I. A Most Frequently Asked Question

In cable selection and use, one question is repeatedly asked:

"What is the maximum current this cable can withstand?"

This question itself is incorrect.

The correct question is:

"Under a given overload current, how long can the cable operate safely?"

The short-term overload capacity of a cable is determined by both time and current, not by current alone. Discussing overload capacity while ignoring the time dimension is meaningless in engineering.

II. The Thermal Process of Cables Under Overload

Cables generate Joule heat when carrying current. The amount of heat generated is determined by the following formula:

Q = I² × R × t
Where Q is the heat generated, I is the current, R is the conductor resistance, and t is time.

The current affects the heat generated with a square relationship. Doubling the current quadruples the heat generated.

However, the key point is: if the time is short enough, even with a large current, the total heat generated may be very small.

A cable is not a fuse. Fuses are designed to melt within milliseconds. Cables are systems with significant thermal inertia—both conductors and insulation materials require time to reach dangerous temperatures.

A short-term high-current surge may only cause a temperature rise of a few degrees Celsius. However, prolonged small overloads can lead to thermal aging or even thermal breakdown of the insulation.

III. Failure Sequence: Insulation Before Conductor

A common misconception is that cable overloads will "burn through the copper wires."

This is incorrect.

In actual overload scenarios, the insulation layer fails first, not the conductor.

The melting point of copper conductors is approximately 1085°C. The long-term allowable operating temperature of XLPE insulation is only 90°C, and even considering short-term overloads, its withstand temperature does not exceed 250°C. The long-term allowable operating temperature of PVC insulation is 70°C, and its short-term withstand temperature is approximately 160°C.

Comparing these figures clearly shows that before the copper conductor reaches its melting point, the insulation material has already undergone thermal softening, carbonization, or even complete loss of insulation performance.

Once the insulation fails, a short circuit occurs between the conductors, generating an electric arc and localized high temperatures—only then might the copper conductor melt. However, this is a secondary failure, not a direct consequence of overload.

Therefore, discussing cable overload capacity in engineering essentially means discussing: for what period of time, conductor heating will not cause the insulation temperature to exceed its short-time withstand limit?

IV. Short-Time Overload Capacity of XLPE Insulated Cables

Based on IEC 60364-5-54 and thermal dynamic calculations in engineering practice, for XLPE insulated copper conductor cables, under the premise of an initial temperature of 90°C (full load normal state), the short-time overload capacity is approximately as follows:

When the overload multiple is 150%, the cable can typically withstand several minutes to tens of minutes. This time range mainly depends on the rate of heat accumulation in the insulation material.

When the overload multiple is 200%, the cable can withstand tens of seconds to several minutes. The limiting factor here is mainly the rate of temperature rise on the insulation surface.

When the overload multiple is 300%, the cable can withstand several seconds to more than ten seconds. At this point, the temperature at the conductor-insulation interface rises rapidly, becoming the primary limiting factor.

When the overload reaches 500% or higher, the cable can only withstand it for 1 to 5 seconds. Under these conditions, the insulation material will rapidly carbonize, leaving almost no safety margin.

It should be noted that the above values ​​are only engineering estimates. Precise values ​​depend on the cable cross-section, laying method, initial temperature, and the specific formulation of the insulation material. The lower the initial temperature, the longer the withstand time—cold starts are much safer than hot starts. Better heat dissipation also extends the withstand time—air-laid installation is superior to conduit installation.

V. Cable Overload Check for Direct Motor Start-up

Taking a 132kW motor as an example. Its rated current is approximately 240A (in a 400V system). During direct start-up, the starting current is approximately 6 times the rated current, i.e., 1440A. The starting duration is typically 6 seconds.

The matching cable is 95mm² XLPE copper cable. The rated current carrying capacity of this cable in a 40°C environment under conduit installation conditions is approximately 300A.

The verification process is as follows:

First, determine the initial temperature. Assume the cable has been running under rated load for some time, with an initial temperature of approximately 90°C.

Then calculate the heat generated during startup. The heat generated equals the square of the current multiplied by the resistance multiplied by the time, i.e., 1440² × R × 6.

Compare this value with the heat generated under rated operating conditions. Under rated conditions, with a current of 300A for 1 hour (3600 seconds), the heat generated is 300² × R × 3600.

The actual calculation results show that the heat generated during a 6-second startup process is equivalent to only about 15 to 20 seconds of heat generated under rated conditions. This corresponds to a temperature rise of approximately 15 to 20°C.

This temperature rise is far below the short-time withstand temperature limit of XLPE insulation (approximately 250°C). Therefore, the startup process will not cause insulation damage.

This is why in many direct-start motor applications, the cable specification does not need to be increased due to the starting current—provided the startup time is short enough, typically within 5 to 8 seconds.

VI. Three Judgment Criteria in Engineering Practice

First, distinguish between steady-state overload and transient overload.

A steady-state overload refers to a situation where the current exceeds the rated value and lasts for several minutes or more. The main risk of this type of overload is insulation thermal aging, which can lead to long-term cumulative damage.

A transient overload refers to a situation where the current is several times the rated value but lasts only for a few seconds. This type of overload can usually be withstood by the cable unless it occurs repeatedly.

Second, use insulation temperature as a failure criterion.

The basis for judging whether a cable is overloaded is not "whether the copper has burned out," but "whether the insulation temperature exceeds the short-time withstand limit." For XLPE insulation, the short-time withstand temperature is usually taken as 250°C, based on the conductor temperature.

Third, consider the cumulative effect.

If the equipment is frequently started and stopped, such as a crane or reciprocating compressor, the temperature rise from each start will accumulate. In this case, it is not enough to only look at the temperature rise of a single start; the cumulative temperature rise effect under thermal cycling needs to be calculated.

VII. Selection Recommendations
For equipment with high starting current, such as motors, transformers, and welding machines, there are four common coping strategies.

The first method is to increase the cable specification. This method is suitable for scenarios with long start-up times (over 10 seconds) or frequent starts. However, it is more expensive, especially for long-distance laying.

The second method is to install a soft starter. This method is suitable for scenarios with medium start-up times (3 to 10 seconds) and where current surge reduction is needed. The cost is moderate.

The third method is to install a frequency converter. This method is suitable for scenarios with very frequent starts or where precise speed control is required. It offers the most comprehensive functionality but is also the most expensive.

The fourth method is to leave it as is and use the original specification. This method is suitable for scenarios with very short start-up times (no more than 5 seconds) and infrequent starts. The cost is zero, but this is contingent on safety being verified.

A common engineering mistake is blindly increasing the cable specification to handle start-up overload. This is often not the optimal solution. The correct approach is to first calculate the actual heat generated during start-up. In many cases, calculations show that the existing cable is sufficient.

If verification is required, the following parameters should be prepared: cable cross-section, material, and insulation type; the start-up current-time curve provided by the equipment manufacturer; and the laying method and initial temperature.

VIII. Key Conclusions

First, the short-term overload capacity of a cable is determined by both time and current. Asking "How much current can it withstand?" is meaningless; one must also ask "How long can it withstand?".

Second, under overload, the insulation fails first, not the conductor. The temperature limit of insulation is far below the melting point of copper.

Third, XLPE insulated cables can typically withstand tens of seconds to several minutes under 200% overload, depending on the initial temperature and heat dissipation conditions.

Fourth, for short-term impacts from direct motor starting, in most cases, it is not necessary to increase the cable specification, provided that the starting time does not exceed 5 to 8 seconds and is infrequent.

Fifth, engineering decisions should be based on calculations, not intuition. Blindly increasing cable specifications wastes costs, while neglecting verification may create hidden dangers.