Cable Sizing Calculator — AS/NZS 3008 🇦🇺
AS/NZS 3008.1.1:2017 is the principal cable sizing standard used across Australia and New Zealand. Unlike its European and British counterparts, AS/NZS 3008 specifies a 40°C reference ambient temperature—reflecting the harsher thermal environment found throughout most of the continent—rather than the 30°C baseline used by BS 7671 and IEC 60364.
The standard covers selection of cables from 1 mm² to 630 mm² for voltages up to and including 0.6/1 kV. It defines current-carrying capacities for single-core and multicore cables in a wide range of installation methods, then applies derating factors for ambient temperature (Table 22), grouping (Table 23), depth of burial (Table 25), and thermal resistivity of soil (Table 26). Voltage drop limits are verified using the millivolt-per-ampere-per-metre values in Table 35, with the maximum permissible drop set by AS/NZS 3000:2018 Clause 3.6.2 at 5% from the point of supply.
This calculator automates the full AS/NZS 3008 methodology—from design current through to final cable selection—with every intermediate step and clause reference shown in the output. Enter your circuit parameters and receive a compliant cable size in seconds.
How Cable Sizing Works Under AS/NZS 3008
Step 1 — Determine the Design Current (Ib)
Calculate the maximum sustained load current the circuit will carry under normal
operating conditions. For single-phase circuits at 230V:
Ib = P / (V × cosφ). For three-phase balanced loads
at 400V: Ib = P / (√3 × VL × cosφ).
AS/NZS 3000:2018 Clause 2.2.2 requires the design current to account for diversity
where applicable.
Step 2 — Select Protective Device Rating (In)
Choose the circuit breaker or fuse rating such that In ≥ Ib
per AS/NZS 3000:2018 Clause 2.5.4. The protective device must also satisfy the
disconnection time requirements of Clause 2.6.3 for the specific circuit type.
Step 3 — Identify the Installation Method
AS/NZS 3008 Table 3 defines installation methods that determine which current-rating table to use. Common methods include: enclosed in conduit on a wall (Column 4), clipped direct to a surface (Column 6), on an unperforated cable tray (Column 7), and direct buried in ground (Column 9). The installation method determines the thermal dissipation characteristics of the cable.
Step 4 — Apply Derating Factors
The cable’s tabulated current-carrying capacity must be adjusted by multiplying the relevant derating factors:
- Ambient temperature (Ca): Table 22 for cables in air, Table 25 for buried cables. The reference temperature for cables in air is 40°C (Table 4) or 25°C for buried cables (Table 9). At 50°C ambient, the derating factor for 90°C-rated thermoplastic is 0.82.
- Grouping (Cg): Table 23 provides factors for multiple circuits installed together. Four single-phase circuits touching on a cable tray gives Cg = 0.65.
- Thermal insulation (Ci): Where cables are surrounded by thermal insulation for more than 0.5 m, Table 24 factors apply—a cable totally surrounded requires Ci = 0.50.
- Soil thermal resistivity (Cs): Table 26 adjusts for soil conditions other than the reference 1.2 K·m/W.
Step 5 — Calculate Required Current Rating (Iz)
The minimum cable current rating is:
Iz ≥ In / (Ca × Cg × Ci × Cs).
Select the smallest cable from Table 4 (single-core) or Table 13 (multicore) whose
tabulated rating meets or exceeds Iz. The calculator checks both the
active conductor and the separate earth conductor per AS/NZS 3000 Table 5.1.
Step 6 — Verify Voltage Drop
Using Table 35 (millivolt-per-ampere-per-metre values), calculate:
Vdrop = (mV/A/m × Ib × L) / 1000.
AS/NZS 3000:2018 Clause 3.6.2 limits the total voltage drop from the point of supply
to 5% of nominal voltage (i.e. 11.5V for 230V single-phase). If the voltage drop
exceeds the limit, increase the cable size and re-verify.
Step 7 — Confirm Earth Fault Loop Impedance
The selected cable must satisfy the earth fault loop impedance requirements of AS/NZS 3000 Clause 2.6.3.3 to ensure the protective device disconnects within the required time. The calculator verifies Zs using the conductor resistance values from AS/NZS 3008 Table 30 and reactance values from Table 34.
Key Reference Tables
Table 3 — Installation Methods
Defines 9 standard installation methods (e.g. enclosed in conduit, clipped direct, cable tray, underground) that determine the applicable current-rating column in Tables 4–21.
Identify how the cable will be installed to select the correct current-rating column. A cable in conduit uses a different column than one clipped direct, significantly affecting the permitted ampacity.
Table 4 — Current-Carrying Capacity, Single-Core Cables
Provides maximum continuous current ratings for single-core thermoplastic (V-75) and thermosetting (X-90) copper and aluminium cables at 40°C ambient temperature in air, across all installation methods.
After identifying the installation method, look up the minimum cable cross-section whose tabulated current rating meets or exceeds the required I_z after derating.
Table 13 — Current-Carrying Capacity, Multicore Cables
Equivalent to Table 4 but for multicore cables (2-core, 3-core, 4-core). Current ratings are lower than single-core due to mutual heating between conductors within the same sheath.
Used instead of Table 4 when the circuit uses multicore cable (e.g. TPS flat cable commonly used in Australian domestic wiring).
Table 22 — Ambient Temperature Derating Factors (In Air)
Correction factors for ambient temperatures other than the 40°C reference for cables installed in air. Factors range from 1.18 at 25°C to 0.50 at 70°C for 75°C-rated insulation.
Multiply the cable’s tabulated current rating by the correction factor for the actual site ambient temperature. Roof spaces in summer can exceed 60°C in many Australian regions.
Table 23 — Grouping Derating Factors
Factors for cables installed in groups (multiple circuits in the same conduit, cable tray, or trench). The more circuits grouped together, the lower the factor due to mutual heating.
Count the number of grouped circuits and apply the factor to each cable’s current rating. Six circuits on a tray have a grouping factor of 0.57.
Table 35 — Voltage Drop (mV/A/m)
Three-phase and single-phase millivolt-per-ampere-per-metre values for copper and aluminium conductors from 1 mm² to 630 mm², at different power factors.
Calculate voltage drop as V_drop = mV/A/m × I_b × L / 1000. Compare against the 5% limit from AS/NZS 3000 Clause 3.6.2.
Worked Example — AS/NZS 3008 Cable Sizing
Scenario
A 32A cooker circuit in a residential kitchen in Sydney. The cable runs 25 metres from the switchboard through PVC conduit on the wall. Ambient temperature is 40°C (Australian reference). Two other circuits share the same conduit. Supply is 230V single-phase. Cable type: V-75 (thermoplastic) multicore copper.
Determine design current
The cooker has a maximum demand of 7 kW at unity power factor on a 230V single-phase supply.
I_b = P / V = 7000 / 230 = 30.4AI_b = 30.4A
Select protective device
Choose a 32A MCB (I_n ≥ I_b). Per AS/NZS 3000 Clause 2.5.4, 32A ≥ 30.4A is satisfactory.
I_n = 32A (MCB Type B)
Identify installation method
Multicore cable enclosed in PVC conduit fixed to a wall corresponds to AS/NZS 3008 Table 3, Installation Method 1 (enclosed in conduit on a wall), using Table 13 Column 3.
Installation Method 1, Table 13 Column 3
Determine derating factors
Ambient temperature: 40°C is the reference for cables in air, so C_a = 1.00 (Table 22). Grouping: 3 circuits in the same conduit gives C_g = 0.70 (Table 23, 3 circuits, enclosed). No thermal insulation: C_i = 1.00.
C_total = C_a × C_g × C_i = 1.00 × 0.70 × 1.00 = 0.70C_total = 0.70
Calculate required cable current rating
The cable must carry at least I_n divided by the combined derating factor.
I_z ≥ I_n / C_total = 32 / 0.70 = 45.7AI_z ≥ 45.7A
Select cable size from Table 13
From AS/NZS 3008 Table 13, Column 3 (enclosed in conduit), V-75 copper multicore: 4 mm² = 32A (too low), 6 mm² = 41A (too low), 10 mm² = 58A (satisfactory, 58A ≥ 45.7A).
Selected cable: 10 mm² V-75 copper multicore (58A rating)
Verify voltage drop
From AS/NZS 3008 Table 35, the single-phase mV/A/m for 10 mm² copper at 0.8 power factor is approximately 4.01 mV/A/m.
V_drop = (mV/A/m × I_b × L) / 1000 = (4.01 × 30.4 × 25) / 1000 = 3.05VV_drop = 3.05V (1.33% of 230V) — well within the 5% limit (11.5V). PASS.
A 10 mm² V-75 copper multicore cable satisfies both the current-carrying capacity and voltage drop requirements for this 32A cooker circuit. The cable is derated from its base 58A rating due to grouping with two other circuits in the same conduit. The voltage drop of 1.33% leaves significant headroom below the 5% AS/NZS 3000 limit.
Common Mistakes When Using AS/NZS 3008
- 1
Using 30°C as the reference ambient temperature. AS/NZS 3008 uses 40°C for cables in air (unlike BS 7671 and IEC 60364 which use 30°C). Applying the wrong baseline means the cable appears to need less derating than it actually does, leading to undersized cables in hot conditions.
- 2
Confusing Table 4 (single-core) with Table 13 (multicore). The current ratings differ substantially—a 6 mm² V-75 single-core cable might be rated at 50A clipped direct, while the same conductor in a multicore cable is only rated at 41A. Using the wrong table overestimates the cable’s capacity.
- 3
Ignoring grouping derating in ceiling spaces. Australian ceiling voids regularly reach 50–60°C in summer, and multiple circuits are often bundled together. Both the ambient temperature factor and grouping factor must be applied simultaneously, which can reduce effective capacity by 40–50%.
- 4
Not checking voltage drop after selecting for current capacity. A cable that passes on ampacity can still fail on voltage drop, especially on long residential runs. AS/NZS 3000 Clause 3.6.2 imposes a 5% limit from the point of supply. Runs to sheds, granny flats, or pool equipment often exceed this.
- 5
Forgetting the 25°C reference ambient for buried cables. While cables in air use 40°C, buried cables (Tables 9–21) reference 25°C soil temperature. Applying the wrong reference temperature to underground circuits produces incorrect derating calculations.
How Does AS/NZS 3008 Compare?
AS/NZS 3008 is distinct from BS 7671 and IEC 60364 primarily in its 40°C reference ambient (vs 30°C), reflecting Australian climate conditions. This means cables are already rated for higher temperatures, so less derating is needed in moderate climates but the base ratings are inherently lower. Installation methods are numbered differently from the IEC lettering system (Method 1–9 vs A1/B1/C/D etc.). The standard also uses V-75 and X-90 insulation designations unique to the Australian market. NEC (US) is the most different, using AWG conductor sizes and an entirely different table structure.
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