Power over Ethernet: sizing the budget for cameras, phones and access points.

Power over Ethernet carries power and data on one cable, which makes installation cleaner. The catch is that switches have a total power limit that is often lower than the sum of all ports.

The standards

• 802.3af (PoE): up to about 15 watts at the switch port per device.
• 802.3at (PoE+): up to about 30 watts.
• 802.3bt (PoE++): up to about 60 and 90 watts, for PTZ cameras, large access points and thin clients.

Devices are assigned classes, and the switch allocates power by class or by actual draw.

The budget problem

A 48-port switch is not necessarily able to deliver full power on all 48 ports. Its power supply sets the total. If you plug in more demand than the budget, some devices lose power or never start.

How to size

1. List each powered device and its maximum draw, not its average.
2. Add them up per switch, and add headroom for growth.
3. Compare with the switch’s total PoE budget.
4. Consider infrared illumination, heaters and PTZ motors, which raise the draw in cold or dark conditions.

Other considerations

Cable quality matters, since poor cable loses power as heat. Use a UPS for the switch, since cameras and phones fail with it. Monitor PoE usage so you see trouble before users do.

A rule of thumb

Plan to use no more than about 80 per cent of the budget on day one.