Electrical load

Watts / voltage = amps

For lamps incorporating ballasts like fluorescent lamps multiply by 1.6

Can you claarify that, multiply what by 1.6? Omarromy can you tell use what you are doing and where you live. If this is a single fixture in a residence we use 14/2 wire with a 15 amp breaker pretty much standard.

In BS7671 it is mentioned to multiply the current by 1.6 to select the wire size.Breaker size depends on quantity of lamps connected or load. Normally in homes and offices 6A mcb is used. In factories where many fittings are connected to a single switch the engineer should calculate or consultant should specify in drawings

Not sure what "BS7671" is but, in the USA, lets look at a typical residential circuit. 14 gauge wire is rated to carry 15 amps at 120 vac. There is no number in this sequence of 14, 15 and 120 that you can multiply by 1.6 and get 14 gauge wire.

in BS practicing countries,lighting circuits are fed at 240V 50Hz and it is the normal practice to use 1mm^2 pvc insulated cables or sometimes 1.5mm^2 depending on the load

The load in a lamp circuit incorporating a ballast(fluorescent) is the lamp watts plus the loss in the ballast. That's why a multiplying factor of 1.6 is recommended by the Institution of electrical engineers,UK.