Let us then assume that a final circuit is loaded in accordance with the load profile of the system which it is part of. The highest occurring current is equal to the maximum permissible current Iz. For a cable of 3*1.5 mm² cross-section in installation method B1 with normal load profiles for homes, industry and agriculture, this produces lost energies of around 12 kWh a year for every metre of cable length. The results are similar for a five-conductor cable with three loaded wires (Table 7, upper section “Permissible load”). While it is true that there is power loss in three instead of just two wires here, this is less per wire in accordance with the lower current carrying capability.
If the cross-section is increased by just one standard step to 2.5 mm², the lost energy drops to values of around 7 kWh/(m*a). The additional costs may pay back in around four months!
For the reasons given, night storage heating represents an exception. Here, the payback periods are around 2 years. Nonetheless, for this reason upgrading the supply line would be of limited use, since here electrical energy is used to generate heat (the sense of doing this could also be “placed under general suspicion”; unless you live in Norway). Moving the release of heat from night to day justifies an economy measure only to a – very – limited extent: Almost as an aside it should also be noted here that the differences between electrical heating tariffs and the normal household tariffs are no longer as great as they once were. If you want to save not (just) money but (also) fuel and CO2, the equation no longer applies that power stations are fully utilised at peak load times at midday or in the early evening while being underutilised at night. That a “green” supply is not in principle provided at night is only true of solar energy; with wind, this is merely dependent on the weather, in other words on chance. This reduces the lump-sum diurnal spreading of the value of electrical energy as well as the prices to be paid for it (which does not necessarily correlate) in the direction of stochastic parameters.
A further “outlier” is load profile G1 for a facility that only operates on working days – and then only during business hours. Because of the relatively short loading times, to some extent the same applies here as for night storage heating. The payback periods for G1 are just 1/3 as long as with HZ0, but around 3 times as long as in the other profiles.
The corresponding observation can be made for the other industrial and agricultural profiles, but there, too, the results tend to correspond to the selection made here by way of example.
When “playing” with the Excel table it can also be seen that even an upgrade from 1.5 mm² to 16 mm² is worthwhile! Even then, the payback periods are only around 1.5 years, for profile G1 barely 4 years and for profile HZ0 around 13 years. On the other hand, in profile G3, which approximates base load, the upgrade pays back in just 0.6 years.
However, as mentioned, this only applies assuming that the valid standardised load profile for a system equally applies to each final circuit in the system and the maximum permissible current is reached at least once a year. However, this is hardly ever the case; simply because cables and lines are only available with specific standard cross-sections and the next larger one must be selected. Furthermore, reserve and safety factors are always built in. If the cable is only utilised up to half its current carrying capability in practice, the heating drops to a quarter, and the payback period rises e.g. from six months to two years. And yet: What is two years in the life of an installation cable? This generally lasts as long as the entire building. So one number larger is always worthwhile!