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Electrical Applications

Cable Selection


External Influences

A wiring system will be selected and built so that it is suitable for the external influences it may encounter. External influences recognised by the BS7671 are:

Ambient temperature:

The temperature of the surrounding environment. In this country we assume that it is 20°C.


Presence of water or high humidity:

The presence of water can damage the cables insulation and lower its insulation resistance. Water also lowers people’s resistance making them of susceptible to shock.


Presence of solid foreign bodies:

A solid foreign body can be anything from dust to gravel to swarf. These bodies can block ventilation systems and therefore stop the dissipation of heat. Fine partials of dust can cause an explosive atmosphere and this can stop the dissipation of heat.


Presence of corrosive or polluting substances:

Corrosive substance can be such thing like water, salt and acid’s. What ever wiring system is chosen, its external cover must be able to with stand the corrosive substances.



If the wiring system is going to be subject to mechanical damage the wire chosen much be able to sustain this in a way that dose not damage the circuit.



Most machinery produces vibration. To make sure this is not transmitted in to the wiring system where it may break fittings, the machines supply is fed by a flexible cable.


Other mechanical stresses:

As metal heats up it expands and this expansion can be damaging if there is no allowance made for it in the wiring system. As the load increases in a wiring system so dose the heat generated in the conductor, this causes the conductor to expand. This could put mechanical strain on the fixtures.


Presence of flora or mould growth:

Flora is anything that grows it can be any thing from mould to bacterial growth. Mould and bacteria like warm humid conditions and if there are any cracks or crevasses you would expect to find some there. This is often a problem in green houses.





Presence of fauna:

Fauna is a general word for animals. In a situation where animals are ever present wiring systems must be placed where they can not be damaged by the animals. 


Solar radiation and UV radiation:

Most plastics when exposed to sunlight for long periods of time will degrade and become brittle. The plastic losses it properties and when put under mechanical pressure it will break.


Building Design

The design of a building may determine what types of wiring systems are used. E.g. If the building has a suspended floor to run cables under.


Terms used when selecting conductor sizes in a electrical circuit.


Design current (Ib):

This is the amount of current a load will take


Nominal rating of protective device (In):

The current rating of the fuse of circuit breaker


Nominal Voltage:

The mains voltage supply usually 230V


Volt Drop:

The voltage lost over a load due to resistance. The maximum permissible voltage drop in an installation is 4% of the nominal voltage.


Thermal constraints:

When sizing a protective conductor it should be big enough to handle the rise in temperature in fault conditions. Without damage to its surroundings.


Shock protection:

The circuit is designed so that the resistance of both phase and CPC is low enough to allow the protective device to disconnect the circuit quickly and prevent danger from shock.


Voltage Drop

The maximum permissible voltage drop allowed on a 110v installation is 4.4 volts. The maximum possible voltage allowed on a 110v installation is 114.4 volts. The minimum possible voltage allowed on a 110v installation is 105.6 volts.


Factors effecting cable size selection:

When selecting the size of a cable the following factors have to be taken into account:


When cables are grouped together the cables capacity to dissipate heat is limited.


Ambient Temperature:

If the ambient temperature increases it takes longer for the heat to dissipate as there is a smaller difference if temperature.


Thermal Insulation:

This is like rapping a blanket round the cable. It limits the cables capacity to dissipate heat.


Type of protective device (fuse):

BS3036 is a semi enclosed fuse. It takes 2 times its nominal current rating (Id) to cause it to operate. Where as a BS1362 cartridge fuse, which operates at 1.45 times its nominal current rating. This means that a circuit protected by a BS3036 fuse will rise to a higher temperature before the fuse will blow. Therefore you  will need to select a larger cable size.

Shaun James Parry