What Is Derating Factors For Underground Cables?

Each cable has a current carrying capacity (or cable ampacity). The value of this current is given in cables ampacity tables and depends on some factors which are called derating or correction factors.

I work with cables for more than 13 years now. The cables we use in my workplace are XLPE copper cables. Selecting the right cable for the load is economical, technical, and safety factor.

If we choose a cable with a lower ampacity than the load, it heats up and gets overloaded. If we choose a cable with a much higher ampacity than the load, it costs more.

Derating cable current is essential to choose the suitable cable for the load.

What is the Derating factor in cable sizing?

De-rating factor, also known as the correction factor, is a multiplier that is applied to a cable ampacity rating in order to change the value depending on various operating conditions.

For example ambient air temperature, soil temperature, and laying method of the cable. These factors make a cable’s current carrying capacity lower than the designed value.

Cable de-rating factors are used to assess a cable’s safe power dissipation capability at high temperatures. 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.

Why is cable derating necessary?

A cable in service without correction factors may fail catastrophically.

Cable current carrying capacity varies depending on the working conditions such as surrounding temperature, laying depth, soil thermal resistivity, number of cables in the same path, and some other factors.

The same cable size ampacity increases and decreases depending on these factors. The de-rating factor is used to illustrate how much the cable ampacity will be under the working conditions.

De-rating factors manage the cable’s current carrying capacity and increase the safety margin between the cable’s specification limit and the maximum load, providing extra protection to ensure 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 designed to carry. Its purpose is to protect the insulation of the wire against deterioration caused by extreme heat.

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.

For more information about cable ampacity, you can find it in my other article here.

When should you derate 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 covering of the cable 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?

You always need to think about adjusting a cable’s capacity when choosing the right cable for a job, whether it’s for a home, business, or industry.

But there are cases where you don’t have to make any adjustments. This happens when the factors affecting the cable are all normal. In these cases, adjusting the cable’s capacity won’t make any difference because everything is already as it should be.

For example, if we bury a cable underground at a depth of 0.5 meters the ground temperature is 35°C, and there are no other cables nearby, then we don’t need to adjust the cable’s capacity. The factors affecting it are all normal, and they don’t change its current-carrying ability.

In simple terms, you should check each situation to see if the cable needs adjusting or not. If all the factors are normal (unity), then you’re good to go. One important thing to remember is that these adjustments are only for cables that carry electricity; the earth wire, which doesn’t carry electricity, doesn’t need any adjustments.

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.

air temperature derating
air temperature derating

Ground temperature derating:

Just like the air temperature, the ground temperature affects electrical cables’ ampacity.

ground temperature derating
ground temperature derating

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.

Burial depth derating
Burial depth derating

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.

soil thermal resistivity
soil thermal resistivity

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.

trefoil formation

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?

You can put less than three cables in a conduit without derating. Furthermore, as per the NEC rule, if you are going to use a number of cables over three, you must have to drive the capacity of each cable.

This clause of NEC rules shows that if the number of cables is less than three in a conduit, you can ignore the derating factor.

Why does the depth of burial affect cable ampacity?

Due to thermal resistance, the impact on the soil surrounding the cable usually increases with increasing the burial depth. That affects the current-carrying capacity (ampacity) as the cable is buried deeper in the soil.

As expected, the current carrying capacity diminishes with increasing burial depth. It is clear that cables buried at deeper depths have a significant impact on the cable’s ampacity rating.

Underground cables are used to contact the soil to dissipate the temperature. The soil’s ability to dissipate heat varies with the depth. This is why burial depth affects the cable ampacity.

Why does cable grouping affect the correction factor?

Cable grouping affects the correction factor due to the thermal effect of each cable on the others because Power cables installed beside each other (in the form of the group) cannot tolerate as much current as a single piece of cable.

In this situation, if one conductor carries more current than its capacity, heat dissipation occurs in this cable; due to this heat, all other grouped cables’ correction factors will be affected.

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?

Overhead power lines need a derating factor to correct their current carrying capacity/ampacity.

This happens in the hot climatic regions that have higher ambient temperatures; these temperatures can affect the cables installed overhead.

Warm surrounding the cables becomes a reason for heat dissipation from the cables resulting in a hotter cable.

For more information about OHTL, read my other detailed article here.