PoE Cable Voltage Drop Calculator

Power over Ethernet elegantly solves the problem of running both data and power to a device (like a security camera or Wi-Fi access point) over a single cable, but that convenience comes with a real electrical constraint that's easy to overlook until it causes a genuinely confusing intermittent failure: every length of copper wire has resistance, and that resistance causes a real voltage drop proportional to both the current flowing through it and the distance it travels — a basic consequence of Ohm's law that becomes a practical concern specifically at the longer cable lengths and higher power draws that modern PoE installations increasingly push toward.

The calculation itself follows directly from Ohm's law (voltage equals current times resistance), applied to the wire's total resistance over its length — but with one detail that's easy to miss if reasoning from first principles without double-checking against an actual formula: the relevant length isn't the one-way distance from the switch to the device, but effectively double that, because the electrical circuit is a complete loop — current flows out to the device along one conductor and returns along another, and voltage drop occurs on both legs of that round trip. A cable run described as '200 feet' to a camera actually represents 400 feet of total conductor length that the current must traverse, and forgetting this doubling is a common source of underestimating real voltage drop by exactly half.

Wire gauge — the physical thickness of the copper conductor — is the primary variable an installer actually controls to manage voltage drop, and the relationship is intuitive once stated plainly: thicker wire (a lower AWG number, confusingly, since American Wire Gauge numbering runs inversely to physical thickness) has lower resistance per unit length, directly reducing voltage drop for the same current and distance. This is exactly why long PoE runs, or runs powering higher-draw devices like PTZ cameras or high-power Wi-Fi access points, sometimes call for a heavier gauge cable than standard installations use, even though standard Cat5e/Cat6 cable (typically 24 AWG) is perfectly adequate for shorter runs or lower-power devices.

The practical stakes of getting this wrong are genuinely disruptive rather than merely theoretical. PoE-powered devices have a minimum input voltage below which they won't power on reliably, or may power on but behave erratically — browning out, unexpectedly rebooting, or failing to start at all, especially under a momentary higher current draw (a PTZ camera moving its motor, for instance, or a device starting up and briefly drawing more current than its steady-state operation). These symptoms can look like a device fault, a switch problem, or an intermittent cable issue, and diagnosing them as a voltage drop problem specifically requires actually calculating expected drop for the installation's real cable length, gauge, and current draw — exactly the kind of check this calculator supports, and one well worth doing proactively during installation planning for any run approaching Ethernet's practical maximum distance, rather than discovering the problem after a device is already installed and behaving unreliably in the field.