When higher power levels are being carried on data cabling, derating your cable may sometimes be necessary.

Depending on the installation environment, some channel parameters may need to be adjusted or derated to ensure that the data cabling performs as intended. When you derate a cable, you don’t get the full performance benefits – such as reach – that it offers.

Certain conditions prevent a cable from providing its advertised performance. In these cases, the cable must be operated at less than its rated maximum capability to avoid performance issues (this is what’s known as “cable derating”).

One condition that creates the need for cable derating is high cable temperatures. When a cable’s temperature rises, cabling resistance and insertion loss also increase. Since adjusting the input power is not possible, the cabling in the channel must have sufficient insertion loss margin. Without this added margin, the only option is to shorten the channel (derate the cable) to meet the insertion loss limit.

The Impact of PoE on Cable Temperature

With IEEE 802.3bt (the 100W Power over Ethernet (PoE) standard) now approved, paying attention to cable temperatures will be even more important. Along with these much-needed, higher PoE wattage levels come higher current levels that are being transported through the cable. This increases the amount of power dissipated in the cabling, creating higher temperature conditions, which could cause cable performance issues.

So when can a channel fulfill its 100 m reach (the channel length recommended by industry standards) under higher temperatures without cable derating? Only when the manufacturer designs and manufactures the cable to do so.

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Cable Performance Under High Temperatures

To prove this theory, Belden conducted its own series of cable performance tests. Belden cables, along with a handful of other well-known industry cables, were placed in an environmental chamber to measure cable temperature change and associated insertion loss.

These temperature and insertion-loss changes were recorded. Then, the data gathered during these tests was combined with insertion-loss levels of connectors and patch cords to determine the maximum length that a typical channel could still reach while maintaining channel insertion loss compliance.

Belden used a full 100 m channel with 10 m of patch cords and an initial permanent link length of 90 m. During these tests, we assumed that the connectors and patch cords were being used at room temperature with the same insertion-loss levels. Permanent links were assumed to be at 60 degrees C (to align with TSB-184-A, where the ambient temperature is 45 degrees C and temperature rise due to PoE current and cable bundling is 15 degrees C).

The end result? We found that Belden’s 10GXS Category 6A Cable offers 8% insertion loss margin. This extra margin ensures that your channel will still reach 100 m under maximum PoE load – even when bundled and faced with higher cable temperatures.

When this cable gets hot, whether it’s from high current levels due to PoE or because of tightly packed cable trays and the lack of heat dissipation, it may reach the maximum operating temperature of 60 degrees C – but there’s still enough insertion loss margin to reach the full 100 m without cable derating.

In comparison, most other Category 6A cables have an insertion loss margin of 3% – which isn’t high enough to allow you to obtain the full 100 m reach. In fact, the guidance from ANSI/TIA-568.2-D, Appendix I, states that the maximum channel length under the same conditions would be 85 m.

A Note About 22 AWG Cable

Although 22 AWG cable has less resistance, it doesn’t have enough insertion loss margin to support PoE – so a channel using 22AWG cable would have to be derated in full PoE load situations.

Want to learn more about achieving a 100 m channel length with a full PoE load? Send us your questions or comment below. We’re happy to help!

An up-close look at recent PoE Legislation