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High temps and cable

One way to beat the heat is to install cables with additional headroom for Insertion Loss.


September 1, 2006  


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I was driving home one evening in July when I received a call from Paul Barker reminding me that a column was due by the end of the week. It was sweltering outside and I was fortunate to have air conditioning in the car to beat the heat of summer.

It gave me the idea for the theme of this month’s article, namely what effects does high temperatures have on the performance of cabling? It is an important factor to consider when designing and installing a network cabling system.

The environment where a cable is installed can be susceptible to high temperatures. A study by the University of Berkley showed that the temperature in the ceiling space under a roof can be as high as 50C (122F), even if the building is air-conditioned.

Cables installed in this ceiling space will experience some degradation in performance. This is recognized in the TIA standard for Category 6 cabling. Annex G of ANSI/TIA/EIA-568-B.2-1-2002 provides a table for horizontal cable length de-rating for different temperatures. As an example, if a cable is installed in an environment where the temperature averaged over the length of the cable is as high as 50C, then the maximum horizontal cable distance should be reduced from 90 meters to 79.5 meters.

This is because the cable Insertion Loss increases about 4% for every 10C temperature rise.

One way to beat the heat is to install cables with additional headroom for Insertion Loss. Check the manufacturer’s specifications for Insertion Loss headroom beyond the minimum requirements of the standard.

A 12% headroom will allow for a temperature increase of 30C above the ambient temperature of 20C. Manufacturers are able to offer more headroom by designing cables with a larger conductor diameter than the nominal 24 AWG specified in the standard.

Many high performance Category 6 cables in the market today are manufactured using 23 AWG conductors, including augmented Category 6 cables for the emerging 10 Gb/s Ethernet application.

Also, the choice of insulation and jacketing materials can significantly affect the Insertion Loss performance at higher temperatures and higher frequencies. For example, limited combustible cables exhibit significantly less variation with temperature.

I have also been asked about the effect of low temperatures on cable performance. In general, the transmission performance improves as the temperature decreases. It works in your favor. On the other hand, the mechanical handling characteristics of cables are worse at low temperatures. The cable is stiffer and more difficult to work with.

There is another emerging standard that can impact cabling performance and can raise the heat, so to speak. IEEE 802.3at task force is working on the next generation of “Power over Ethernet” standard called PoE Plus.

The objective is to be able to deliver up to 30 Watts of power over 2-pairs and possibly as high as 50 Watts when powering over all 4-pairs.

The power is applied as a direct current (DC) power with a voltage of 48 volts across 2-pairs at the power sourcing equipment (PSE). The DC power is applied coincident with the data signal. The PoE standard today limits the power to a maximum of 15 watts at the PSE, which translates into a current of 0.175 Amps per conductor. The intent in the standards group is to be able to increase the current level to as high as 0.42 Amps per conductor.

The laws of physics dictate that the higher the current flowing in the conductor, the higher the heating effect on the cable. The heating effect is proportional to the square of the current (I2) times the DC Resistance (Rdc) or I2Rdc.

The results of some recent tests on the heating effects of cables can be found at the IEEE 802.3 at web site. They show that a current of 0.42 Amps per conductor can heat the cable from 8C to 15C above ambient, depending on the cable design.

Cable designs using larger conductor diameters can support higher currents with less heating. This heating effect can be easily estimated from the DC resistance (Rdc), which is proportional to the cross-sectional area of the copper (see Table above).

Can my network cabling survive the temperature extremes and deliver the power required without overheating? It is something to think about on that drive home in the heat of summer. On second thought, leave it to the experts and take some time to relax with a cold drink on a hot September day.

Gauge Size Cond. Dia. (mm) Cond. Area (mm2) Rdc nominal (?/m) Heating Effect
24 0.511 0.205 8.4 100 %
23 0.574 0.259 6.7 79%
22 0.643 0.324 5.3 63 %

Paul Kish is Director, Systems & Standards, at Belden CDT.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence