Each cable has a current carrying capacity (or cable ampacity). The value of this current is given in cable ampacity tables and depends on some factors which are called derating or correction factors.
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What is the Derating factor in cable sizing?
The derating factor of a cable is a numerical value that is used to adjust the current-carrying capacity of the cable based on various factors such as ambient temperature, installation method, depth of installation, and the presence of other cables in proximity.
It is essentially a safety margin applied to the ampacity of the cable to ensure that it operates within its temperature limits.
The specific derating factors can vary depending on the type of cable, the installation environment, and the standards or regulations being followed.
These factors are usually provided by cable manufacturers or specified in industry standards such as the National Electrical Code (NEC) in the United States or the International Electrotechnical Commission (IEC) standards.
For example, when considering the ambient temperature, if the cable is expected to operate in a higher temperature environment than the standard rating, a derating factor is applied to reduce the current-carrying capacity of the cable.
The derating factor accounts for the decrease in the cable’s ability to dissipate heat, preventing it from reaching temperatures that could cause damage or create a safety hazard.
It is crucial to consult the specific standards and guidelines relevant to the installation to determine the appropriate derating factors and ensure that the cables are sized correctly to handle the expected operating conditions safely and efficiently.
Why is cable derating necessary?
Cable derating is necessary to ensure that electrical cables operate within their specified temperature limits, thereby promoting safety and preventing potential damage or hazards. Several factors can lead to the need for cable derating:
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Ambient temperature: High ambient temperatures can decrease a cable’s ability to dissipate heat. When a cable operates in such conditions, the current-carrying capacity must be reduced to prevent the cable from exceeding its temperature limits.
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Grouping of cables: When multiple cables are placed close together, they can affect each other’s ability to dissipate heat. This proximity can lead to an increase in temperature, necessitating a derating factor to ensure that the combined heat does not surpass the temperature limits of the cables.
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Depth of installation: Cables installed at greater depths might experience higher temperatures due to the surrounding environment. In such cases, derating is necessary to prevent overheating.
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Type of installation: The method of cable installation, such as direct burial, conduit, or cable trays, can influence heat dissipation. Depending on the installation type, derating may be required to maintain safe operating temperatures.
By applying the appropriate derating factors, engineers can adjust the current-carrying capacity of the cable to align with the specific environmental and installation conditions.
This precautionary measure ensures that the cable does not overheat, minimizing the risk of electrical failures, fire hazards, or damage to the insulation and other components. It is a crucial practice to guarantee the safe and reliable operation of electrical systems.
For more information about cable ampacity, you can find it in my other article here.
When should you do derating for cables?
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 circuits are close to each other, they can make the wires get hot because of electricity and how they’re positioned.
If you put cables close together or use them in places where the temperature changes a lot, they won’t be able to get as hot or work as well.
To figure out how much electricity a cable can handle, we need to know the temperature around it. If the actual temperature is different from what it’s designed for, we have to make adjustments to keep the cable safe and working properly.
So, we have to consider all these things when we decide how much we should adjust the cable’s capacity to make sure it doesn’t get too hot and lasts a long time.
What will happen if I don’t derate my cable?
If you don’t adjust your cable’s capacity, it can get too hot because it’s trying to carry too much electricity. The more electricity it has to carry beyond its capacity, the hotter it gets.
When we adjust the cable’s capacity, we’re basically making sure it can handle the electricity without getting too hot. This helps protect the cable covering from getting damaged by excessive heat.
The main reason we adjust cables is to prevent them from overheating. When electricity flows through a cable, it encounters resistance, which creates heat. If there are many cables close together, they can make each other hotter because of how they interact physically and electrically.
As the temperature of the cable rises, so does the linear resistance, resulting in a higher voltage drop and decreased system output.
When cable derating is not required?
Cable derating is not always necessary and may not be required in certain circumstances. Here are some situations when cable derating might not be necessary:
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Operating within standard temperature limits: If the cables are expected to operate within their standard temperature limits and there are no significant factors affecting their heat dissipation, derating may not be required.
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Low ambient temperatures: In environments where the ambient temperatures are low and well below the maximum temperature rating of the cable, derating may not be necessary.
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Single cable installations: If cables are installed individually and not in close proximity to other heat-generating sources, the need for derating might be minimal.
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Shallow cable installations: In cases where cables are installed at shallow depths, the surrounding environment might not significantly affect their temperature, negating the need for derating.
However, it is essential to consult relevant industry standards, local electrical codes, and specific project requirements to determine whether derating is necessary.
Even if derating might not be explicitly required in some situations, it is important to consider the potential environmental and installation factors that could affect the cables’ performance and safety.
Always ensure that the installation adheres to safety regulations and best practices to maintain the reliability and longevity of the electrical system.
Various factors of derating cables
There are various factors that can necessitate the derating of cables, including:
Air ambient temperature derating:
Cable temperature is one of the most important factors which affect current carrying capacity. The higher the temperature, the lower the current the cable can carry.
The ampacity of a cable should be equal to or greater than the maximum current that it’s 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.
Cable ampacity derating is also applied for both ambient and ground temperatures.
Ground temperature derating:
Just like the air temperature, the ground temperature affects electrical cables’ ampacity.
Burial depth of cable:
Buried cables depend on soil to reduce cable temperature. So the current of the cable should be derated according to soil temperature to protect the cable from overheating.
Each cable has a designed laying depth, If the laying depth is not the same as the designed value, then you should apply the Laying Depth factor.
Soil thermal resistivity correction table:
Soil thermal resistivity is one of the most important factors in cable laying underground. This importance is because it affects directly soil conductivity and its ability to reduce cable temperature.
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.
Cables formation factors
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. In the below tables, the formation of the cables affects their current capacity.
How to calculate cable derating?
Say we have 95 mm2 cable. If we lay this cable underground in soil with a temperature of 40°C and thermal resistivity of 150°C. cm/watt and the laying depth is 1.5 m.
Then the cable ampacity should be multiplied by three correction factors.
- laying depth (K1)
- soil temperature (K2)
- soil resistivity (K3)
Derating factor formula = Cable current * K1*K2*K3*K4…*Kn, While K’s are the derating factors of the cable.
Total derating factors = K1*K2*K3 = 0.95*0.9*0.91 =0.77, Then multiply this value by the cable current. The resulting current should be the new current carrying capacity for the cable.
After applying all derating factors we check if the cable is still suitable for the load current. If the cable’s 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 for the load current.
When Should I apply cable temperature derating?
According to the National Electrical Code (NEC), temperature derating must be applied in two circumstances: when the ambient temperature reaches 30 degrees Celsius and when there are more than three cables bundled together in one conduit. This will enhance the cable’s operation and safety.
Wires can be affected by the environment to an extent by the ampacity of their conductors.
Based on an ambient temperature of 86°F, ampacities are calculated in accordance with NEC® Table 310.15 (B) (16).
In the case that the temperature exceeds 86°F, the ampacity from the table must be corrected according to the values contained in NEC Table 310.15 (B) (2) (a).
Among the factors considered in derating tables are the amount of heat produced by the current, the ambient temperature, and the temperature of the surrounding wires.
How many wires can you put in a conduit without derating?
The number of wires that can be safely installed in a conduit without derating depends on various factors, including the type of wires, the size of the conduit, the ambient temperature, and the insulation type of the wires.
The National Electrical Code (NEC) provides guidelines for the maximum fill capacity of conductors in conduits to prevent overheating.
According to NEC standards, for typical power and lighting branch circuit conductors, the maximum fill capacities are as follows:
- For non-metallic sheathed cables (NM), the maximum fill capacity is generally 40% of the total area inside the conduit.
- For other types of conductors, such as THHN or THWN, the fill capacities can vary based on the size of the conduit and the diameter of the individual wires.
It’s crucial to consult the specific NEC guidelines, which provide detailed tables for the allowable conduit fill capacities based on the type of conduit and the size and type of conductors used.
Adhering to these guidelines is essential to ensure that the wires do not overheat, maintaining the safety and integrity of the electrical installation.
Always consult with a licensed electrician or refer to the latest edition of the NEC for precise information on conduit fill capacities and derating requirements.
Why does the depth of burial affect cable ampacity?
The depth of burial can affect cable ampacity due to the variation in temperature. When cables are buried underground, the surrounding soil acts as a thermal insulator.
The deeper the cables are buried, the more the soil insulates them from external temperature fluctuations, including seasonal variations.
At greater depths, the temperature is relatively stable compared to the surface. However, the temperature at these depths is often higher than the average air temperature.
As a result, the cables experience a higher ambient temperature, which can impact their ampacity.
When the temperature surrounding the cable increases, the cable’s ability to dissipate heat is affected. This reduced heat dissipation capacity can lead to an increase in the cable’s operating temperature.
As per the ampacity ratings of cables, the current-carrying capacity of the cable needs to be adjusted to ensure that the cable does not exceed its temperature limits, thereby maintaining safe and efficient operation.
Therefore, considering the depth of burial is essential when calculating the ampacity of buried cables, as it helps to determine the appropriate derating factor necessary to adjust the current-carrying capacity of the cable to match the actual operating conditions.
Why does cable grouping affect the correction factor?
Cable grouping can significantly affect the correction factor or derating factor applied to cables. When multiple cables are installed closely together, they can influence each other’s ability to dissipate heat.
This proximity can result in a higher ambient temperature around the cables, leading to an increase in the overall temperature of the conductors.
The increased temperature can impact the ampacity of the cables, potentially causing them to operate at higher temperatures than they are designed for.
To ensure safe and efficient operation, the ampacity or current-carrying capacity of the cables must be adjusted using a correction factor. This correction factor, often referred to as a derating factor, is applied to the cables to compensate for the heat buildup caused by the proximity of other cables.
By applying the appropriate correction factor, engineers can account for the increased ambient temperature resulting from cable grouping and ensure that the cables operate within their temperature limits.
This practice is crucial for preventing overheating, maintaining the integrity of the cables, and ensuring the safety and reliability of the electrical system as a whole.
How does ambient temperature affect cable rating?
The current rating of all cables decreases when the ambient (environmental) temperature, whether air or soil, approaches the maximum conductor temperature limit. The higher the temperature the higher the cable resistance the lower the cable ampacity.
The relationship between current rating and air temperature for cables in the air is a non-linear function of the excess of cable surface temperature above ambient air temperature and cable size (outer surface area) also matters.
The rate of change in current rating with ambient air temperature varies with cable size (diameters).
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.
Do overhead power lines need a derating factor?
Derating factors are typically not applied to overhead power lines in the same way they are applied to cables in enclosed spaces or underground installations.
Overhead power lines are exposed to ambient air, and their heat dissipation is not as restricted as that of cables in confined spaces.
However, some factors can indirectly affect the ampacity or current-carrying capacity of overhead power lines, such as:
- Ambient temperature: Extreme weather conditions, such as high temperatures, can impact the performance of overhead power lines. While derating factors might not be explicitly applied, the thermal expansion of the conductors and the increased resistance of the wires at higher temperatures can affect their overall transmission capacity.
- Wind and ice loading: Overhead power lines are subject to the mechanical stresses induced by wind, ice, and other environmental factors. Although this doesn’t directly affect derating in the traditional sense, it can impact the mechanical design and safety considerations of the lines.
While derating factors might not be applied directly to overhead power lines, these lines are designed with built-in safety factors to account for temperature variations and mechanical stresses.
Electrical engineers and utility companies consider various factors in designing and installing overhead power lines to ensure they operate within safe limits and can withstand the expected environmental conditions over their operational lifespan.
For more information about OHTL, read my other detailed article here.
