fi 6 access points, standard CCA (Copper Clad Aluminum) cable will result in catastrophic voltage drops. You absolutely must use pure bare copper Cat6a cable (AWG 23). The thicker copper conductor significantly reduces resistance, keeping your power loss in the green zone. If your cable run exceeds the standard 100-meter limit, you will need to insert a Gigabit PoE Extender midway to regenerate the data signal and pass the remaining power through.
Understanding PoE Voltage Drop
Power over Ethernet (PoE) injects DC voltage (typically 48V to 54V) directly into your Ethernet cables alongside your network data. Because Ethernet cables are made of thin copper wires (usually 23 or 24 AWG), they act like very weak resistors. As the power travels down the length of the cable, some of it is lost as heat (I²R loss). This is known as "voltage drop." If the run is too long, the cable is too thin, or the powered device (PD) draws too much wattage, the voltage reaching the device will dip below its minimum operating threshold. The device will fail to power on, reboot randomly, or damage its internal components. WattDrop calculates the exact loop resistance of your cable run, computes the power dissipated as heat, and determines exactly how many watts will survive the journey to reach your device.
Frequently Asked Questions
What is PoE voltage drop and why does it matter?
Power over Ethernet sends DC power through copper cable alongside data. Copper has resistance that increases with distance and decreases with thicker wire gauge. A 100-meter Cat5e run can lose 10-15V of the original 48-57V supply. If voltage drops too low, the powered device (camera, access point, phone) fails to boot or operates erratically.
What is the maximum PoE cable length?
The Ethernet standard limits cable runs to 100 meters (328 feet) for both data and power. However, PoE voltage drop may prevent device operation well before 100 meters — especially for high-power devices on thin cable. A 90-meter run of Cat5e with a 25.5W device can drop voltage below the device's minimum requirement. Use Cat6/6A for long runs.
What is the difference between PoE, PoE+, and PoE++ standards?
IEEE 802.3af (PoE) delivers up to 15.4W at the source, 12.95W at the device. IEEE 802.3at (PoE+) delivers up to 30W source, 25.5W device. IEEE 802.3bt (PoE++) delivers up to 60W (Type 3) or 90W (Type 4). Higher power standards use all 4 pairs instead of 2, reducing resistance and voltage drop per pair.