Quote Originally Posted by fgf View Post
Good question* I would make guess* cable inside conduit in 'air'.

If cable surround by dirt/clay when buried* harder for temperature to dissipate. In tunnel* air still flow and can cool cable temperature.

For this calculation I would apply ambient temperature derating factor as underground tunnel temperature higher I guess than normal 'air' and cable proximity derating factor.

Cable size guide standards to use-> BS7671 or IEC