PoE & voltage drop calculator
Check every camera run before the cable is specified. For PoE, the run length is compared with 100m and the camera’s draw with its power budget; for 12V, the voltage dropped along the copper is calculated. Any failing run is returned with the corrective measure.
Run acceptable
- At 30m, the run complies with the 100m Ethernet limit.
- 6W required against a 12.95W 802.3af (PoE) device budget: 6.9W headroom.
- Please also check the switch total, as many 8-port switches cannot supply eight cameras at their maximum draw.
PoE switches available
View all networking- 4 portGoodTop PoE-Extender-Repeater-4-Port-Gigabit-1000M 4 Port PoE Extender 1000M, 100m Reach£14.33
- 8 portHikvision DS-3E0510P-E 8-Port Gigabit PoE Unmanaged Switch, SFP Uplink, 110W Budget£77.59
- 9 portHikvision DS-3E0109P-E/M 9-Port Unmanaged PoE Switch, 8×PoE 100Mbps, Extend Mode 300m£55.44
- 16 portHikvision DS-3E0518P-E 16-Port Gigabit PoE Unmanaged Switch, SFP Uplink, 230W Budget£196.47
- 24 portHikvision DS-3E0326P-E/M(B) 24-Port Unmanaged PoE Network Switch, Gigabit Uplinks£198.69
Specify the switch by its total PoE budget rather than by port count: the combined draw of the cameras must fall within it; port ratings alone do not show this.
Basis of the calculation
The maximum PoE run is 100m on Cat5e or Cat6 cable. Device power by standard: 802.3af provides 12.95W, 802.3at provides 25.5W and 802.3bt provides 51W. On most jobs the constraint is the switch rather than an individual camera, so the draws of all cameras on a switch should be totalled and checked against its whole-unit PoE budget, rather than the per-port rating.
On 12V DC, the limiting factor is the resistance of the copper. The drop increases with run length, counted out and back, and moving up one conductor size halves it. Most cameras operate down to about −10%, or 10.8V, but infrared at night can double the draw, so a run that tests correctly at 3pm can fail to hold the camera at midnight. Corrective options: move the run to PoE, specify 24V equipment, site a PSU at the camera end, or install thicker cable.
