1️⃣ The current in the cable core varies throughout the day. When performing the thermal calculation of the cable line and selecting the core cross-section, the calculations traditionally include the maximum current value (Ic) of the daily load graph, not taking into account that this value is usually reached only for a limited time (say, 30-60 minutes a day).

Simple estimates (using the example of a 110 kV class cable) show that the cable heating/cooling processes are quite inertial:
✅ XLPE has a time constant τ ≈ 45 min, and the insulation heating/cooling processes will be completed in 3τ ≈ 135 min (> 2 hours);
✅ for the soil (when laying in the ground), the time constant τ >10 hours, and the processes will be completed in 3τ >30 hours (> 1 day).

The large inertia of processes suggests that cables whose cores are selected for current of the maximum of load graph (Ic) will not have time to warm up sufficiently in a short time of the existence of this maximum load, i.e. the insulation temperature is not able to reach 90°С. The lowered (< 90°С) real insulation temperature makes it possible to reduce the core cross-section compared to the case when the current (Ic) is assumed to pass for the entire 24 hours. The reduction can be 1-3 steps in the range of standard values of the core cross-section. So, accounting for “load factor” is a crucial and may help to save a lot on the cost of the cable.

2️⃣ The effect of taking into account the inertia of the cable heating/cooling processes will increase if we recall that in fact the insulation allows not only a temperature of 90°С, but also overheating up to 105°С (for some types of cables – up to 130°С). The overheating time is limited to 100 hours per case, but not more than 1000 hours per year (other values are possible, depending on the country and regulations). The possibility of a slight increase in temperature (say, from 90°С to 105°С) allows for even greater overloading of cables with current compared to the case when there was a limit of 90°С.

Well, we can conclude that the word “overload” can be understood as one of the following:
1️⃣ an increase in current (above ampacity), without overheating above 90°С.
2️⃣ an increase in temperature (overheating above 90°С).

Overload of type 1️⃣ is quite acceptable, and in some cases the current in the core can increase up to 1.5-2.0 times above ampacity, for up to several hours, without any danger for the cable (without overheating above 90°С).

Overload of type 2️⃣ is acceptable but in a narrow temperature range – by no more than 15°С (from 90°С to 105°С), and such an overload should be significantly limited in time.

Understanding overloads 1️⃣ and 2️⃣ helps to save on the core cross-section. At the same time, control over the actual temperature of the cable can be assigned to the temperature monitoring system (for this purpose, an optical fibre is integrated into the screen).

You can read about cable overloads in Chapter 7.3 of the book “High Voltage Cable Lines“.