In the case of multi-core cables, the cross sectional area of all three cores is taken into account. If multiple cables are pulled at the same time into a common conduit, it is usually assumed that the pulling force does not act on all cables at the same time and a reduced number is considered (e.g. in Switzerland, if three cables are pulled into a common duct only 2 cables are considered being pulled.
Recommendations are not harmonized in the various regions and countries, for example US and Canada consider IEEE Standard 1185 and AEIC Pub G5-90, European countries (UK, Germany, etc.) have similar values for which we refer to VDE. Special requirements or recommendations may apply.