GUIDE TO CHOOSING ELECTRICAL MATERIALS AND DEVICES USED FOR HOME

GUIDE TO CHOOSING ELECTRICAL MATERIALS AND DEVICES USED FOR HOME

How to Choose the Right Wire and Cable For Your Home?

Cables consist of conductors surrounded by insulation. The conductors should be preferable to copper. Aluminium conductors are sometimes used to reduce cost but they are inferior to copper cables as uninsulated portions such as connections to switches tend to oxidize and become brittle thus giving trouble in the long run. If possible, aluminium cables should not be used for wiring. Copper-covered aluminium cables are also available and they are considered better than plain aluminium cables as they do not get oxidized so easily. But they are not as good as copper cables. Formerly, single-wire cables were used. Nowadays stranded wires are commonly used for better performance.

Even though rubber was once considered the standard insulating material, nowadays plastics (especially PVC covered cables) are very much popular. In moist or wet situations, PVC insulation fares better than rubber.

Sizes. At present, cables are manufactured in metric sizes and are referred by the number and diameter of the conductor. Thus, the cable designated as 1/1.13 cable contains one conductor of diameter 1.13 mm giving an area of 1.0 sq mm. The table below gives the recommended sizes of wires for different uses.

What are the different sizes of wire and their uses?

Circuit	Fuse	Cable size (area)	Cable current, rating (amp)
Lighting	5 amp	1.0 to 1.5 mm²	12 to 15
Immersion heater	15-20	2.5 mm²	21
Ring circuit and spurs	30	2.5 mm²	21
Radial circuit	20	2.5 mm²	21
Radial circuit	30	4 mm²	27
Cooker	45	6 to 10 mm²	35 to 48

Notes:

(a) The cable current rating indicated above is for rewirable fuses. Rating should be uprated by one third if cartridge fuse or mini circuit-breakers are used.

(b) Nowadays it is more common to use multistorey cables instead of single-strand wire (also read the article: ).

What is Fuse?

A fuse protects appliances and cables from damage by electrical faults and is similar to a safety valve in a boiler system. Nowadays miniature circuit breakers (MCB) are used instead of fuses in all classes of buildings. The current rating of fuses should be lower than that of the cable it has to protect.

There are mainly the following two types of fuses:

(a) Rewirable or semi-enclosed fuses

(b) Cartridge fuses

Rewirable fuses: Rewirable fuses are cheap. It takes twice the current rating of a rewirable fuse to blow it. Thus, a 5 A rewirable fuse requires a current of 10 A to blow it. As in the same holder, a different capacity fuse wire is used, one should take care that the same capacity fuse wire is used during the rewiring. This can be done either by using colour-coded fuse wire or making special markings on the fuse holder with the same colour code as given below:

5 A	White
15 A	Blue
20 A	Yellow
30 A	Red
45 A	Green

Cartridge fuses: Cartridge fuses of different ratings (different ratings are of different sizes) are available (Fuse of different ratings are of different sizes.). It is not possible to fit a 20 A cartridge fuse in a 5 A fuse holder. A 20 A fuse blows at 1.5 times its current rating. Thus, a 30 A fuse will blow out at 45 A. It is not generally possible by simple inspection to tellavlien a cartridge fuse has blown.

Why do we need a miniature circuit breaker?

Miniature circuit breakers are single pole switch which automatically switches off when excess current flows through its operating coil and thus flows in the circuit (overloading or short circuit). It has many advantages over fuses but initial costs are high and hence, they are not generally used for low-cost constructions.

It is a modern alternative to the fuses described above. An MCB trips at 1.25 times its rated current capacity. Thus, a 30 A MCB trips at 37.5 A (compared to 45 A in case of a cartridge and 60 A in case of a rewirable fuse). So an MCB is more sensitive than a fuse. It is important to note that MCB should not be confused with earth leakage circuit breaker (ELCB). An MCB is only an overload device.

Also Read: Wire Sizing And MCB Selection For Residential Building

Why do we need an Earth Leakage Circuit Breaker (ELCB)?

If an electric appliance with a fault is connected to the line and if its metal parts are not earthed, the metal may become live without the fuse blowing. Such a situation can produce an electric shock to the person touching it. However, if the metal body is earthed, then the current will flow through the earth wire. For such a condition, it is common to use a tripping device called the earth leakage circuit breaker (ELCB). It is like the main switch which automatically trips even when only a small current flows through the earth wire and the current supply will be cut off completely. This switch is connected to the neutral line of the electric supply in the main distribution board and before the main switch. Hence, when it trips, the supply to the building will be completely cut off.

There are two principal types of ELCBs used in home installations—the current operated and the voltage-operated. Current-operated ELCBs work on the principle of a core balanced transformer and are more expensive than voltage-operated ELCBs. It has two ratings—firstly as the main switch (60 or 100 A) and secondly as its operating tripping current rating capacity usually is 30 mA. The voltage-operated device has many disadvantages.

Why do we need a Residual Current Circuit Breaker (RCCB)?

This is a much more positive protection device that can be provided instead of ELCB to avoid electric shocks and also current leakage in the wiring of buildings. An RCCB is connected to all three phases and the neutral. It is more expensive than ELCB and provides good protection against even minor defects in the wiring of a building. However, the wiring should be perfect, otherwise, even with a small defect, it will trip until it is made good.

What are Plug Fuses?

As indicated earlier in many instances, it is advantageous to provide a low-rated fuse at the plug outlet and hence, modern plugs or sockets are also provided with plug fuses. These are interchangeable fuses and are used to connect expensive electric equipment such as televisions, refrigerators, air conditioners, etc.

Note: When installing junction boxes, switchboards, etc. to walls, it will be better, if possible, to fix them on the inside walls of the rooms rather than the outside walls as the outer walls may get wet during very heavy rains. Alternatively, all such exterior walls, which have electrical switchboards, should be protected by a continuous chajja or painted with good exterior grade waterproof paint to protect them from penetration of rainwater.

Also read: List Of Electrical Materials Used For Residential Building