POE+ : The effects of increased current on cable bundles

Heating effect, bundle size and channel lengths

It has been apparent for a number of years that the number of devices powered via the LAN using Power over Ethernet (POE) has been on the increase.

The advantages of POE to the network manager are enormous but in order to increase the number of devices that can be powered, this requires an increase in available power from the current 48v 400mA (12.95W). Some equipment manufacturers have already “tweaked” this limit to 15W and possibly higher.

POEHowever, this increase in power can have an undesirable heating effect on the copper conductors which posed a problem for the IEEE committee responsible.

Nexans, being an active member of the committee offered to investigate the problem and extensive research was carried out at the Nexans Research Centre in France.

After presentation of the Nexans results, the IEEE committee were able to establish that a new POE+ standard delivering 57v and 600mA (25W) was achievable whilst limiting the temperature rise to 10 degC. This was on the basis that for Cat5e and higher cabling systems, the maximum cable bundle should be limited to 100 cables.

The advantages are clear and will overcome several obstacles:

  • Central power management and backup
  • Remote management of the device
  • Removal of power transformers and consequential heating effect

However, there will be a limited heating effect to the building itself as the cable runs will still warm up. It will also be necessary to ensure existing cable runs do not exceed the 100 cables per bundle limit and a site audit may be necessary.

Benefits of Higher Performance Cabling

Additional results from the Nexans research demonstrated that if a Category 7A cable were to be used, the temperature rise would be negligible (at approx 3 degC).

The use of higher performance Category 7A cabling therefore provides the following benefits:

  1. Less energy is required to provide the extra cooling needed to compensate for the heat generated in the cabling system.
  2. the number of cables in the bundle could be increased.
  3. the cabling system has the potential to double the power supported to 50W.