Electrical data for electrical appliances and machines is normally available on its nameplate. However, sometimes the nameplate is missing or damaged, or only certain items of information are available.
In the case of motors, all the data may be available but you still have to do some extra calculations.
Another application where further calculation is necessary is where the maximum current of an electrical supply point is limited by a circuit breaker. This is the case at all Eskom supply points where the kVA demand of a customer is limited by the ampere rating of a circuit breaker installed at the point of supply.
It is important that you know how to calculate the ampere rating of the circuit breaker that must be installed for a particular load.
EXAMPLE 1
Ten 100 W lamps are connected in parallel at a supply voltage of 220V.
1 lamp = 100 watts
10 lamps = 1000 watts
Therefore, the current drawn by ten lamps:
P = V x I
I = P = 1000 = 4,55 Amps
V 220
Where: P = Watts
V = Volts
I = Current
You will not find a 4,55 amp circuit breaker in the Eskom stores or anywhere else! They are only manufactured in standard ampere ratings and the nearest to 4,55 Amps is a circuit breaker with a rating of 5 Amps, the perfect size for this job.
EXAMPLE 2
A 220 V stove has the following cooking plates:
2 x 1000 W plates
2 x 1500 W plates
2 x 1500 W oven elements
Determine the size of circuit breaker to be installed.
2 x 1000 = 2 000 W
2 x 1500 = 3 000 W
2 x 1500 = 3 000 W Total = 8 000 W
P = V x 1
I = P = 8000 = 36,36 Amps Thus, install one 40 A circuit breaker
V 220
EXAMPLE 3
Installations are not all as simple as those described in Example 1 and 2. Normally there is a wide variety of appliances and machines connected to a system.
Consider a typical 220V, one-phase housing installation.
You might have all of the following items to connect:
1 stove 8kW
1 water heater 3kW
10 x 16A plugs 4kW
14 light fittings 30 x 100 W lamps
1 pump (swi
