1. Current carrying capacity of conductors
Dipartimento di ELETTROTECNICA ed AUTOMAZIONE
Istituto Tecnico Industriale A. MALIGNANI - UDINE
Current carrying capacity of conductors

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All cables have electrical resistance, so there must be an energy loss when they carry current and this loss appears as heat… …and the temperature of the cable rises.

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The rate of heat loss is a function of the difference between the conductor and the surrounding temperatures, so as the conductor temperature rises, so does the rate at which it loses heat.

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A cable carrying a steady current, which produces a fixed heating effect, will get hotter until it reaches the balance temperature where heat input is equal to heat loss (see figure).

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The final temperature achieved by the cable will thus depends on the current carried, how easily heat is dissipated from the cable and the temperature of the cable surroundings. P.V.C. is probably the most usual form of insulation, and is very susceptible to damage by high temperatures. At about 80°C the material becomes soft, at about 115°C P.V.C. becomes corrosive to cooper and aluminium.

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It is very important that P.V.C.. insulation should not be allowed normally to exceed 70°C, so the current ratings of cables are designed to ensure that this will not happen; higher temperatures (up to 160°C) are permitted under very short term fault conditions.

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It should be noticed that the cable voltage drop will be higher if more current is carried. Insulation cannot tolerate high temperatures since they will lead to deterioration and eventually failure.

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There are different Tables and columns for different types of cables, with differing conditions of installation, degrees of grouping and so on For example, mineral insulation does not deteriorate, even at very high temperatures.

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Methods of cable installation We have seen in the previous subsection that the rating of a cable depends on its ability to lose the heat produced in it by the current it carries. This ability depends to some extent on the way the cable is installed.

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For example, a cable clipped to a surface will more easily be able to dissipate heat than a similar cable which is installed with others in a conduit.

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Determination of the current-carrying
capacity value
According to the type of cable installation, tables T3/1 and T3/2 make it possible to determine the current-carrying capacities in continuous operation, defined in the standard as lz, in the copper or aluminium conductors and cables in free air and in ground installation in compliance with the IEC Standard.

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Determination of the current-carrying capacity value
When there are several circuits close together, or ambient temperatures other than the standard values (30°C for installation in free air and 20°C for installation in the ground), the capacity values shown in table T3/2 must be multiplied by the correction factors indicated in the tables of paragraphs 3.2. and 3.3.

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Now I’ll show you a simple example of calculation...

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And now...
Thank you !