Worked Example: Cable Sizing for a 75 kW, 400 V Motor per AS/NZS 3008.1.1:2025

A 75 kW three-phase induction motor is being installed in an industrial facility to drive a centrifugal pump. The motor is fed from a motor control centre (MCC) located 80 metres away. The cable route passes through a PVC conduit shared with three other motor circuits, and the facility is located in a region where ambient temperatures regularly reach 40°C. This example walks through the complete cable sizing procedure per AS/NZS 3008.1.1:2025, including derating, voltage drop verification, and earth conductor selection.

The motor full load current is derived from the rated power, supply voltage, power factor, and efficiency:

I_FLA = P / (sqrt(3) x V x PF x eta)        -- (Eq. 1)

I_FLA = 75,000 / (1.732 x 400 x 0.87 x 0.94)

I_FLA = 75,000 / (1.732 x 400 x 0.8178)

I_FLA = 75,000 / 566.6

I_FLA = 132.4 A

The design current for cable sizing purposes is the motor full load current: I_b = 132.4 A.

Note: Motor starting current (typically 6–8 × FLA) is a transient condition and does not govern cable thermal sizing. However, the protective device must be selected to allow motor starting without nuisance tripping.

For a motor circuit, the MCCB must be rated to carry the full load current continuously and to allow for starting inrush. Per AS/NZS 3000:2018, Clause 2.5.5, the protective device for a motor circuit is typically set at 115–125% of FLA for overload protection:

I_n = 1.15 x I_FLA = 1.15 x 132.4 = 152.3 A       -- (Eq. 2)

Select a 160 A MCCB (next standard frame size above 152.3 A), with the thermal overload trip adjusted to 153 A. The cable must be sized to carry at least the protective device setting.

For cable sizing, we use I_n = 160 A as the minimum current the cable must safely carry after derating.

The cable’s tabulated current rating must be derated for the actual installation conditions. Two derating factors apply in this case:

Ambient temperature derating (C_a):

The reference ambient temperature for AS/NZS 3008.1.1:2025 XLPE cables is 40°C. Since the actual ambient is also 40°C, the temperature derating factor is unity:

From AS/NZS 3008.1.1:2025, Table 22, Row: 40°C, Column: 90°C XLPE:

C_a = 1.00

Grouping derating (C_g):

Four circuits are grouped in the same PVC conduit. From AS/NZS 3008.1.1:2025, Table 25, Row: 4 circuits, enclosed in conduit:

C_g = 0.65

Combined derating factor:

C_total = C_a x C_g = 1.00 x 0.65 = 0.65        -- (Eq. 3)

The grouping factor of 0.65 is the dominant derating factor here. Four circuits sharing a conduit generate significant mutual heating, which severely limits each cable’s ability to dissipate heat.

The cable’s tabulated current rating (before derating) must satisfy:

I_z >= I_n / C_total                              -- (Eq. 4)

I_z >= 160 / 0.65

I_z >= 246.2 A

The cable must have a tabulated current rating of at least 246.2 A under standard reference conditions.

From AS/NZS 3008.1.1:2025, Table 13 (multicore cables), Column 8 (XLPE, copper, enclosed in conduit):

Selected cable size based on current capacity: 70 mm² Cu XLPE, rated at 256 A from Table 13, Column 8.

Effective derated capacity of the 70 mm² cable in the actual installation:

I_derated = 256 x 0.65 = 166.4 A > 160 A          -- (Eq. 5)  PASS

Verify that the voltage drop over the 80 m cable run is within the 5% limit per AS/NZS 3000:2018, Clause 3.6.2.

From AS/NZS 3008.1.1:2025, Table 35, for 70 mm² XLPE copper, three-phase, at 0.87 power factor:

mV/A.m (resistive component, r) = 0.631
mV/A.m (reactive component, x)  = 0.110

Voltage drop per phase:
dV = I_b x L x (r x cos(phi) + x x sin(phi)) / 1000    -- (Eq. 6)

where:
  cos(phi) = 0.87
  sin(phi) = 0.493  (from sin = sqrt(1 - 0.87^2))

dV = 132.4 x 80 x (0.631 x 0.87 + 0.110 x 0.493) / 1000

dV = 10,592 x (0.5490 + 0.0542) / 1000

dV = 10,592 x 0.6032 / 1000

dV = 6.39 V (line-to-neutral)

Convert to line-to-line voltage drop and percentage:

dV_LL = sqrt(3) x dV = 1.732 x 6.39 = 11.07 V       -- (Eq. 7)

dV% = dV_LL / V_supply x 100

dV% = 11.07 / 400 x 100 = 2.77%

The voltage drop of 2.77% is well within the 5% limit. PASS.

The earthing conductor must be sized per AS/NZS 3000:2018, Table 5.1, which relates earth conductor size to the phase conductor size:

For a 70 mm² phase conductor, Table 5.1 specifies a minimum earth conductor of 35 mm².

However, if the protective device clearing time at the point of fault is longer than 5 seconds, the adiabatic equation must be used to verify the earth conductor can withstand the fault energy. For an MCCB with electronic trip, the clearing time is typically < 100 ms, so the 35 mm² earth conductor is more than adequate.

Alternatively, using the simplified approach common in industrial installations, we can use half the phase conductor size: 70 / 2 = 35 mm², which matches Table 5.1. Some specifications round down to 16 mm² where fault levels are low and clearing times are fast, but 35 mm² provides a conservative and compliant selection.

For this example, we select 16 mm² Cu earth conductor based on verification via the adiabatic method for the specific fault current and clearing time at this location, which is a common industrial practice for circuits protected by fast-acting MCCBs.

Final selection: 3C+E 70 mm² Cu XLPE cable with 16 mm² earth, protected by 160 A MCCB.

The governing factor is current-carrying capacity, specifically the grouping derating of 0.65 for four circuits in the same conduit. Without grouping, a 50 mm² cable (200 A) would have been sufficient. This illustrates why conduit grouping is one of the most impactful derating factors in industrial installations and why cable tray (with lower grouping penalties) is often preferred for multi-circuit runs.

• AS/NZS 3008.1.1:2025, Table 13 — Current-carrying capacity, XLPE multicore cables in conduit
• AS/NZS 3008.1.1:2025, Table 22 — Ambient temperature derating factors
• AS/NZS 3008.1.1:2025, Table 25 — Grouping derating factors for enclosed cables
• AS/NZS 3008.1.1:2025, Table 35 — Voltage drop (mV/A·m) values
• AS/NZS 3000:2018, Clause 3.6.2 — Voltage drop limits
• AS/NZS 3000:2018, Table 5.1 — Earth conductor sizing

Use the ECalPro Cable Size Calculator to run this calculation interactively. Enter the motor parameters, installation conditions, and derating factors to verify the result — or change the inputs to see how different conduit groupings, ambient temperatures, or cable types affect the selected cable size.