PoE & voltage drop calculator
Check a camera run before you pull the cable: PoE budget and the 100m limit on one side, 12V copper voltage drop on the other — with the fix when a run fails.
This run works
- Run length 30m is within the 100m Ethernet limit.
- 6W camera fits the 802.3af (PoE) device budget of 12.95W (6.9W headroom).
- Also check the switch TOTAL budget: eight cameras at full draw need more than many 8-port switches supply across all ports.
The rules of thumb behind it
PoE is specified to 100m over Cat5e/Cat6, with the device budget set by the standard: 12.95W at the camera for 802.3af, 25.5W for 802.3at, 51W for 802.3bt. The usual failure is not one camera but the switch total — sum every camera's draw and compare it to the switch's whole-unit PoE budget, not just the per-port rating.
On 12V DC the enemy is copper resistance: the drop doubles with run length (the current goes out and comes back) and halves with each step up in conductor size. Cameras typically tolerate about −10% (10.8V), but infrared switching at night can double the draw — a run that works at 3pm can brown-out at midnight. When a run fails: thicker cable, a PSU nearer the camera, 24V equipment, or move the run to PoE.