Selecting the current

rating

of an RCD

1 Revision 01

AS/NZS 3000:2007 (Amendment 2)

3 Revision 01

To Calculate the loading for the lighting portion of the installation use

Load group A (i) - lighting

• A lighting point is where the fixed wiring terminates

• E.g. a twin Fluorescent light fitting is counted as one point

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4 Revision 01

Load group A - lighting

Loading is calculated on the basis of:

– Average size lamp

– Not all lights will be on at the same time

– As part of the overall diversity of the installation

Lighting

For example if a domestic installation has 46 lights this equals 46 points

Using the above table A(i) allows:

3 A for the first 20 points (1 to 20)

2 A for the next 20 points (21 to 40

2 A for the remaining 6 points (41 to 60)

7 A total maximum demand for A(i)

Load group A – lighting

Particular attention must be paid to the footnotes when using table C1

To Calculate the loading for the lighting portion of the installation use

Load group B – socket outlets

• Load group B covers All socket outlets for general purpose use

• Permanently connected equipment not covered in other load groups

• One point represents one outlet

• E.g. a 4 gang outlet is equal to 4 points

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8 Revision 01

If an installation has 10 double and 12 single socket outlets the maximum demand would be

10 x double outlets = 20 point + 12 single outlets = 32 points

B(i) allows

10A for first 20 outlets (1 to 20)

5A for next 12 outlets (21 to 40)

Total demand = 15A for this portion of the installation.

Example 2.1

To calculate the current rating of this RCD the maximum demand of this portion of the installation must be calculated using Table C1

From the switchboard schedule in example 2.1

CCT 4 has 12 points (socket outlets)

CCT 5 has 13 points (socket outlets)

CCT 6 has 20 lighting points

Note: Individual final sub circuit loadings calculated using table C8 are not used to calculate the maximum demand of a portion of an installation.

Maximum demand for this portion of the installation

Step 1; calculate the socket outlets maximum demand using Table C1

Total points = (CCT 4),12 points + (CCT 5),13 points = 25 points for this portion of the installation

Maximum demand calculation

10A for first 20 points (1 to 20)

5A for next 5 points (21 to 40)15A is the maximum demand for the sockets outlets in this portion of the installation

12 Revision 01

Step 2; calculate the lighting maximum demand using Table C1

CCT 6 = 20 points

Maximum demand calculation A(i)

3A for first 20 points therefore maximum demand for the lighting of this portion of the installation = 3A

Step 3 add the socket outlet max demand 15A to the lighting max demand 3A the total maximum demand of this portion of this installation is 18 Amps

AS/NZS:3000:2007 Clause 2.6.2.1 General

Any device for the provision of additional protection shall be capable of interrupting the part of the circuit protected by the device when an earth leakage current is above a predetermined value.

The load current rating of an RCD shall be not less than the greater of the following—

(a) the maximum demand of the portion of the electrical installation being protected by the device; or

(b) the highest current rating of any overload protective device on the portion of the electrical installation being protected.

As the calculated maximum demand for the portion of the installation being protected by the first RCD has been calculated to be 18A and the highest rated circuit breaker being protected by the RCD is 20A the RCD must have a minimum current rating of 20A

If the maximum demand was calculated to be above 20A then the maximum demand value would be the minimum RCD rating