PoE Voltage Drop Calculator

Loop resistance, voltage at the powered device, cable loss and maximum run length for IEEE 802.3af, 802.3at and 802.3bt Type 3 / Type 4 — with AWG, patch cords and conductor temperature taken into account.

PoE standard

PSE output voltage (V)

PD power required (W)

Horizontal cable run

Horizontal conductor

Patch cords, both ends (26 AWG stranded)

Conductor temperature (°C)

Design against the TIA limit for 24 AWG (9.38 Ω/100 m instead of 8.42 Ω/100 m nominal). Use this when you do not know the installed cable's measured resistance.

How this is calculated

A powered device draws constant power, not constant current. The current therefore depends on the voltage that survives the cable, and that voltage depends on the current. Calculators that use I = P / V_PSE ignore this and under-report the drop. The real operating point is the lower root of:

R_loop · I² − V_PSE · I + P_PD = 0 → I = ( V_PSE − √( V_PSE² − 4·R_loop·P_PD ) ) / ( 2·R_loop )

If the discriminant is negative there is no operating point at all: the link cannot deliver that power at that supply voltage, whatever the PD negotiates.

Loop resistance

PoE returns its current through the cable, so the loop counts both directions. In 2-pair mode (Type 1 and Type 2) one pair carries the current out and one returns it, putting two conductors in parallel each way. In 4-pair mode (Type 3 and Type 4) there are four conductors each way, which halves the loop:

2-pair: R_loop = R_conductor 4-pair: R_loop = R_conductor / 2

Patch cords matter more than people expect. A 26 AWG stranded cord carries roughly 1.7 times the resistance per metre of 24 AWG solid horizontal cable, so 10 m of cords is electrically similar to adding about 17 m of permanent link. Leaving them out is the most common reason a calculated budget passes and the installed link does not.

Temperature

Copper resistance rises about 0.393 % per °C above 20 °C. A cable in a hot ceiling void at 50 °C has roughly 12 % more resistance than the same cable on a bench. A cable sitting in a large bundle that is carrying PoE heats itself further; bundle heating is addressed in TIA TSB-184-A, and the practical approach is to enter the elevated temperature you actually expect rather than assume 20 °C.

IEEE 802.3 PoE limits used here

Type	Pairs	PSE max	PD guaranteed	PD min voltage	Max current

PSE output is 44–57 V for Type 1 and 50–57 V for Types 2–4. Most switches sit near 53 V, which is why the defaults here are not the bare minimum: at exactly 50 V a full Type 4 load over a long worst-case channel will not meet the PD minimum voltage. Authoritative values are in IEEE 802.3, Clause 33 (Types 1–2) and Clause 145 (Types 3–4).

Conductor resistance

DC resistance of one conductor at 20 °C, computed from AWG geometry with ρ = 1.724×10⁻⁸ Ω·m. Stranded conductors are taken as 5 % higher than solid.

Gauge	Ω / 100 m	Ω / 1000 ft	Typical use

Limitations

This computes steady-state DC behaviour of a balanced channel. It does not model inrush at PD power-up, PSE port efficiency, resistance unbalance between the conductors of a pairset, or self-heating from the PoE current itself — enter a raised conductor temperature for that. For cable already in the wall, a measured loop resistance always beats a calculated one.