By R. (Dick) Hardie Pr Eng
Technical Marketing Manager
Aberdare Cables (Pty) Ltd
Good engineering design has always incorporated factors of safety. For mechanical design, the maximum design load would typically range from 25% of the ultimate tensile strength (U.T.S.) of the material, up to perhaps 70% of the U.T.S. In civil engineering, the Factors of Safety are sometimes even more conservative, for example in the design of concrete structures. A similar philosophy is required when selecting electrical components and equipment in order to design reliably, thus preventing overheating and eventual failure. In so doing, reliable engineering design implies that the equipment will never be called upon to perform beyond its rated capacity.
CURRENT CARRYING CAPACITY OF ELECTRIC CABLE
The ACTUAL current rating of an electric cable is based on the thermal environment in which the cable is installed. In particular, the ambient temperature, depth of burial in the ground, presence and spacing in relation to other cables or other heat sources, and type of soil will have a profound effect on the actual current rating of an electric cable. After the application of applicable derating factors (see tables below) the cable’s actual current rating will usually be lower than the standard (un-derated) value quoted by manufacturer’s brochures.
To assist in determining the rated capacity of motors, transformers and cables, there are a number of formulae, charts and tables available from the equipment manufacturer. These allow compensation for factors which may enhance or detract from the rated capacity of the equipment. In the case of electric cables, manufacturers quote and publish tables of current ratings which are based on standard conditions of installation, not taking any of the derating factors, mentioned above, into account (ie: un-derated; based on standard conditions of installation).
Table 1 Parameters for standard current rating (as published by manufacturer) for low voltage cable
|Maximum sustained conductor temperature||70˚||90˚|
|Ambient air temperature (free Air shaded)||30˚||30˚|
|Ground thermal resistivity||1,2 K.m/W||1,2 K.m/W|
|Depth of laying to top of cable or duct (Low Voltage Cable)||500mm||500mm|
DERATING FACTORS FOR NON STANDARD CONDITIONS FOR LOW VOLATGE CABLE(applicable to multi-core cable up to 300mm2)
Table 2.1 Derating factors for depth of laying
|Depth of laying(mm)||Direct in ground||In single way ducts|
Table 2.2 Derating factors for ground thermal resistivity
|Thermal Resistivity(K.m/W)||Direct in ground||In single way ducts|
Table 2.3 Derating factors for grouping of cables in horizontal formation, at standard depths of laying and in standard soil conditions
|No of||Direct in ground||In Single way ducts|
|cables||Axial spacing (mm)||Axial spacing (mm)|
Table 2.4 Derating factors for ground temperature
|Maximum||Ground Temperatures (°C )|
Table 2.5 Derating factors for air temperature
|Maximum||Air Temperatures (°C )|
Table 2.6 Derating factors for grouping of multicore cable installed horizontally in the air
|No. of cables||1||2||3||6||9|
|Clearance D* between cables||1||0.95||0.9||0.88||0.85|
D* is the overall diameter of one cable
Cables may be grouped in air without derating, provided that the cables are installed on ladders, and that:-
(a) For horizontal formationThe clearance is greater than 6 x D (or 150mm, whichever is the least) for multi core cables, and 2 x D or 150mm for single core cables.
(b) For vertical formation
(i) The clearance from a vertical wall is greater than 20mm, and
(ii) The vertical clearance between cables is greater than 150mm.
(c) If the number of cables > 4, they are installed in the horizontal plane.
Table 2.7 Derating factors for exposure to solar radiation
|Cross-sectional area ofConductormm²||Correction Factor|
|1000 W/m²(Coastal)||1250 W/m²(Highveld)|
|1,5 – 10 16 – 35 50 – 95 120 – 400 240 – 400||0,700,680,650,620,59||0,620,570,530,490,44|
The correction factor applicable to cables exposed to direct solar radiation (ie: exposed to sun) is an extremely important derating factor which is often overlooked.
Assume that we need to select a Low Voltage PVC insulated electric cable to be installed 1250mm below ground, the cable will be together with 3 other similar cables in the trench, the soil thermal resistivity has been measured, and the sand was found to have a thermal resistivity of 2,5 K.m/W. The slope of the land is north facing, as a result the soil temperature can reach a temperature of 35°C on hot summer days.
Calculating the overall derating factor
Depth of burial: Table 2.1 yields a derating factor of 0.94
Group derating for 4 cables touching: Table 2.3 yield a derating factor of 0.63
Ground thermal resistances: Table 2.2 yields a derating factor of 0.78
Ground temperature: Table 2.4 yields a derating factor of 0.9
The overall derating factor is 0,94 x 0,63 x 0,78 x 0,9 = 0,41
Now apply this factor to the standard current rating tables as published by electric cable manufacturers.
It soon becomes apparent, that a cable with a current rating under “standard” conditions of say 100 Amps, would only have a current rating of 41 Amps under the conditions listed above.In other words, such a cable will be fully loaded when carrying only 41 Amps.
For Information on the INTERMITTENT LOADING OF ELECTRIC CABLE