PoE calculator: line length and voltage drop

PoE power is lost to conductor resistance over twisted pair — the device receives less than the switch outputs, and on a long line a PoE camera or access point might not power up. Choose a PoE standard and conductor gauge (AWG) — the calculator estimates voltage drop, voltage at the device, and the maximum length.

Top — how many volts are lost in the cable and how many reach the device. Bottom — line length on the channel scale; the dashed line marks the maximum length, beyond which power runs out.

Input data

PoE standardThe PoE standard sets the cable's power and current: af — up to 15 W, at (PoE+) — 30 W, bt — 60–90 W. af/at use 2 pairs, bt uses all 4. Conductor gaugeAWG — the American Wire Gauge: the higher the number, the thinner the conductor and the greater the loss. AWG 24 is typical for Cat5e/Cat6; AWG 26 is a thin patch cord. Line length, L м PoE current, I blank — taken from the standard А Source voltage, U blank — taken from the standard В

Result

Loop resistance — Ом

Voltage at the deviceHow many volts reach the device after cable losses. It must be no lower than the minimum for its PoE standard, or the device won't power up (red). — В

Fill in the fields — the verdict will appear here

Voltage drop, ΔU — В

Maximum length — м

An estimate based on DC copper conductor resistance at 20 °C; CCA (copper-clad aluminum) and heating cause greater losses. Maximum length is limited by both power and the data channel (100 m).

How it's calculated

Loop resistance is taken from conductor gauge and length. In 802.3af/at, power runs over two pairs; in 802.3bt, over four, with conductors working in parallel.
Voltage drop is the PoE current times this resistance; what's left at the device is the source voltage minus the loss.
What remains must be enough for the chosen standard's minimum, and the channel length is capped at 100 m regardless.

Example: AWG 24, PoE+ (802.3at), 50 m → a 4.21 Ω loop, a 2.53 V drop, 47.5 V at the device against a minimum allowed 42.5 V.

Copper conductor resistance by AWG gauge, Ω/m (DC, 20 °C)

Gauge	Cross-section, mm²	Resistance, Ω/m
AWG 26	0.129	0.1345
AWG 24	0.205	0.0842
AWG 23	0.258	0.0668
AWG 22	0.326	0.053

Power over IEEE 802.3: how many pairs carry current, the current itself, source voltage, and minimum at the device

Standard	Pairs used for power	Current, A	Source U, V	Minimum at device, V
802.3af (PoE, 15.4 W)	2	0.35	48	37
802.3at (PoE+, 30 W)	2	0.6	50	42.5
802.3bt Type 3 (PoE++, 60 W)	4	0.6	50	42.5
802.3bt Type 4 (90 W)	4	0.96	52	41.1

ΔU = I·Rloop; PoE 802.3af/at — 2 pairs (Rloop = Rcond·L), 802.3bt — 4 pairs (Rcond·L/2). Channel length is capped at 100 m per ISO/IEC 11801 and GOST R 53246.

This is an estimate

Verify final values against current standards, the project design, and equipment nameplates.

Explained in the article Основы сетей: модель OSI, IP-адреса и кабели

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