PoE & voltage drop calculator
Run the numbers before you pull cable. PoE runs are checked against the 100m limit and the device budget; 12V runs for volt drop in the copper. A failing run comes back with the fix.
Run OK
- 30m run — inside the 100m Ethernet limit.
- 6W load on 802.3af (PoE): 12.95W device budget, 6.9W spare.
- And check the switch total: plenty of 8-port switches can’t carry eight full-draw cameras across every port.
PoE switches from stock
All networking- 4 port4 Port Gigabit POE Extender 1000M Network Switch Repeater IEEE802.3af/at Plug&Play for PoE Switch NVR IP Camera APAvailable
- 16 portDahua PFS4218-16ET-240 Desktop , 16-Port Managed 2-Layer PoE Switch, 16 × RJ45 10-100M, 240WAvailable
Ports are the easy bit. Add up the cameras’ draw and spec the switch on its total PoE budget, not the per-port rating — the total runs out first.
Rules of thumb for the install
PoE: 100m max over Cat5e/Cat6. The standard sets the device budget — 12.95W at the camera on 802.3af, 25.5W on 802.3at, 51W on 802.3bt. On site it is rarely one camera that trips it; it is the switch total. Add up every camera on the switch and check the sum against the whole-unit PoE budget, not the per-port figure.
12V DC: copper resistance is what kills it. Volt drop climbs with run length, and you count the run twice (out and back); every step up in conductor size halves it. Expect most cameras to cope with a sag of about −10% (10.8V). The catch is night: with the IR on, draw can double, so a run that tests fine at 3pm browns out at midnight. Failing run? Move it to PoE, go to 24V kit, put the PSU nearer the camera, or pull heavier cable.