Cable Selection Process

The following three main factors influence the selection of a particular cable to

satisfy the circuit requirements:

(a) Current-carrying capacity—dependent upon the method of

installation and the presence of external influences, such as thermal

insulation, which restrict the operating temperature of the cable.

(b) Voltage drop—dependent upon the impedance of the cable, the

magnitude of the load current and the load power factor.

(c) Short-circuit temperature limit—dependent upon energy produced

during the short-circuit condition. The minimum cable size will be the

smallest cable that satisfies the three requirements.

However, with experience it will become apparent that the different nature of

installations will determine which of the requirements predominate. In general, the

current-carrying capacity requirement will be the most demanding in the relatively

shorter route lengths of domestic premises and the like where factors such as semi-

enclosed re-wirer able fuse protection, cable grouping, and thermal insulation

occur. On the other hand the voltage drop limitation is usually the deciding factor

for longer route lengths which are not subject to the factors mentioned above. The

need to increase cable size to meet the short-circuit temperature rise requirements

will only occur in special situations for the voltage ratings of the cables covered by

AS/NZS 3008.

DETERMINATION OF MINIMUM CABLE SIZE BASED ON CURRENT

CARRYING CAPACITY CONSIDERATIONS

To satisfy the current-carrying capacity requirements of a circuit it is necessary to take into

account a number of factors, as follows:

(a) Determine the current requirements of the circuit.

NOTES:

a. AS/NZS 3000 makes requirements concerning the relationship between the current

required by the load connected to the circuit, the type and current rating of the

overcurrent protective device, and the current-carrying capacity of the cable. Such

factors will invariably determine the minimum current requirements for the

application of AS/NZS 3008.

b. Where re-wirable fuses form the circuit-protection, AS 3000 makes provision for de-

rating factor to be applied to the current-carrying capacity of cable determined from

this Standard. This de-rating factor is necessary because of the desire to limit the

maximum permissible temperature rise under overload conditions.

(b) Determine the method of cable installation to be used, as follows:

For a single circuit, determine if the method of installation requires the

application of a de-rating factor selected from Tables 22, 23 or 24 AS/NZS

3008. Where applicable, divide the value of current determined in Step (a) by

the de-rating factor so determined.

For a group of circuits, determine if the method of installation requires the

application of a de-rating factor selected from Tables 22 to 26 AS/NZS 3008.

Where applicable, divide the value of current determined by Step (a) by the

de-rating factor so determined.

NOTE: Tables 2(1), 2(2), 2(3) and 2(4) AS/NZS 3008 provide guidance to the

installation methods and de-rating factors applicable to the common elastomer

or thermoplastic-insulated cables.

(c) Determine the environmental conditions in the vicinity of the cable installation.

Where applicable, divide the value of current determined in Step (b) by—

(i) the ambient air or soil temperature rating factor selected from Tables 27(1)

and 27(2) AS/NZS 3008;

(ii) the depth of laying rating factor selected from Tables 28(1) and 28(2) AS/NZS 3008; and

(iii) the soil thermal resistivity rating factor selected from Table 29

AS/NZS 3008.

(d) The resulting value of current represents the minimum current-carrying

capacity required of the circuit. Refer to the tables of current-carrying capacity for

the different cable types, Tables 3 to 21 AS/NZS 3008. Taking into account the

method of installation employed, the smallest conductor size which has a tabulated

current-carrying capacity equal to or in excess of this predetermined minimum

value will be considered to be the minimum cable size satisfying the current-

carrying capacity requirement.

Using the AS/NZS 3008.1 series to select cable conductor size based on current-carrying capacity

The following examples illustrate how to determine minimum size cables from the current-

carrying capacity Tables in the AS/NZS 3008.1 series.

Example 2.1

A thermoplastic V75 insulated 2 core copper cable is to be installed for the circuit to a fixed

appliance rated at 30 A. The cable will be installed on a surface unenclosed in air. What is the

minimum size cable that will meet the current-carrying requirements?

Step 1. Use the appropriate Table 2 to find which current-carrying capacity Table (and

Column) to use

Step 2. In Table 2(1), go down Column 2 to Row 12 for two-core cables installed on a surface

unenclosed in air, then across Row to Columns 3 and 5 to check the installation method, in

this case ‘clipped to a surface’

Step 3. The reader is referred to current-carrying capacity Tables 9 and 10, Columns 4 and 5

Step 4. Go to the referred Table for V-75 cable i.e. in this example Table 9

Step 5. Columns 4 and 5 in the Table refer to

the installation method ‘unenclosed’ and

‘touching’ (a surface). The conductor is

specified as copper so move down Column 4

to a current not less than the current the circuit

is designed for i.e. 30 A. The current in Table

9 that meets this requirement is 34 A.

Step 6. Move across the Row showing 34 A to

Column 1 headed ‘Conductor size’. The Table

shows that the minimum conductor size for

the cable in this example is 4 mm2

Using the AS/NZS 3000 to select cable conductor size based on current-carrying capacity

With the introduction of 2007 version of the wiring rule the above charts were added. If you

compare the size of the conductors selected in AS/NZS 3008 to those selected if using

AS/NZS 3000 you will find that the selection using the later is a smaller cable. As either

method is considered acceptable, you can now have two answers to the same question and it

would seem that both are right.

So Which method do I choose?

As a general rule I tend to think that on small or simple jobs where the cable is not subject to

long runs or installed in such a way that it must be de-rated, (i.e. In bulk thermal insulation or

where Bulk thermal insulation would normally be installed) use AS/NZS 3000, in all other

cases use the more accurate AS/NZS 3008 series standards. In any case if your required to

select cable, you should have the latest version of both AS/NZS 3000 and AS/NZS 3008 on

hand and document the method of cable selection and the standard to which you referred.

Reference AS/NZS 3008 AS/NZS 3000 & HB300 Copyright Standards Australia

DETERMINATION OF MINIMUM CABLE SIZE BASED ON VOLTAGE DROP

CONSIDERATIONS

To satisfy the voltage drop limitations of a circuit, it is necessary to take into

account the current required by the load and the route length of the circuit, as

follows:

(a) Determine the current (I) requirements of the circuit.

(b) Determine the route length (L) of the circuit.

(c) Determine the maximum voltage drop (Vd) permitted on the

circuit run.

NOTE: AS/NZS 3000 generally limits the fall in voltage from the consumers’

terminals to any point of an installation to 5% of the nominal voltage at the

consumers’ terminals.

(d) Determine the voltage drop (Vc) in millivolts per ampere metre

(mV/A.m) using Equation 4.2(1) and the values of I, L and Vd

determined in Steps (a), (b) and (c).

(e) Refer to the tables of voltage drop (mV/A.m) for the different

cable types, Tables 40 to 50. Taking into account the method of

installation, maximum conductor operating temperature and load

power factor, the smallest conductor size which has a tabulated

voltage drop (mV/A.m) value nearest to, but not exceeding, the value

determined in Step (d) will be considered to be the minimum cable

size satisfying the voltage drop limitation. This simplified method

gives an approximate but conservative solution assuming maximum

cable operating temperatures and the most onerous relationship

between load and cable power factors. A more accurate assessment

can be made of the actual voltage drop (Vd) using the appropriate

equation of Clause 4.5, the cable reactance determined from Tables

30 to 33, the cable a.c. resistance determined from Tables 34 to 39

using the approximate conductor operating temperature assessed from

Equation 4.4(1), and the load power factor.

NOTES:

1 If the value of voltage drop assessed using the appropriate equation of Clause 4.5AS/NZS 3008 is significantly

lower than the equivalent value determined using the simplified method suggested in Steps (a) to (e),

consideration should be given to the calculation of voltage drop for the next smaller cable size.

2 Because of the need to make an initial set of assumptions relating to cable size, the calculation method of

Clause 4.5 will normally only be of use to check the accuracy of the simplified method or to check the voltage

drop on an existing or known cable installation

For mains cables 2% volt drop

Vc = Volts x 1000

= 230 x 0.02 x 1000

L x I Length of Run x Current

For sub-circuits cables 3% volt drop

Vc = Volts x 1000

= 230 x 0.03 x 1000

L x I Length of Run x Current

where

Vc = the millivolts drop per ampere-metre route length of circuit

Vd = actual voltage drop, in volts

L = route length of circuit, in metres (i.e. the distance measured along

the circuit from the origin to the connected load)

I = the current to be carried by the cable, in amperes.

Using the AS/NZS 3008.1 series to select cable conductor size based on VOLTAGE DROP CONSIDERATIONS

Examples

Consider the electrical installation illustrated below where the conductor

sizes are to be determined for the final sub-circuits for:

1. A three-phase 30 A appliance: 2. A single-phase 30 A appliance. The

three-phase voltage drop in the consumers mains is 3 V and the installation

generally operates as a balanced load, i.e. the current in the consumers

mains neutral can be disregarded in this case.

Both the sub-circuits used for the voltage calculation are to be wired with

multi-core V75 insulated and sheathed copper conductors installed in single

circuit configuration unenclosed in air clipped to a wall. Each circuit will be

protected by a 30 A circuit breaker. What size conductor will satisfy the

voltage drop limitation for each circuit?

1. Three-phase circuit

A 6 mm2 conductor size is chosen in accordance with the requirements for a

30 Amp load and a circuit length of 90 metres. This conductor size for the

cable and installation conditions has a current-carrying capacity of 37 A in

accordance with Table 12 Column 4 of the AS/NZS 3008.1 series. If the

mains cable has a voltage drop of 3 volts and the maximum total voltage

drop must not exceed 5%, the Voltage drop limitation for the sub-circuit is

determined as follows:

(i) Maximum permissible voltage drop in the final sub-circuit

including consumers mains is:

5% x 400 = 20V

(ii) Permitted voltage drop in the 3 phase final sub-circuit is:

20 V – 3 V = 17 V

(iii) The maximum unit value of voltage drop is determined as

follows:

L = 90 m; I = 30 A; Vd = 17 V; Vc = ?

Vc = Volts drop x 1000

= 17 x 1000

L x I 90 x 30

= 6.30mV/A.m

The calculated value is the unit value (Vc) applicable to the

voltage drop applicable to a voltage drop of 17 V in the final

sub-circuit..

(iv) The minimum allowable conductor size is selected from

unit value (Vc) in Table 40 to 50 of the

AS/NZS 3008.1 series as follows:

(1) Select the Table for mulit-core cables with copper

conductors, in this example Table 42

(Figure 2.14).

(2) Select 75°C Column for normal operating temperature of

V75 cables.

(3) Select the nearest lower unit value to the calculated value

of 6.3 mV/A.m i.e. 3.86 mV/A.m.

(4) The smallest cable size that will not exceed a voltage drop

of is 10 mm2 in accordance

with Column 1 of Table 42.

2. Single-phase circuit

A 4mm2 conductor size is chosen in accordance with requirements of the 30

A load. This conductor size for the cable and installation conditions has a

current-carrying capacity of 34 A in accordance with Table 9 Column 4. If

the mains cable has a voltage drop of 3 volts and the maximum total voltage

drop must not exceed 5%, the Voltage drop limitation for the sub-circuit is

determined as follows:

(i) Maximum permissible single-phase voltage drop in final sub-circuit

including consumers

mains is:

5% x 230 V = 11.5 V

(ii) Convert the 3 φ voltage drop in the consumers main to single-phase

value

Single-phase voltage drop value = Three-phase voltage drop.

1.73

= 3

1.73

= 1.73 volts

(iii) Calculated permitted single-phase voltage drop in the final sub-circuit

is:

11.5 V – 1.73 V = 9.77 V

(iv) The maximum unit value of voltage drop is determined as follows:

L = 90 m; I = 30 A; Vd = 9.77 V; Vc = ?

Vc =

Volts drop x

1000 = 9.77 x 1000

L x I 90 x 30

= 3.62mV/A.m

The calculated value is the maximum single-phase unit value (Vc)

applicable to a voltage drop of 9.77 V in the final sub-circuit.

The calculated value is for single-phase and must be converted to a three-

phase value to align with those given in Table 42.

Vc3φ = Vc1φ x 0.866

= 3.62 x 0.866

= 3.14 mV/A.m

(v) Determine the minimum conductor size from the unit values in Table 42

of the AS/NZS 3008.1 series.

(1) Select 75°C Column, in Table 42, for normal operating temperature of

V75 cables.

(2) Select the nearest lower unit value to the calculated value of 3.14

mV/Am i.e.

2.43 mV/A.m.

(3) The smallest cable size that will not exceed a voltage drop of 9.77 V is

16 mm2 in accordance with Column 1 of Table 42.

Summary:

In both final sub-circuits the cable size had to be increased to comply with

the voltage drop

requirements of the Wiring rules. The example assumed a balanced load in

the three-phase

consumers mains. Where the currents in each phase can be shown to be of

different

magnitudes for consistent periods voltage drop calculations can be

performed on a single-phase

basis, by geometrically summing the voltage drop in the heaviest loaded

phase and

the voltage drop in the neutral, as shown in the AS/NZS 3008.1 series. See

Table 2.3 for a

summary of the methods of determining voltage and how they are applied

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Using the AS/NZS 3000:2007 to select cable conductor size based on VOLTAGE DROP CONSIDERATIONS

Read clause C4.2 AS/NZS 3000 : 2007 this shows examples of calculations

and intended use of this chart

Reference AS/NZS 3008 & HB300 Copyright Standards Australia

DETERMINATION OF MINIMUM CABLE SIZE BASED ON THE SHORT-

CIRCUIT TEMPERATURE CONSIDERATIONS

To satisfy the short-circuit temperature limit it is necessary to take into account the

energy producing the temperature rise (I2t) and the initial and final temperatures,

as follows:

(a) Determine the maximum duration and value of the prospective

short-circuit current.

(b) Determine the initial and final conductor temperatures and select

an appropriate value of the constant (K) from Table 51.

(c) Calculate the minimum cross-sectional area of the cable using

Equation 5.3(1). This cable size represents the minimum size required

to satisfy the short-circuit temperature rise requirements.

Reference AS/NZS 3008 Copyright Standards Australia