Voltage Drop PoE Calculator

Enter Project Inputs

Use this to estimate DC drop and available device voltage for PoE links.

New calculation

PoE profile

Profiles suggest typical power and pair count.

Cable option

Values are typical DC resistance at 20°C.

Conductor resistance @20°C (Ω/100m)

Use custom if your datasheet differs.

Length

Enter the one-way cable length.

Ambient temperature (°C)

Resistance increases as temperature rises.

PSE voltage (V)

Most PoE systems operate near 48–57 V.

PSE power (W)

Use the planned output power at the source.

Pairs used for power

2 pairs for af/at, 4 pairs for bt.

Parallel cables

Rare option: multiple identical runs in parallel.

Safety margin (%)

Inflates current to cover unknowns.

Minimum PD voltage (V)

Set your device or design limit.

Project notes (optional)

Saved in downloads with the result.

Tip: If you fail voltage limits, reduce length, use thicker conductors, lower power, or move to 4-pair delivery when supported.

Example Data

Profile	Cable	Length	Temp	Pairs	PSE	Voltage Drop	Voltage at PD
IEEE 802.3af	Cat 6 (23 AWG)	80 m	30°C	2	48 V / 15.4 W	≈ 3.0 V	≈ 45.0 V
IEEE 802.3at	Cat 6 (23 AWG)	100 m	40°C	2	48 V / 30 W	≈ 8.6 V	≈ 39.4 V
IEEE 802.3bt Type 4	Cat 7 (22 AWG)	90 m	35°C	4	52 V / 90 W	≈ 3.8 V	≈ 48.2 V

Examples are illustrative and depend on exact cable construction and operating conditions.

Formula Used

This tool models DC voltage drop using conductor resistance and how PoE places pairs in parallel.

1) Temperature-corrected resistance

R_T = R₂₀ × (1 + α × (T − 20))

α is the copper temperature coefficient (≈ 0.00393 / °C). R₂₀ is the conductor resistance at 20°C.

2) Effective loop resistance

R_loop = 2 × R_T × L
R_eff = (R_loop / Pairs) / Parallel

The DC loop uses two conductors. Using 2 or 4 pairs places loops in parallel.

3) Current and voltage drop

I = (W_PSE / V_PSE) × (1 + Margin%)
V_drop = I × R_eff
V_PD = V_PSE − V_drop

Margin inflates current for conservative design.

4) Loss and available power

P_loss = I² × R_eff
P_PD ≈ V_PD × I

This is a practical approximation for planning, not device certification.

How to Use This Calculator

Select a PoE profile or choose custom for your design.
Choose a cable option, then verify the resistance value.
Enter the one-way link length and ambient temperature.
Set source voltage and planned source power output.
Pick how many pairs carry power and any safety margin.
Define the minimum device voltage you want to maintain.
Press Calculate, then download CSV or PDF if needed.

Voltage Drop in PoE Links

Power over Ethernet is popular on construction sites because it simplifies installation for IP cameras, access points, intercoms, and sensors. One cable can deliver both data and DC power, reducing conduit fill and minimizing separate power circuits. The practical limitation is voltage drop. As cable length increases, conductor resistance causes a loss of voltage and power along the run. If the device voltage falls below its operating threshold, the load may reboot, fail to start, or behave intermittently.

This calculator estimates the DC drop from the source equipment (PSE) to the powered device (PD). It uses typical copper resistance values for common cable categories and adjusts resistance for temperature. Warmer environments increase resistance, which increases drop. The tool also models how PoE uses pairs: with two-pair delivery the loop resistance is higher than four-pair delivery, because four pairs place loops in parallel and reduce effective resistance. A safety margin is included to cover real-world variation such as connector losses, patch leads, and installation tolerances.

Use the inputs to match your design intent. Select the PoE profile that reflects your planned power level, or choose custom for nonstandard loads. Confirm the conductor resistance from your cable datasheet when accuracy matters. Enter the one-way length, because the calculator forms the loop internally. Set an appropriate minimum PD voltage based on the device requirement or your project standard. The status result helps you identify when a link is likely to work reliably.

Example use: a site camera requiring 30 W on a 100 m Cat 6 run at 40°C can experience a noticeable drop that reduces the voltage at the device. In the Example Data table above, the 802.3at case shows the impact of longer length and higher temperature. If you need more headroom, shorten the run, choose a lower-loss conductor size, reduce the load, or select equipment that supports four-pair delivery. Another practical option is to move the switch closer, add an intermediate powered enclosure, or use a PoE extender that is approved for your system.

For construction documentation, save the results as CSV or PDF and attach them to the low-voltage package. Documenting expected voltage at the device is useful during commissioning, especially when troubleshooting marginal links. Always verify with field measurements for critical systems, and align your design with applicable codes, manufacturer guidance, and your project specifications.

FAQs

1) Should I enter the total loop length?

No. Enter the one-way cable length. The calculator automatically accounts for the return path by doubling conductor resistance inside the loop model.

2) Why does temperature change the result?

Copper resistance increases with temperature. Higher resistance raises voltage drop for the same power level, so warmer sites or cabinets reduce available voltage at the device.

3) When should I use four pairs?

Use four pairs when your equipment supports it and you need higher power or lower drop. Four-pair delivery reduces effective resistance by placing more conductors in parallel.

4) Do patch cords and connectors matter?

Yes. Patch leads, jacks, and terminations add resistance. Apply a safety margin or include the extra length so the estimate better matches real installations.

5) What minimum device voltage should I choose?

Use the device datasheet minimum input voltage, or your project standard. If you are unsure, select a conservative limit to reduce nuisance resets during peak load.

6) Is this result suitable for certification?

It is a planning estimate. Final compliance depends on the PoE standard, equipment behavior, and installation details. Confirm performance with commissioning tests and manufacturer guidance.

7) How can I fix a failing link?

Reduce length, use a lower-resistance cable, reduce load, or move the source closer. For approved systems, consider four-pair power or a compliant extender strategy.