Ing. Stefano Comuzzi I.T.I. Malignani - Udine Current carrying capacity of conductors Dipartimento di ELETTROTECNICA ed AUTOMAZIONE Istituto Tecnico Industriale

Ing. Stefano Comuzzi I.T.I. Malignani - Udine

Current carrying capacityof conductors

Dipartimento di ELETTROTECNICA ed AUTOMAZIONE

Istituto Tecnico Industriale A. MALIGNANI - UDINE


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.



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.



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).



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.



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.



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.



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.



Methods of cable installationMethods 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.



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.




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 3645-523 Standard.



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.





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



And now...

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Ing. Stefano Comuzzi I.T.I. Malignani - Udine Current carrying capacity of conductors Dipartimento di ELETTROTECNICA ed AUTOMAZIONE Istituto Tecnico Industriale