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 dependent upon the method of installation installation and the presence of external influences, such as thermal insulation, which restrict the operating temperature of the cable. (b) Voltage drop — dependent dependent upon the impedance of the cable, the magnitude of the load current and the load power factor. (c) Short-circuit temperature limit — dependent 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 semienclosed re-wirer able fuse protection, cable grouping, and thermal insulation occur. On the other hand the voltage drop limitation limitation is usually the deciding factor for longer route lengths which are not subject to the factors mentioned above. T he 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 derating 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 t he 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 currentcarrying 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 currentcarrying 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 2 the cable in this example is 4 mm
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 ( V d) permitted on the circuit run. NOTE: AS/NZS 3000 generally limits the fall in voltage from the consumer s’ terminals to any point of an installation to 5% of the nominal voltage at the consumers’ terminals. (d) Determine the voltage drop ( V c) in millivolts per ampere metre (mV/A.m) using Equation 4.2(1) and the values of I , L and V d 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 ( V d) 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 Volts x 1000 LxI
Vc =
=
230 x 0.02 x 1000 Length of Run x Current
For sub-circuits cables 3% volt drop Vc =
Volts x 1000
LxI
=
230 x 0.03 x 1000 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 = LxI =
17 x 1000 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 = ?
Volts drop x Vc = 1000 LxI
=
9.77 x 1000 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 threephase 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 _____________________________________________________________ ________________________________
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 SHORTCIRCUIT TEMPERATURE CONSIDERATIONS
To satisfy the short-circuit temperature limit it is necessary to take into account the energy producing the temperature rise ( I 2t ) 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
