Cable losses in power-over-ethernet jobs revealed

THE EXTENT of energy losses in the cabling of power-over-ethernet projects has been revealed by the US Government.

In what will be seen a blow to the reputation of the emerging technology of PoE, the report by the US Department of Energy says that the low voltages involved and the relatively high currents combined with small-gauge  ethernet cables could compromise the energy savings from LED lighting.

The authors of the study say that cable runs in connected lighting installations could experience more than 5 per cent power loss over runs longer than 50m.

The DOE tested a variety of cable at different lengths and measured power lost due to the resistance of the wiring. Such cables – generically referred to as Cat5 or Cat6, sometimes with alphabetic suffixes that specify other features such as fire rating or shielding –  have varying resistance to direct current, but few cable manufacturers publish this data.

The American National Standards Institute standards body, however, has published a standard that specifies some characteristics of cables that would generally meet the needs of high-speed ethernet signals and PoE.

For the most part, the insitute’s work focused on minimum diameters of individual wires or what is commonly called American Wire Gauge in the US.

The Department of Energy measurements reveal that for installations that use ANSI-compliant cable and keep cable runs to a maximum of 50m, losses will be capped at 5 per cent. Lower losses would be better and could be accomplished by shorter runs or larger wires.

It’s worth noting that AC power distribution also suffers losses, but with higher voltage levels and lower current, the transmission loss is lower than in the DC case.

Still, the centralised power conversion in a PoE system can offer better efficiency relative to AC/DC conversion in a driver located in each luminaire.

Power over ethernet as a concept has been gaining ground as a concept in recent years. It allows both power and data to be conveyed over standard ethernet cables. The luminaires receive low-voltage power – usually 24V – from the cables and data acquired by the sensors – such as occupancy detectors – in the lighting fixtures provides information about the building's use and operations to building managers. 

Earlier this month one of the largest European power-over-ethernet projects has opened in Copenhagen. Over 400 LED luminaires at the ‘smart office’ of the Albertslund Municipality are connected, powered and communicated with using Ethernet cables.


  • Read the full PoE report HERE.
  • This year's Lighting Fixture Design  Conference takes place on 20 and 21 June 2018 . Organised by Lux and LEDs magazine, the event takes place at the Cavendish Conference Centre in London. For more information and to reserve you place, click HERE.





Comments 1

This makes a lot of sense. In applications where the load is relatively low, POE could make sense and in applications where there is a need for data on top of the lighting requirement. However, where there is a lighting only need and the luminous flux required is relatively high say above 1000 to 1500 lumens per point POE is going to be awkward as the number of luminaires per port becomes quite low. So the moment we approach commercial lighting outputs we are pretty much down to one luminaire per port which expands POE rack sizes and cabling requirements dramatically. In low flux applications, mostly residential the expense of POE switches could be prohibitive. Every project would need a very good cost-benefit calculation a concept stage.

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