Khairul Nizar Ismail

09 March 2011Chapter 2: Final Circuit & Load Estimation

Definition of a final circuit

• A circuit connected directly to current using equipment, or to a socket outlet or socket outlets or other points for the connection of such equipment

• An outlet is defined as the termination of fixed wiring feeding a luminaire, socket, or any current consuming appliance. From this it will be seen that a final circuit might consist of a pair of 1.5mm2 cables feeding a few lights or a very 3 core cable feeding a large motor direct from a circuit breaker or main switchboard.

• Socket outlet: A device, provided with female contacts, which is intended to be installed with the fixed wiring, and intended to a receive plug. A luminaire track system is not regarded as a socket outlet system

Definition of a final circuit

Final Circuit Distribution

• Final circuits can be divided into the following types, all of which will need different treatment when planning the size of the conductors and the rating of the overcurrent devices:The general of final circuits are:

1. Final circuit feeding fixed equipment or 2A sockets.2. Final circuit feeding 13A sockets to BS 13633. Final circuit feeding sockets to BS 196 (5A, 15A, and

30A)4. Final circuit feeding sockets to BS EN 60309-2 (industrial

types 16A, 32A, 63A and 125A)5. Final circuit feeding fluorescent or types of discharge

lighting6. Final circuit feeding motors7. Final circuit feeding cookers

Sockets

Final circuit feeding fixed equipment or 2A sockets • the number of points supplied is limited by their aggregate

demand, as determined from Table 1 • No diversity is permitted

Point of utilisation of current using equipment Current demand to be assumeda. Socket outlets other than 2A • Rated current

b. 2A socket outlets • At least 0.5A

c. Lighting outlet* • Current equivalent to the connected load, with a minimum of 100W perlampholder

d. Electric clock, electric shaver supply unit, shaver socket outlet (complying with BS 4573), bell transformer, and current using equipment of a rating not greater than 5VA

• May be neglected

e. Household cooking appliance • The first 10A of the rated current plus 30% of the reminder of the rated current plus 5A if a socket outlet is incorporated in the control unit

f. All other stationary equipment • British Standard rated current, or normal current

Final circuit feeding 13A sockets to BS 1363

• The main advantages of the 13A socket with fused plug are that any appliance with a loading not exceeding 3.12kW (13A at 240V) may be connected with perfect safety to any 13A socket, and under certain conditions an unlimited number of socket may be connected to any one circuit

• One point which must be borne in mind by the designer is the question of the use of

outdoor equipment. • 13A socket outlets circuits can be fed by

either radial or ring circuits.

Final circuit feeding 13A sockets to BS 1363(Radial circuit arrangement)

Final circuit feeding 13A sockets to BS 1363(Ring circuit arrangement)

• A ring circuits utilises one additional conductor to loop back to the sending end. In other words, the socket outlets in the ring circuit are fed by two parallel conductors.

• The sharing of the load between the two parallel conductors will depend on the load distribution within the ring.

Final circuit feeding 13A sockets to BS 1363

• Recommendations exist in the IEE On Site Guide for standard circuit arrangements with 13A sockets. These permit 13A sockets to be wired on the final circuits as follows (subject to any derating factors for ambient temperature, grouping or voltage drop):

a. An unlimited number of socket outlets connected to a final circuit serving a floor area not exceeding 100m2 wired with 2.5mm2 PVC insulated cables in the form of a ring (or 1.5mm2 MI cables in the form of a ring), and protected by 30A or 32A overcurrent protective device. In actual practice, 10 number of socket outlets connected to a final circuit wired with 2.5mm2 PVC insulated cables in the form of a ring.

Final circuit feeding 13A sockets to BS 1363

b. An unlimited number of socket outlets connected to a final circuit serving in floor area not exceeding 50m2 with 4mm2 PVC insulated cables on a radial circuit (or 2.5mm2 MI cables on a radial circuit) and protected by an overcurrent device of 30A or 32A rating.

c. An unlimited number of sockets connected to a final circuit serving a floor area not exceeding 20m2 with 2.5mm2 PVC insulated cables on a radial circuit (or 1.5mm2 MI cables on a radial circuit) and protected by an overcurrent device not exceeding 20A.

Malaysia Practices for 13A Socket Outlet (BS 1363)

Types of 13A Socket Outlets

Area Malaysia Practices

Size of wires Fuse/Circuit Breaker Rating

1. Single socket outlet 20m2 • 2.5mm2 PVC cables

16A

2. Double socket outlet 20m2 • 2.5mm2 PVC cables

20A

3. Ring (10 Nos 13A socket outlet provided they are all located within an area of not more than 1000 sq feet)

100m2 • 2.5mm2 PVC cables

32A

4. Radial (Max 6 Socket Outlets)

50m2 • 4mm2 PVC cables

32A

Final circuit for socket outlets to BS 196

(537-05, 553-01 & 553-02) • These are industrial type sockets and

plugs, made three size, 5A, 15A and 30A with a maximum voltage of 250V. As these sockets may be used for any voltage up to 250V they must be marked with voltage and current. By a system of keyways the plug of a given voltage cannot be inserted into a socket of a different voltage. Quite a large selection of supply voltages from 6V to 250V are catered for by this system of keyways. All sockets are 2 poles with a scraping earth, and the plugs are reversible.

• Plugs can be fitted with single pole or double pole fuses made to BS 1361 for fuses in the body of the plug, or to BS 196 for plugs with fused pins.

Final circuit for socket outlets to BS EN 60309-2

• Diversity factor, DF is the ratio of the sum of the maximum power demands of the subdivisions, parts of a system, to the maximum demand of the whole system or part of the system under consideration.

Diversity Factor

IEE ON SITE GUIDE DIVERSITY FACTOR

• Maximum demand (often referred to as MD) is the largest current normally carried by circuits, switches and protective devices; it does not include the levels of current flowing under overload or short circuit conditions

Maximum Demand

(DF) FactorDiversity x CL Load Connected MDDemand, Maximum