PoE Voltage Drop: What Causes It and Why It Matters

PoE voltage drop occurs as electrical current travels through the copper conductors in an Ethernet cable. Because copper has electrical resistance, some of the power supplied by the Power Sourcing Equipment (PSE) is lost before it reaches the Powered Device (PD).

This is a normal part of Power over Ethernet operation and one reason PoE standards distinguish between the power supplied at the PSE and the power available at the connected device. Cable length, conductor resistance and the amount of current being carried can all influence the amount of loss.

Why Does Voltage Drop Occur in PoE?

Every copper conductor has some electrical resistance. When current passes through that resistance, the voltage measured at the far end of the conductor is lower than at the source.

In a PoE system, Ethernet cable carries electrical current from a PoE switch or other PSE to a powered device. As the cable gets longer, the current travels through more conductor, increasing the total resistance of the circuit.

Higher current also increases the effect of conductor resistance. This makes cable characteristics increasingly important as PoE is used to support higher-power equipment.

Does PoE Lose Power Over Distance?

Yes. Some power is lost as heat within the cable as electricity travels between the PSE and PD. The amount of power available at the device is therefore lower than the amount initially supplied by the PSE.

PoE standards account for this. The maximum PSE output and maximum power available to the PD are not identical because the system must allow for losses through the Ethernet channel.

This is also why our guide to PoE power levels distinguishes between power supplied by the network equipment and power available to the connected device.

Does a Longer Ethernet Cable Cause More Voltage Drop?

Generally, yes. A longer copper cable has more conductor resistance than a shorter cable of the same type and gauge. That increased resistance can result in greater voltage drop and power loss.

However, this does not mean a properly designed PoE connection becomes unreliable simply because it approaches the normal Ethernet distance limit. Standards-based PoE is designed around defined channel conditions and maximum distances.

Copper Ethernet channels are generally limited to 100 meters (328 feet). Our PoE distance guide explains that limit and the options for devices located farther away.

Does Ethernet Cable Type Affect PoE Voltage Drop?

Yes. The physical characteristics of the cable affect its electrical resistance. Conductor size is particularly important: larger copper conductors generally have lower resistance than smaller conductors.

Cable construction and conductor material also matter. Solid copper Ethernet cable is generally preferred for permanent PoE installations. Copper-clad aluminum (CCA) cable has greater electrical resistance than solid copper and can create additional concerns when carrying PoE power.

Higher-power PoE also makes heat within cable bundles an important design consideration. Our PoE cable guide covers cable category, conductor material and other selection factors in more detail.

Is Voltage Drop More Important With PoE++?

Voltage drop becomes increasingly important as more power is delivered over Ethernet. IEEE 802.3bt uses all four pairs of an Ethernet cable for power delivery and supports substantially more power than earlier PoE standards.

Higher-power applications can place greater electrical demands on the cabling infrastructure, making conductor resistance, cable quality and installation practices important considerations.

That does not mean PoE++ inherently creates excessive voltage drop. A properly designed standards-based system accounts for allowable losses between the PSE and PD.

Can PoE Voltage Drop Be Calculated?

Yes. Voltage drop is fundamentally related to current and circuit resistance. Using Ohm’s law, the relationship can be expressed as:

Voltage Drop = Current × Resistance

The practical calculation for an Ethernet channel is more complicated than simply measuring the length of one conductor because PoE distributes current across conductor pairs and different PoE types use different power-delivery arrangements.

For most system design work, equipment specifications and standards-based cabling requirements provide the appropriate limits. Detailed calculations may be useful when evaluating unusual cable lengths, conductor sizes or higher-power applications.

Does Voltage Drop Affect a PoE Power Budget?

Voltage drop and cable loss help explain why the power available at a PD is lower than the maximum power supplied at the PSE. However, they should not be treated as an arbitrary amount that is simply subtracted from every switch’s rated power capacity.

When calculating a system, designers need to consider both the power capabilities of the PSE and the amount of power required by the connected devices.

Our PoE power budget guide explains how total device requirements, per-port capacity and overall switch capacity work together.

Why Does PoE Voltage Drop Matter in Intelligent Buildings?

Intelligent buildings can contain large numbers of PoE-powered devices spread across substantial floor areas. Lighting, sensors, controls and other equipment may operate through network infrastructure with many individual Ethernet channels.

Understanding voltage drop helps designers recognize why cable length, conductor quality and available device power matter as PoE moves beyond conventional IT equipment into building systems.

For applications such as PoE lighting, reliable power delivery depends on the entire path—from the PoE switch, through the structured cabling, to the controller, node or powered device. Proper system design keeps those components operating within their intended electrical and network limits.