This is not actually straight answer. Ability of wire to transfer current is different when transmitting over free air, inside concrete wall, how close two wire together, in trunking, etc. It also depends in ambient temperature.

In general, it is about 20A for 2.5mm cable. The ring circuit can carry more just because the current can be supplied in both direction from the Main Distributor Box. You can refer from this for Malaysia Guideline from Suruhanjaya Tenaga (ST)

http://www.st.gov.my/index.php/en/download...ntial-buildings

The 4mm can carry 26A under 30C ambient temperature. I'm no sifu 

Yeah i am aware that house all using radial circuit... im just stating what i read from some guidelines that if you have ring circuit your cable can take more amps... though not much people using ring.. even in offices =D