Specific wire sizes are approved for terminal blocks in relation to their current-carrying capacity - the higher the approved rated current, the higher the maximum permissible rated wire size. Testing institutes only accept "unprepared stripped wires" for terminal block approval purposes. If necessary, stranded wires can first be drilled for easier insertion into the terminal block.

Approval can be granted for the use of stranded wires with ferrules in connection with the approval of the relevant terminal devices. The rated values for each terminal block are indicated in the "Ratings" table of the relevant PTR product page. The "Wire sizes" table lists the permitted wire sizes, including when stranded wires with ferrules are used.