In older electrical installation, the wiring is divided into a number of circuits each having a specific current rating. The current ratings are determined by the current ratings of the circuit fuses or MCBs in the consumer unit and protecting the circuits. There are five old standard ratings: 5, 15,20,30 and 45 amps but rarely does a consumer unit include all five ratings. Some homes only have 5A circuits, for lighting only, but most have 5A. and 30A fuses: the 30A being for ring circuits supplying 13A socket outlets, and a separate 30A fuse for the cooker and another 30A fuse for the instantaneous shower unit. Immersion heaters are usually supplied from either a 15A or 20A fuseway and night storage heaters from 20A fuse usually from a separate consumer unit under times witch control set for the overnight cheap rate. Where the cooker is of extra large size it is supplied from a 45A fuse.
Circuit fuses
Circuit fuses are in two types: rewirable, using fuse wires, and cartridge type where the fusing element is enclosed in a cartridge similar to but physically larger than the fuse in a 13A plug. The cartridges are of different dimensions according to their current rating. This, unlike a rewirable fuse makes it impossible to uprate a fuse by inserting one of higher rating which could lead to overloading the circuit wiring, which in turn could result in a fire. The fuse units of both types are colour coded according to their current rating. The old colours are: white (5A); blue (15A); yellow (20A); red (30A);and green (45A).