Ringing the changes

The UK, Ireland and some Commonwealth and Asian principalities are the only countries using ring circuits. They are not used anywhere else in Europe. Have we got it right or is this yet another of the UK's outdated insular customs?

Birth of the ring circuit

The ring circuit was invented in the 1940s to meet improved standards for post-war housing. Electricity was becoming a basic necessity and old methods of wiring concentrated on lighting circuits, with some 2 A two or three-pin sockets, 5 A sockets and maybe two 15 A sockets, one downstairs and one upstairs. There was perhaps one socket on the cooker panel in the kitchen for a kettle or iron. Irons were very often plugged into a lampholder using a bayonet cap adaptor. Some homes may have had a one-bar fire, real luxury would have meant a two-bar fire. Central heating was virtually unknown in the average home and it was not anticipated that whole house heating would ever be electrical.

The principle behind the ring circuit system was that a householder could take an electric heater from room to room throughout the property without having to use long extension leads. A socket outlet could be installed in each room so that there was a point available wherever needed.

It was considered that a kettle plus one or two heaters would have a maximum demand of 30 A, so the idea was to run a 28 A cable around the house in the form of a ring, with a socket in each room able to take a maximum 3 kW load.

As the average small household was probably no greater than 90-110 m2 the philosophy was that the load was unlikely to exceed 7 kW. Even if it did, it was only likely to last a short time and a 30 A rewirable fuse would be able to cope with a small overload for a limited period. 30 A ring circuits were never intended to be used in commercial and industrial premises. It was purely a domestic facility (see figure 1, over).

Things have changed considerably since those days and there are now very good reasons for considering alternative wiring systems.

Domestic and commercial consumers now own a multitude of low current appliances. New installations need many sockets, flexibility is needed to allow furniture to be moved around and provision needs to be made for alterations and additions.

Extending or breaking into a ring circuit is not a straightforward exercise. Many domestic ring circuits have been modified incorrectly by DIY persons, so are no longer a ring and are likely to be unsafe. Common dangerous faults are: an incomplete ring on one or all conductors; two circuits cross- or inter-connected.

It is now recommended that kitchens are treated as a separate entity and have at least one ring circuit with an availability of 30 A.

Two 20 A radial circuits in a kitchen will use less cable than a ring circuit and provide greater capacity as long as care is taken to ensure that fixed loads such as washing machines are not all on one circuit.

A ring circuit does not lend itself to the separate control of groups of socket outlets. Unless thorough testing is carried out on a new or modified circuit, wiring faults may go undetected and invalidate the basic safety principles of the system.

The radial alternative

Radial and tree circuits offer alternatives. Figure 2, over, shows a typical radial circuit. A 20 A circuit will serve a 50 m2 floor area and a 30 A circuit, 75 m2. These are based on the maximum anticipated load in these areas not exceeding 5 or 7 kW respectively.

The limiting factor in such areas is the cable length, voltage drop and the earth loop impedance of the circuit. In the future all socket outlet circuits in domestic situations are likely to require rcd protection, so cable length will not be such an important issue. The design criterion will then be the maximum anticipated load that would be used in the area.

A tree circuit (figure 3, over) is simply a radial circuit with branches. A 20 A tree circuit wired with 2·5 mm2 cables would be far more versatile than a straight radial circuit and probably far more practical.

Points could be placed economically wherever they are likely to be used; the limitation would still be the maximum load to be used in the area, not the number of sockets. The added bonus is an opportunity to control sections of the circuit separately with switches, timers and, in the near future, a mobile phone through a home bus system. This could also apply to commercial premises.

Typically, a standard three-bedroom domestic property could be adequately served by two 20 A 2·5 mm2 radial or tree circuits, and one 32 A 4·0 mm2 radial or tree circuit in the kitchen.

The benefits for the contractor are easy to see:

• less cable would be needed, leading to less time to install and lower installation cost;
• less drain on the world's resources of copper;
• the problems created by Regulation 433-02-04 of distributing the load evenly in a ring circuit would disappear, thus reducing the exposure of the contractor to a breach of regulations claim;
• much less time required for inspection and testing;
• any breaks or loose connections will be readily identified when testing.

On the downside, an extra circuit-breaker will probably be required which may mean a larger consumer unit is needed.

What price change?

What are the cost implications of moving from ring circuits to radial or tree circuits? Let's take the case of a typical three-bedroom house with socket outlets as follows:

• upstairs - master bedroom, three double outlets; second bedroom, three double outlets; third bedroom, two double outlets;
• downstairs living areas - lounge, four double outlets; dining room, three double outlets; hallway and study, three double outlets;
• kitchen, six double outlets and utility room, two double outlets, where heavy loading requires a 30 A circuit or two 20 A circuits.

The variables are cable and the size of the consumer unit. Accessories are common to all.

Option 1

• Using three 32 A ring circuits: kitchen - eight outlets; ground floor - ten outlets; first floor - eight outlets.
• A total of 130 m of 2·5 mm2 cable would be required, with eight circuits used and a ten-way split consumer unit.
• Typical cost £650.00 without accessories.

Option 2

• Kitchen - two 20 A radial circuits serving eight outlets.
• Other rooms - two 20 A tree circuits of nine outlets each serving part ground and part first floor.
• A total of 90 m of 2·5 mm2 cable would be required, with nine circuits used and a ten-way split consumer unit.
• Typical cost £510.00 without accessories.

Option 3

• Kitchen - one 32 A, 4·0 mm2 radial or tree circuit serving eight outlets.
• Other rooms - two 32 A, 4·0 mm2 tree circuits each serving nine outlets.
• A total of 90 m of 4·0 mm2 cable would be required, with nine circuits and a ten-way split consumer unit.
• Typical cost £620.00 without accessories.

The costings demonstrate that replacing three ring circuits with four 20 A radial/tree circuits could achieve cost savings in the order of 20%. Replacing three ring circuits with three 32 A radial circuits wired with 4·0 mm2 cable achieves a much lesser saving of approximately 5%.

In this example a ten-way consumer unit facilitated all systems. It is likely with some properties that if radial/tree circuits were used a larger consumer unit would be required. The additional cost of a 12-way split consumer unit over a ten-way unit is under £12.

The findings indicate that on large properties radial circuits are likely to be far more cost-effective than ring circuits, but on smaller properties savings may not be as significant. It must be remembered that experience shows radial circuits are likely to be a safer option than ring circuits.

As a matter of interest I have discussed ring circuits with many friends and associates in the industry. Many agree that the ring circuit is outdated and the benefits to users and contractors are limited. Some say that ring circuits should be banned because of their inherent dangers. A number of large contractors will not use them in commercial or industrial premises and one supermarket chain I know will not permit ring circuits in its properties.

There are definite benefits to be had by contractors when using radial or tree circuits instead of ring circuits. Why not give them a try?