What is a cable’s de-rating factor?
Cables de-rating factor is a factor used to assess a cable’s safe power dissipation capability at high temperatures. De-rating factor, also known as correction factor, is a multiplier that is applied to an ampacity rating in order to change the value depending on various operation conditions.
The multiplier might be less than one, equal to one, or more than one. Cables are designed for normal or specified environmental conditions, assuming adequate ventilation. De-rating is a method of designing a cable for high-performance systems.
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What is the significance of the De-rating factor?
A cable in service without correction factors may fail catastrophically.
If the temperature of the surrounding environment rises, or if the cable is needed to be utilized in temperatures, humidity, enclosures, or other circumstances outside of its range, it will not operate to its full potential. The de-rating factor is used to illustrate how much the cable may be stressed/used under such conditions.
This may be multiplied by actual design ratings. Under these situations, derating factors or correction factors are utilized to generate limits for the cable.
De-rating manage the cable’s current carrying capacity and increases the safety margin between the cable’s specification limit and the maximum load, providing extra protection and guaranteeing that the cable does not fail when temperature or other external conditions change.
- When we make proper cable derating, we are lowering the maximum current that cables are authorized to carry. Its purpose is to protect the insulation of the wire against deterioration caused by extreme heat.
Derating factor and cable temperature
When current flows through a wire, it generates heat. The quantity of heat produced increases as current levels climb. The National Electrical Code (NEC) mandates conductor derating in two situations: when the ambient temperature reaches 30 degrees Celsius and when more than three cables are bundled in a conduit.
Because of electromagnetic and physical proximity effects, many circuits operating in close proximity may raise the temperature of the conductors. When cables are stacked close together or deployed in a temperature-varying environment, their ability to transfer heat and achieve a higher working temperature is hampered.
Cable current ampacity is determined according to a specific ambient or ground temperature. If the temperature of the laying ground or the ambient is not the same as the design one, Then temperature derating is necessary to enhance the cable operation and safety.
All of these factors must be taken into account when deciding the cable de-rating that will keep cable temperatures within design requirements while enhancing cable insulation life.
Why is a de-rating factor used for cable sizing?
Cable selection comprises choosing the suitable conductor type as well as the appropriate conductor size/cross-section area/diameter dependent on the application. To begin with, it is vital to understand the relevance of cable size and selection.
Following that, the selection criteria will be assessed, taking into consideration all of the derating elements that may have an influence on cable ampacity.
Aside from the size of the conductor, a variety of additional conductor kinds will be studied.
So, what exactly is ampacity? The maximum current that an insulated conductor may safely take without exceeding its insulator and sheath temperature limits is described as ampacity.
External disturbances, such as cable ampacity, influence a cable’s current rating. In such cases, existing ratings must be enhanced by including particular relevant criteria known as derating factors.
Because there are several derating factors, the values of all derating factors are multiplied to provide an average value. The key derating considerations to consider while choosing cable size are as follows.
The quantity of heat produced increases as the amount of current flowing through a conductor increases. The conductor’s heat must be evacuated into the atmosphere. If the heat does not escape, the conductor’s temperature will rise until it exceeds the cable’s temperature tolerance, at which point the cable will fail.
The ampacity of a cable should be equal to or greater than the maximum current that it is expected to carry during its service life while not exceeding its temperature rating.
The temperature rating is determined by the heat resistance of the components used in the cable’s coating and sheath. The greater the thermal electrical resistivity, the less likely it is to fail at higher temperatures. Cable ampacity derating is also applied for both ambient and ground temperature.
The cable manufacturers provide derating factors for a variety of options, including the number of installed cables in a layer, the number of layers, and cable spacing horizontally and vertically. The true situation is the result of an insufficient mix of cable installation and cable manufacturer characteristics.
International standards (IEC, ERA, etc.) have been developed to address this issue. Similarly, rules established a deduction factor for a group of cables in a cable ladder or inside a duct bank.
It is common for cables to encounter a variety of climatic conditions as they travel. The environment with the largest rated current derating factor should be chosen and applied to the whole cable path. This criterion is generally eased if the length of the cable route is less than 0.35 m
Ground Thermal Resistance
Cables buried in the ground must emit heat into the environment.
Soil heat conductivity varies substantially based on soil characteristics such as closeness to a water source, coastal area, dry soil, and desert sand.
The higher the level of thermal resistance, the more difficult it is to remove heat from the cable. As a result, the cable size becomes a significant consideration when applying the de-rating factor.
Underground Laying Depth
Cables laying depth is another derating factor. Each cable has a designed laying depth, If the laying depth is not the same as the designed value, then you should apply Laying Depth factor.
What variables influence cable derating?
Rated current may be carried by a conductor section size without causing cable insulation damage. The rated current will be specified in the product catalogue of the cable manufacturer.
The rated current should be read in relation to the surrounding conditions. However, actual site conditions are seldom the same as what the manufacturer states.
Furthermore, there will be times throughout a project when cable data is unavailable. IEC standards also include cable ampacity, although under their normal ambient and installation circumstances.
Ambient Air Temperature, Ground Temperature, Ground Thermal Resistivity, and Cable Arrangement are the cable derating variables to consider.
Derating Factor Calculation
Consider a 95 mm2 electrical cable. If we bury this wire in 30°C soil at a laying depth of 1.35 m and a thermal resistance of 140°C/watt.
The ampacity of the cable should then be multiplied by three de-rating factors. The first is the laying depth, followed by the soil temperature, and lastly the soil resistivity.
From derating factors tables, Total derating factor = 0.27 = 0.70 (laying depth) *0.56 (soil temperature) *0.69 (soil resistivity).
The result may then be multiplied by the cable current. The generated current should be proportionate to the current carrying capacity of the cable.
After applying all derating factors we check if the cable still suitable to the load current. If the cable new ampacity is less than the load current, then we choose a larger size cable. Then we apply the same derating factor again to the new cable current and check if it is suitable to the load current.
Medium voltage cable de-rating factor.
Derating factors and tables of medium voltage cables are the same as the low voltage ones.
Current rates for medium voltage cables are established by the International Electro-technical Commission standard IEC 60502. The derating factor is used to calculate the cable rating.
Voltages up to 1 kV are covered in Part 1 of the IEC 60502 standard, whereas voltages from 1 kV to 30 kV are covered in Part 2. Part 1 does not include any current capacity sizing since IEC 60364 is expected to take care of it. Component 2 Annex B provides a method for calculating current capacity at voltages covered by this component.
The current rating tables include three phase wires placed both above and below ground. Derating considerations for buried cables include ambient temperature, depth of laying, and soil thermal resistance, as well as cable grouping.
There are also multiple de-rating factors for various variables.
As an example,
- The de-rating factors for MV cables may be found in the standard’s related tables.
- A table will provide the de-rating factor for ambient ground temperature.
- A table will also include the reduction factor for cable grouping in the ground.
- A table will also provide a de-rating factor for soil thermal resistance.
- Table will also include a de-rating factor for laying depth.
- The same is true for wires that are above ground.
To recapitulate, the first step in incorporating rating elements into the ampacity value of a cable is to identify and grasp the application, as well as to consult the standards or authority with jurisdiction. Using this knowledge, choosing the best conductor size for your application can save you time and money on unnecessary repairs or material replacement.
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