Is two 120mm ² cables equal to one 240mm ² cable?

In the design or renovation of communication power supply and power distribution systems, we often encounter issues with cable selection and configuration. Recently, a seemingly simple but representative question has been repeatedly mentioned: "Can using two cables with a cross-sectional area of 120 square millimeters in parallel be equivalent to using a cable with a cross-sectional area of 240 square millimeters?''

In mathematics, the cross-sectional area is indeed equal; But in practical engineering, this is by no means a simple equal sign relationship. This is a typical problem of "theoretically feasible" but "engineering requires caution". Below, we will conduct a comprehensive analysis from multiple dimensions.

1.Theoretical calculation and practical situation of current carrying capacity

From the perspective of the DC resistance of a pure conductor, the total cross-sectional area of two 120mm ² cables connected in parallel is 240mm ², and their ideal current carrying capacity can theoretically approach that of a 240mm ² cable of the same material and insulation type. Many cable current carrying capacity tables also seem to support this inference.

However, the key issue is that "parallel operation" introduces complexity:

1】Uneven current distribution: In actual operation, small differences in the length of two cables, the laying path, and the contact resistance of the connecting terminals can result in the inability to achieve the ideal 50%/50% current distribution. One cable may carry 55% or even more of the current, while the other has a lighter load. The cable that is overloaded for a long time will accelerate aging.

2】 Deterioration of heat dissipation conditions: When two cables are tightly laid in parallel, their heat dissipation conditions are much worse than when a single cable is laid separately. They will heat up each other, causing an increase in the ambient temperature of the common area. According to the "Design Standards for Power Engineering Cables", cables must undergo a correction factor (K value) conversion under conditions such as group laying and conduit installation. This means that the actual safe current carrying capacity of two parallel 120mm ² cables is usually less than the current carrying capacity of an independently laid 240mm ² cable.

2.System reliability and failure mode analysis

This is the core that must be considered from the perspective of power supply profession.

·Single 240mm ² cable: The system structure is simple, and the fault points are relatively single. But it itself is a single path, and once the cable experiences faults such as intermediate breakdown or external force damage, it will cause a complete interruption of power supply.

·Two 120mm ² cables are connected in parallel to form a redundant path. If one of them is disconnected due to a fault, the other can still temporarily carry all the load (ensuring that the load does not exceed its emergency overload capacity), buying time for fault repair and improving system availability. This is an important advantage in scenarios such as communication core rooms and data centers that require extremely high power continuity.

·Involving risk transfer: The parallel scheme introduces more connection points (each cable needs to be connected to the distribution cabinet busbar or circuit breaker at both ends). Connection points (terminals, connectors) are one of the most common fault points in power supply and distribution systems. Any loose or oxidized contact can cause local overheating or even fire, and troubleshooting is more complex than a single cable.

3.Economic and engineering implementation considerations

·Initial cost: Typically, the material cost of two 120mm ² cables plus additional connection accessories and laying labor costs will be higher than that of one 240mm ² cable. Although the unit price of 240mm ² cable is high, the construction volume is relatively simplified.

·Space and laying difficulty: Two cables mean a larger total outer diameter, requiring wider cable trays or cable trenches, and higher requirements for bending radius and laying channels, especially in communication rooms or old pipelines with tight space, which may become a decisive limiting factor.

·Protection coordination: The configuration of distribution protection devices has become more complex. Is it to configure protective switches for two cables separately, or to share a large switch at their front end? If the switch is shared, it is impossible to provide detailed protection for overload or fault of a single cable; If switches are installed separately, careful calculation and setting of protection settings are required to ensure their selectivity, sensitivity, and coordination with higher-level protection, with a higher technical threshold.

4.It is not prohibited to use cable parallel operation for high current circuits, but there are strict regulations. Usually requires:

1】. Parallel cables should be of the same model, specification, and length.

2】. It is advisable to use equal length laying to achieve consistent impedance as much as possible.

3】. For important loads, it is recommended to install current monitoring at both ends to observe the real-time distribution of current.