Table of Contents
How to Calculate Cable Size Using AS/NZS 3008
This comprehensive guide provides step-by-step instructions for sizing electrical cables in accordance with Australian Standard AS/NZS 3008.1.1:2025. Whether you’re an electrical engineer, contractor, or student, this resource will help you master the essential calculations for selecting the correct cable sizes in various applications. Key topics covered:
Current-carrying capacity calculations
Voltage drop requirements
Short-circuit performance
Neutral and earth cable sizing
We’ll walk through two detailed examples, demonstrating how to:
Calculate load currents
Select initial cable sizes
Determine cable operating temperatures
Check voltage drop compliance
Verify short-circuit performance
Choose appropriate active, neutral, and earth cable sizes
By following this guide, you’ll learn how to accurately size cables for different electrical installations while ensuring compliance with Australian Standards. Whether you’re working on residential, commercial, or industrial projects, these calculations are crucial for safe and efficient electrical systems. Use this article as your go-to reference for cable sizing calculations, and feel confident in your ability to design electrical systems that meet all necessary requirements and safety standards.
If you prefer to perform these AS/NZS 3008.1 cable sizing calculations automatically, you can use our free AS/NZS 3008.1 cable sizing calculator. It calculates current-carrying capacity, voltage drop, and short-circuit performance, and instantly selects compliant active, neutral, and earth conductor sizes.
Example 1 - Three-Phase 400 V System: Cable Sizing for High Current Load Over a Long Run
Problem
Three-phase 400 V system. Load current = 1200 A. Route length = 260 m. The cable type is single-core copper XLPE 90 degrees. Buried in separate conduits. The voltage drop limit is 3.2 %. The fault level is 15 kA, and the clearing time is 1 second. Based on AS/NZS 3008.1.1:2025, calculate the smallest active conductor, neutral, and earth cable sizes.
Answer
Step 1: Minimum size to meet current-carrying capacity
To determine which cable can carry the 1200 A load, an initial cable size assumption must be made. A practical assumption would be to use 2 sets of 400 mm2 cables, which would equate to 1252 A. Based on this assumption, we would proceed to the subsequent steps to determine whether it could provide sufficient current-carrying capacity, adequate voltage drop, and short-circuit performance.
The correct current rating table must be selected from AS/NZS 3008.1.1:2025 based on the cable type and phases to make a valid assumption. Once the correct reference table has been identified, the appropriate column for the installation method and conductor type must be selected.
The appropriate table and column for this problem are Table 3.13, Column 19.
Step 2: Minimum size to meet voltage drop requirement
The voltage drop for 3 sets of 630 mm2 cables will be calculated with a load current of 1200 A.
The rated current of the assumed cable is:
\(I_R = 2460\ A\) (the value from Table 3.13, Column 19 is multiplied by 3, which is the number of runs chosen)
Determine cable operating temperature
To determine the Voltage Drop for a cable selected, the operating temperature \((\theta_0)\) must first be calculated using the equation:
\(\left(\frac{I_0}{I_R}\right)^2 = \frac{\theta_0 – \theta_A}{\theta_R – \theta_A}\)
Where:
\(I_0\) = operating current, in Amperes
\(I_R\) = rated current given in Tables 3.9 to 3.32 (AS/NZS 3008.1.1:2025), in amperes
\(\theta_R\) = operating temperature of the cable when carrying \(I_R\), in degrees Celsius
\(\theta_A\) = ambient air or soil temperature, in degrees Celsius
The values for these are as follows:
\(I_0 = 1200\,A\)
\(\theta_R = 90°C\) (temperature rating for the XLPE 90 degrees cable)
\(\theta_A = 25°C\) (standard ambient temperature for soil)
\(I_R = 2460\ A\)
From these, the calculated operating temperature is:
\(\theta_0 = 40.47°C\)
Determine cable resistance based on cable operating temperature
\(R_c = 0.0389\ \Omega/km\) (from Table 4.5(A) Column 3 of AS/NZS 3008.1.1:2025, single core cables)
Determine cable reactance
The cable reactance \(X_c\) can also be determined from the cable type (XLPE)
\(X_c = 0.0787\ \Omega/km\) (from Table 4.1(A) Column 3 of AS/NZS 3008.1.1:2025, the appropriate table for this cable type)
Calculate voltage drop based on power factor and cable operating temperature
\(I = 1260\ A\) (Load Current)
\(L=260\, m\) (Route length)
A standard power factor of 0.9 can be used to calculate the power factor angle:
\(\theta = 25.84°\)
From these values, the voltage drop on the cable can be calculated:
\(V_{d3\phi} = 12.49\ V\)
\(\%VD = (12.49\ V/400\ V)*100\%\)
\(\%VD = 3.12\%\)
Step 3: Minimum size to handle the prospective short-circuit current
The fault level dictated by the impedances must be calculated to evaluate the short-circuit performance.
Calculate the total impedance of the cable \(Z_t\) from the values of \(R_c\) and \(X_c\)
\(Z_t=\sqrt{R_c^2 + X_c^2}=0.0878\ \Omega/km\)
\(Z_c=0.0228\ \Omega\)
\(Z_n=400\ V/15000\ A=0.027\ \Omega\)
\(V_{\text{end}}=400\ V-12.49\ V=387.51\ V\)
The fault level can be calculated using the equation:
\(I=\frac{V_{\text{end}}}{Z_{c} + Z_{n}}=\frac{387.51}{0.0228+0.027}\)
\(I=7829.78\ A\)
This is the fault level at the end of the cable, which must be considered in calculating the minimum cross-sectional area.
\(I^2 t = K^2 S^2\)
Where:
\(I=7829.78\ A\)
\(t=1s\)
The initial and final temperatures are needed to determine the K constant. The 45°C operating temperature calculated earlier would be the initial temperature. The final temperature would depend on the insulating materials in contact with the conductors. As per Table 5.2 of AS/NZS 3008.1.1:2025, for XLPE 90, the final temperature would be 250°C.
Using these temperatures, the value of K can be determined from Table 5.1 of AS/NZS 3008.1.1:2025.
\(K=167.4\)
\(S=46.77\ mm^2\)
Step 4: Select the neutral cable size
As per Section 3.5.2 of AS/NZS 3000, the neutral cable will be the same size as the smallest active conductor and meet the load current requirements.
Step 5: Select the earthing cable size
However, the minimum earth cable size capable of handling a phase-to-earth fault must also be checked. The equation for the minimum conductor cross-section used in Step 5 will also be used. The only difference is that the phase-to-earth short circuit current will be used.
\(I_{\text{1ph}} = \frac{I_{\text{3ph}}}{\sqrt{3}} = \frac{7829.78\ A}{\sqrt{3}}\)
\(I_{\text{1ph}}=4520.53\ A\)
\(S=27.00 mm^2\)
Example 2 - 690 V Fire-Rated Cable Selection for 75 kW Load
Problem
The load is 75 kW, 3 phase, with a power factor of 0.9 at 690 V. The cable is Fire-Rated, Cu, single-core length = 5 m, unenclosed touching. There are no derating factors. The maximum allowed voltage drop is 2.5 %, fault level = 15 kA, and fault time (fault clearing time) = 0.15 s.
Answer
Step 1. Minimum size to meet current-carrying capacity
The load current is:
\(I_0 = \frac{P}{\sqrt{3} V \cos(\theta)}\)
\(P=75000\ W\)
\(V=690\ V\)
\(\cos(\theta)=0.9\) (power factor)
To determine which cable can carry the 69.73 A load, an initial cable size assumption must be made. A practical assumption would be to have 10 mm2 cables, which equate to 77 A. Based on this assumption, we would proceed to the subsequent steps to determine whether it could provide sufficient current-carrying capacity, adequate voltage drop, and short-circuit performance.
The appropriate table and column for this problem are Table 3.14, Column 5.
Step 2: Minimum size to meet voltage drop requirement
The voltage drop for 35 mm2 cables will be calculated with a load current of 69.73 A.
Determine cable operating temperature
\(\left(\frac{I_0}{I_R}\right)^2 = \frac{\theta_0 – \theta_A}{\theta_R – \theta_A}\)
Where:
\(\theta_R = 110°C\) (temperature rating for Fire-Rated Copper cables)
\(\theta_A = 40°C\) (standard ambient temperature for air)
\(I_0=69.73\ A\)
\(I_R=176\ A\) (The value is from Table 3.14, Column 5 of AS/NZS 3008.1.1:2025, the appropriate table for single-core, fire-rated cables. The cable size assumption is 35 mm2)
From these, the calculated operating temperature is:
\(\theta_0 = 50.98°C\)
\(\theta_0\) is then raised to the nearest temperature of 45°C, 60°C, 75°C, 80°C, 90°C or 110°C. For the calculated operating temperature, the nearest would be 60°C.
Determine cable resistance based on cable operating temperature
\(R_c = 0.607\ \Omega/km\) (from Table 4.5(A) Column 4 of AS/NZS 3008.1.1:2025, single core cables)
Determine cable reactance
\(X_c = 0.117\ \Omega/km\) (from Table 4.1(A) Column 4 of AS/NZS 3008.1.1:2025, the appropriate table for this cable type)
The \(R_c\) and \(X_c\) values will be used to calculate the voltage drop on the cables, considering the load dictated by the problem. Note that these would need to be converted to Ω/m.
Calculate voltage drop based on power factor and cable operating temperature
\(V_{d3\phi} = IL\left[\sqrt{3}(R_c \cos \theta + X_c \sin \theta)\right]\)
\(I = 69.73\ A\) (Load Current)
\(L=5\ m\) (Length of the Cable)
A standard power factor of 0.9 can be used to calculate the power factor angle:
\(\theta = 25.84°\)
\(V_{d3\phi} = 0.361\ V\)
\(\%VD=(0.361\ V/690\ V)*100\%\)
\(\%VD = 0.052\%\)
Step 3: Minimum size to handle the prospective short-circuit current
The fault level, as dictated by the impedances, must be calculated to evaluate short-circuit performance.
Calculate the total impedance of the cable \(Z_t\) from the values of \(R_c\) and \(X_c\):
\(Z_t=\sqrt{R_c^2 + X_c^2}=0.6182\ \Omega/km\)
Multiply this by the length (5m) to get the cable impedance:
\(Z_c=0.00309\ \Omega\)
The impedance of the network \(Z_n\) can be calculated by Ohm’s law using the supply voltage and the fault level at the supply end of the cable:
\(Z_n=690\ V/15000\ A=0.046\ \Omega\)
The voltage at the end of the cable can also be calculated by subtracting the calculated voltage drop from the supply voltage.
\(V_{\text{end}}=690\ V-0.361\ V=689.639\ V\)
The fault level can be calculated using the equation:
\(I=\frac{V_{\text{end}}}{Z_{c} + Z_{n}}=\frac{689.639}{0.00309+0.046}\)
\(I=14048.67\ A\)
Where:
\(I=14048.67\ A\)
\(t=0.15s\)
\(K=164.7\) (from Table 5.1)
The final temperature is as per Table 5.2 for fire-rated cables (250°C).
The initial temperature is 50°C, the nearest available to the calculated operating temperature.
Using these temperatures, the value of K can be determined from Table 5.1 of AS/NZS 3008.1.1:2025.
\(S=33.04 mm^2\)
The result suggests that the chosen cable (35 mm2) can also handle the specified fault level.
Active Cable Size = 35 mm2
Step 4: Select the neutral cable size
As per Section 3.5.2 of AS/NZS 3000, the neutral cable will be the same size as the smallest active conductor and meet the load current requirements.
Step 5: Select the earthing cable size
However, the minimum earth-cable size capable of handling a phase-to-earth fault must also be checked. The equation for the minimum conductor cross-section used in Step 5 will also be used. The only difference is that the phase-to-earth short circuit current will be used.
\(I_{\text{1ph}} = \frac{I_{\text{3ph}}}{\sqrt{3}} = \frac{14047.38\ A}{\sqrt{3}}\)
\(I_{\text{1ph}} = 8110.52\ A\)
The same values of \(t\), and \(K\) in Step 3 will be used to calculate the minimum conductor cross-section \(S\).
\(S=19.07 mm^2\)
References:
Standards Australia / Standards New Zealand. AS/NZS 3008.1.1:2025 – Electrical Installations – Selection of Cables – Cables for Alternating Voltages up to and Including 0.6/1 kV – Typical Australian Installation Conditions. Sydney: Standards Australia.
Standards Australia / Standards New Zealand. AS/NZS 3000:2018 – Electrical Installations (Wiring Rules). Sydney: Standards Australia.
IEC (International Electrotechnical Commission). IEC 60287 – Electric Cables – Calculation of the Current Rating. Geneva: International Electrotechnical Commission.
IEC (International Electrotechnical Commission). IEC 60949 – Calculation of Thermally Permissible Short-Circuit Currents, Taking into Account Non-Adiabatic Heating Effects. Geneva: International Electrotechnical Commission.
ELEK. Electrical Engineering Calculators. Available at: https://elek.com/calculators/
