Power networks - distribution
Become a winner with correction factors - tips to avoid losses!
Apr 8, 2025
Correction factors for electrical cables - how the load capacity is affected.
When dimensioning electrical cables, it is important not only to look at the load capacity that a cable can theoretically handle, but also to take into account various correction factors that can affect the load capacity. Ignoring these factors can lead to large energy losses, overheating and, in the worst case, material damage or fire risk.
Correction factors are adjustment values that are used to calculate how external circumstances affect a cable's ability to conduct current. Just because a cable can handle a certain current according to a table does not mean that it is always appropriate to use the maximum capacity. The ambient temperature, ventilation conditions and how the cables are laid affect how well they can dissipate heat and thus what load they can withstand in practice. Below are three of the most common correction factors to consider when choosing cables.
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1. Air temperature during indoor installation
The ambient air temperature affects how effectively a cable can dissipate heat. Standard cable load tables assume an ambient temperature of 30°C. If the temperature is higher, for example in a machine room or under a roof, the risk of the cable overheating increases, which reduces its permissible load capacity. To compensate for this, a correction factor is used that reduces the permissible load capacity. For example, at 40°C, a cable's capacity can be reduced by up to 13%.
When cables are laid in pipes buried in the ground, cooling is affected by several factors, such as the thermal conductivity of the ground and the pipe material. A plastic or metal pipe limits the cooling of the cable because heat remains in the pipe rather than being conducted away by the ground. This means that the load capacity of the cable must be adjusted downwards.
The general value of laying a cable in a pipe instead of directly in the ground is 0.80, which means that a cable that can be loaded with 200 A in the ground, for example, can only be loaded with 160 A when laid in a pipe. If the ground also has poor thermal conductivity, for example dry sand, the ability to dissipate heat can be further impaired. If there are different types of ground along the route, the section with the worst ventilation capacity should be used as a starting point unless different cross-sections are joined.
When several cables are installed close together, for example on a cable ladder or in cable trays, they affect each other's cooling capacity. A single cable usually has good ventilation, but when several cables are located side by side, the total heat generation increases, and the cable temperature can become too high. Therefore, a correction factor is used that reduces the permissible load capacity depending on the number of cables and their location. The more cables that share the space, the lower the individual load capacity.
Summary
Correction factors are crucial to reduce losses and ensure safe and efficient cable installation. Air temperature, ground routing and cable co-routing are three of the most critical factors to consider. There are also other factors, such as cable type, insulation material, perforated/unperforated trays and operating conditions, that can affect the load capacity. Therefore, it is always recommended to use the right sizing tool and follow applicable standards to ensure that the cables are not overloaded.
Choosing the right cable from the start can save both energy and maintenance costs in the long run. If you find it complicated or stuck in a table, you can always contact Nexans and we will help you.
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