In the cable catalogs of leading manufacturers of power cables there is a system of correction coefficients for selecting the core cross-section. According to this system, laying cables in pipes reduces the long-term permissible current (current capacity) of the cable line, and the corresponding correction factor is 0.9. I must say that this is incorrect. Laying in pipes in some cases can even increase the current capacity of the cable line, and the correction factor could be up to 1.10-1.15 (instead of 0.9).

Later I will give detailed explanations of it.