Voltage Drop Calculator

When multiple PoE devices share a cable path — like IP cameras daisy-chained through switches or injectors — each segment carries the cumulative current of all downstream devices. This calculator's multi-device mode models exactly that: enter each device's distance from the source and its power draw, and the tool calculates per-segment voltage drop with cumulative current. The last device in the chain always sees the worst voltage — if it drops below 42.5V (802.3at minimum), you need to either upsize the home-run cable, add a midspan injector, or restructure the daisy chain into a star topology.

Each system has a different failure point. Fire alarm NAC circuits operate at 24VDC from the panel, and NFPA 72 requires notification appliances to receive at least 16V — so your maximum allowable drop is roughly 10% when calculated from the full-load supervision voltage of 20.4V. PoE (802.3at) requires a minimum of 42.5V at the powered device, allowing about 12.5V of drop from the 57V source — approximately 15%. Access control systems running 12VDC locks typically fail below 10.5V, so keep drop under 12%. This calculator pre-loads these thresholds when you select the application mode, so you don't have to look them up.

CCA cable has roughly 55-60% higher resistance than solid copper of the same gauge because the aluminum core has significantly higher resistivity. A 300-foot run of CCA Cat5e that would pass voltage drop requirements with solid copper may fail by a wide margin. This is especially dangerous with PoE systems — the cable may test fine for data connectivity but silently starve the powered device of voltage under load. This calculator includes a CCA cable option that uses adjusted resistance values, so you can see the real-world impact before you discover it during commissioning.

Multi-lock circuits are the most common access control voltage drop failure. A single mag lock draws 250-500mA, but a circuit with four locks on a shared pair draws 1-2A total — and the last lock in the daisy chain sees voltage drop from all upstream devices plus its own. Use this calculator's multi-device mode: enter each lock's distance from the power supply and its current draw. If the farthest lock drops below 10.5V, you need to either run a heavier gauge home-run (14 AWG instead of 18 AWG) or split the circuit into two separate power supply outputs.

Yes — this is one of the most common field issues we see. A PoE+ switch delivers 30W at 57V per IEEE 802.3at, but if the cable run is long enough (or uses CCA cable), the voltage at the powered device drops below the 42.5V minimum. The device may boot intermittently, reboot under load, or refuse to power on entirely — and the switch may not report a clear error. The problem gets worse at higher power levels (802.3bt delivering 60-90W) because higher current means proportionally higher voltage drop across the same cable resistance.