PoE Cable Distance & Voltage Drop Calculator (802.3af/at/bt)

Calculate DC voltage drop, resistive cable power dissipation, and switch power budget headroom across Ethernet runs up to 100 meters. Supports IEEE 802.3af (15.4W), 802.3at PoE+ (30W), and 802.3bt PoE++ (60W / 90W) standards with wire gauge options for 23 AWG Cat6/6a and 24 AWG Cat5e.

How Power over Ethernet (PoE) Voltage Drop Calculations Work

Under IEEE 802.3 specifications, Power Sourcing Equipment (PSE) injects direct current (DC) over balanced twisted-pair Ethernet cables. Because copper conductors possess intrinsic electrical resistance (governed by American Wire Gauge cross-sectional diameter), a portion of the voltage drops along the run according to Ohm's Law (ΔV = I × R), and electrical wattage is dissipated as thermal heat (P_loss = I² × R).

Two-Pair vs. Four-Pair Power Delivery

The effective DC loop resistance depends heavily on whether power is delivered over 2 pairs (802.3af/at) or all 4 pairs (802.3bt):

• 2-Pair Delivery (802.3af & 802.3at): Current travels over 2 conductors and returns over 2 conductors. For 23 AWG Cat6a, loop resistance is approximately 14.2 Ω per 100 meters.
• 4-Pair Delivery (802.3bt Type 3 & 4): Current is shared in parallel across all 4 pairs (8 conductors total). Operating pairs in parallel cuts effective loop resistance in half to ~7.1 Ω per 100 meters, significantly lowering voltage drop for high-power devices like PTZ cameras and Wi-Fi 7 APs.

The 100-Meter IEEE Physical Constraint

The maximum certified distance for all standard Ethernet PoE implementations is 100 meters (328 feet), comprising up to 90 meters of solid horizontal structural cabling plus 10 meters of stranded patch cords. Beyond 100 meters, signal attenuation causes packet corruption and voltage drops below the PD minimum operating threshold.

IEEE PoE Standards Voltage & Wattage Reference Table