In principle equipment such as circuit breakers must be connected to thermoplastic insulated cables because their maximum operating temprature is 70° C. But the equipment is also allowed to connect to 90° C thermosetting insulated cables provided that when deciding conductor sizes of the cables, a table from Tables 4D1 to 4D4 of Appendix 4 is refered instead of a table from 4H1 to 4H4. 

Sort of - but it's not the kind of insulation that's the constraining factor, but the temperature the conductor runs at.  Themoplastic (e.g. PVC) cables are only rated for 70 degrees anyway, so no further consideration is needed in that case. Thermosetting (e.g. XLPE) insulated cables are capable of being run at a higher temperature though, so some care is needed to make sure that the conductor temperature doesn't exceed 70 degrees at the terminal - and one easy way of achieving that is to use the 70-degree tables. There are other approaches though - there are methods of calculating conductor temperatures for given currents for a cable (often useful as a first step for accurately calculating actual voltage drop when a cable is used below capacity) which can give more accurate results in some circumstances. In principle the 70 degree requirement also applies where there is no insulation involved (thermoplastic or thermosetting) - e.g. when connecting equipment to bare bus-bars (although in type-tested DBs and the like that's usually covered by the product standard rather than BS 7671).

   - Andy.

In principle equipment such as circuit breakers must be connected to thermoplastic insulated cables because their maximum operating temprature is 70° C. But the equipment is also allowed to connect to 90° C thermosetting insulated cables provided that when deciding conductor sizes of the cables, a table from Tables 4D1 to 4D4 of Appendix 4 is refered instead of a table from 4H1 to 4H4. 

Sort of - but it's not the kind of insulation that's the constraining factor, but the temperature the conductor runs at.  Themoplastic (e.g. PVC) cables are only rated for 70 degrees anyway, so no further consideration is needed in that case. Thermosetting (e.g. XLPE) insulated cables are capable of being run at a higher temperature though, so some care is needed to make sure that the conductor temperature doesn't exceed 70 degrees at the terminal - and one easy way of achieving that is to use the 70-degree tables. There are other approaches though - there are methods of calculating conductor temperatures for given currents for a cable (often useful as a first step for accurately calculating actual voltage drop when a cable is used below capacity) which can give more accurate results in some circumstances. In principle the 70 degree requirement also applies where there is no insulation involved (thermoplastic or thermosetting) - e.g. when connecting equipment to bare bus-bars (although in type-tested DBs and the like that's usually covered by the product standard rather than BS 7671).

   - Andy.

To Andy

You said "one easy way of achieving that is to use the 70-degree tables"

It's very helpful for me to understand what you mean.

Thank you.