What is the operating temperature for electrical cable?

The operating temperature of an electrical cable normally refers to the minimum and maximum temperature that the cable can safely operate at for a sustained period of time. This operating temperature is determined by the insulation material around the cable. 

The ability of an insulation material to withstand a temperature over an acceptable life time gives rise to the continuous conductor operating temperature of a cable. 

Different materials are able to withstand different temperatures, under the assumption of full load. All materials will deteriorate given long term exposure to these temperatures, and will deteriorate at a faster rate at elevated temperature in accordance with the Arrhenius equation, which illustrates the temperature dependence of chemical reactions over time. 

Given that this applies to all cable insulation materials, it's important to understand the long-term stability of these materials to allow an estimation of operational lifetime. This is backed up by evidence of materials such as PVC, PE and XLPE that shows that these lifetime expectancies are valid. 

Typically, 'general purpose' thermoplastic materials such as PE or PVC insulation materials are rated at +70°C. This is because at higher temperatures the materials becomes structurally softer and can be deformed. Thermosetting materials (such as EPR and XLPE), which have undergone chemical cross-linking to create greater thermal stability and structure can be rated at a higher temperature, typically +90°C. Both of these types of materials are suitably thermally resistant to accommodate these temperatures over the expected life of the cable.

So each material type will have an upper and lower range of temperatures within which it is suitable for use. Generally, materials soften at higher temperatures and become rigid at lower temperatures (making some materials less suitable for applications involving flexing at either low or high temperatures).

Considerations need to be given to where these temperatures are exceeded. There is ongoing work to evaluate higher temperature thermoplastic insulation materials and also there are a number of special insulation materials that can withstand significantly higher temperatures. Silicone rubber for instance typically has a temperature range of -60°C to 180°C for fixed applications

Why is the continuous conductor operating temperature important?

The continuous conductor operating temperature rating is important because, as well as being directly related to the acceptable life of a cable, it is directly related to the Current Rating of a cable.  It is due to the greater thermal stability of the insulation with thermosetting materials that allows the conductor to become hotter before risk of thermal degradation, and because of this a higher ampacity is possible at +90°C than at +70°C.

 

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