Quick answer
An 80 m run of 24 AWG copper carrying a 12 W PoE+ device from a 50 V source is estimated to deliver about 48.06 V. The route drops 1.94 V and loses about 0.48 W with the default 40 C cable-temperature and connection-resistance assumptions.
How to use this PoE voltage drop calculator
Measure the one-way cable route from the PoE switch or injector to one powered device. Select the PoE type actually negotiated by both ends, then choose the conductor gauge shown by the cable data sheet. Category alone is not enough because conductor size and DC resistance vary between cable products.
Enter the PSE voltage and the device's maximum operating power. For cameras, include infrared, heater, PTZ, or motorized lens load. For access points, use the maximum power required to enable all intended radios and features.
Open the advanced assumptions when cable temperature, patch cords, connectors, or the device minimum voltage are known. The result checks one cable run. To size all powered ports together, use the PoE Switch Power Budget Calculator.
PoE voltage drop formula
R20 = conductor resistance per 100 m x length / 100 / powered pairsetsRT = R20 x [1 + 0.00393 x (cable temperature C - 20)] + extra resistanceI = [Vs - sqrt(Vs^2 - 4 x RT x Pdevice)] / (2 x RT)Vdevice = Vs - I x RTcable loss = I^2 x RTThe quadratic current formula models the powered device as a constant-power load. As device voltage falls, the route must carry more current to deliver the same watts, which creates additional voltage drop and heat.
Assumptions and methodology
Copper conductor resistance is estimated from standard annealed copper resistivity and AWG cross-sectional area at 20 C. The calculator then applies a copper temperature coefficient of 0.00393 per degree C. A hotter cable has more resistance and therefore more voltage drop at the same load.
- Two-pair PoE uses one powered pairset; four-pair PoE uses two powered pairsets and assumes equal current sharing.
- Device power is treated as constant at the PD, not as a fixed current measured at the PSE.
- Extra effective resistance represents patch cords, connectors, and other route resistance not captured by the horizontal cable estimate.
- The tool checks planning references for source voltage, device power, device voltage, and current per powered pairset.
- It does not model DC resistance unbalance, transient startup, PoE classification, LLDP negotiation, or cable-bundle heating.
Example calculations
PoE cable voltage drop examples
Long camera run: The default 80 m, 24 AWG, 12 W PoE+ example delivers about 48.06 V, draws 250 mA on its powered pairset, and loses 0.48 W in the route.
Thin cable at the PoE+ limit: A 100 m, 26 AWG route attempting to deliver 25.5 W from 50 V falls to about 40.62 V. Estimated pairset current reaches 628 mA, which exposes why cable resistance matters near a PoE power limit.
Four-pair Type 3: A 100 m, 23 AWG route delivering 40 W is estimated at 46.47 V, with current shared across two powered pairsets. Equal sharing is an estimate; resistance unbalance still needs field testing.
PoE voltage and power reference chart
These are planning references for the four common PoE types. Power at the device is lower than maximum source power because the standard allows for channel loss.
| PoE type | Powered pairs | PSE voltage | PD voltage | PSE power | PD power | Current |
|---|---|---|---|---|---|---|
| PoE | 2 pairs | 44-57 V | 37-57 V | 15.4 W | 12.95 W | 350 mA/pairset |
| PoE+ | 2 pairs | 50-57 V | 42.5-57 V | 30 W | 25.5 W | 600 mA/pairset |
| PoE++ Type 3 | 4 pairs | 50-57 V | 42.5-57 V | 60 W | 51 W | 600 mA/pairset |
| PoE++ Type 4 | 4 pairs | 52-57 V | 41.1-57 V | 90 W | 71.3 W | 960 mA/pairset |
Copper conductor resistance by AWG
These are idealized solid-copper estimates at 20 C, not category limits or product certifications. Use the actual cable data-sheet DCR when available, especially for patch cable or unusual conductor construction.
| Conductor | One-conductor DCR | Planning note |
|---|---|---|
| 22 AWG copper | 5.30 ohm/100 m | Low-resistance large conductor estimate. |
| 23 AWG copper | 6.68 ohm/100 m | Common in many Cat 6 and Cat 6A solid-copper cables. |
| 24 AWG copper | 8.42 ohm/100 m | Common planning estimate for solid-copper Ethernet cable. |
| 26 AWG copper | 13.39 ohm/100 m | Higher-resistance thin cable or patch-cord estimate. |
What the PoE voltage drop result means
Voltage and power margin
Delivered voltage must remain above the device requirement, while source power and current must stay within the selected PoE type. A positive voltage margin alone does not prove the port negotiated enough watts.
Cable loss and heat
Cable loss becomes heat. Longer routes, smaller conductors, higher temperature, and more current increase that loss. Bundle size and installation temperature require separate review.
PoE voltage drop vs switch power budget
These are different installation questions. Voltage drop is calculated for one cable run and one device. Switch power budget adds the demand of every powered port and checks whether the switch has enough total watts and enough power per port.
A switch may have plenty of total budget while one long, high-resistance route still causes voltage or current trouble. Conversely, every individual route can look good while the switch total budget is too small for all devices operating together.
When an installed PoE route needs field testing
- The camera reboots when infrared, heat, or PTZ starts.
- The switch reports undervoltage, overload, or power demotion.
- The route includes unknown patch cords, couplers, or damaged cable.
- A four-pair high-power device is close to its current limit.
- The cable is bundled in a hot ceiling, conduit, or outdoor path.
Field testing should look beyond a simple continuity check. DC loop resistance and resistance unbalance are especially important for high-power four-pair PoE because unequal conductors can force one pairset to carry more current than the calculator's equal-sharing assumption.
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FAQ
How do I calculate PoE voltage drop?
Estimate the effective resistance of the powered cable path, then solve voltage, current, and device power together. This calculator treats the powered device as a constant-power load, so current rises as delivered voltage falls. That is more useful for cameras and access points than assuming current never changes.
How much voltage drop is acceptable for PoE?
There is no single percentage that approves every PoE route. The important checks are whether voltage at the device remains above its required input, current per powered pairset stays within the selected PoE type, and the source can provide the required power. Use the device data sheet and selected PoE standard rather than a generic 3% rule.
Does Cat 6 cable have less PoE voltage drop than Cat 5e?
Often, but not because of the category name alone. Many Cat 6 and Cat 6A cables use larger 23 AWG conductors, while many Cat 5e cables use 24 AWG. Actual conductor material, diameter, patch cords, connectors, temperature, and data-sheet DC resistance determine the loss.
Can PoE work beyond 100 meters?
Voltage may still be available beyond 100 m, but standard Ethernet data reach is a separate limit. An extended-distance switch, repeater, fiber link, local power, or other approved design may be needed. This calculator warns above 100 m but does not certify the data link.
Why does four-pair PoE reduce voltage drop?
Four-pair PoE uses two powered pairsets instead of one, so the total device current can be shared across more conductors. Under an equal-sharing estimate, effective cable resistance is approximately halved. Real DC resistance unbalance can prevent perfectly equal sharing.
Why can a PoE camera reboot at night?
Infrared LEDs, heaters, motorized zoom, and PTZ movement can raise camera power above its daytime load. The higher load increases current and cable voltage drop. Calculate with maximum operating power, then check the switch log, measured voltage, cable condition, and PoE negotiation.
Does this calculator support passive PoE?
No. The PoE type checks are based on standard 802.3af, 802.3at, and 802.3bt planning references. Passive PoE can use different voltages and lacks standard negotiation, so it should be evaluated only from the exact injector and device specifications.
Can I use CCA Ethernet cable in this calculator?
The AWG presets assume solid copper. Copper-clad aluminum can have substantially higher resistance and should not be represented by a copper preset. If diagnosing an existing cable, use its verified one-conductor DC resistance in the custom field and check whether the cable is permitted for the intended installation.
Can this result certify an installed PoE cable?
No. It is a planning estimate. Certification and troubleshooting may require measured DC loop resistance, resistance unbalance within and between pairs, wire map, insertion loss, connector inspection, PoE negotiation, and bundle-temperature review.