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What is Cable Sizing?

Cable sizing is the method of determining the correct cross-sectional area (thickness) of electrical cable required for a specific installation. It is not just about making the cable thick enough to carry current. It Is about finding the MINIMUM size that:

• Safely carries the required load without overheating.

• Keeps the voltage drop within limits so equipment works properly.

• Complies with electrical standards (AS/NZS 3008).

• Provides the reasonable cost without wasting material.

Why is Correct Sizing important?

Undersized Cables

• Resistance causes heat & risk of fire.

• Equipment fails from low voltage.

• Safety risk to people and property.

• Violates electrical standards.

• May void coverage.

Oversized Cables 

• Unnecessary material and cost

• Harder to install and terminate

• No added benefit

• Every unnecessary dollar spent

Correct Size

• Appropriate temperature rise

• Proper equipment operation

• Meets all standards

• Best cost-benefit ratio

• Industry best practice

What is AS/NZS 3008?

AS/NZS 3008 is the Australian/New Zealand standard for electrical installations. 

It covers:

• Cable sizing requirements,

• Voltage drop limits,

• Current carrying capacity,

• Installation methods,

• Safety requirements &

• Testing procedures.

Standards like AS/NZS 3008 exist to ensure:

• Cables don’t overheat and cause fires,

• Equipment operates properly,

• Same installation anywhere in Australia/NZ,

• Compliance required for coverage,

• Property compliance adds value &

• Legal requirement for installations.

Voltage Types in Australia & New Zealand

Domestic

230V single-phase (standard home supply)

Commercial 

230V single-phase (general distribution)

400V three-phase (larger installations)

Industrial

400V three-phase (standard)

11,000V (distribution level)

33,000V (transmission level)

Common Load Currents

Single Phase at 230V:

1kW = 4.3A

2kW = 8.7A

3kW = 13A

5kW = 21.7A

10kW = 43.5A

Three Phase at 400V:

1kW = 1.4A

5kW = 7.2A

10kW = 14.5A

15kW = 21.7A

22kW = 32A

30kW = 43.4A

(Calculation: I = P / (V x √3 x 0.85) for 3-phase)

Why Derating?

Cables operate at their best in ideal conditions:

• Moderate temperature (around 40°C ambient),

• Good air circulation &

• No thermal insulation around cable.

Real-world conditions are often different. 

When conditions worsen, we apply derating factors – percentages that reduce the safe current capacity.

Common Derating Conditions

The common derating conditions are:

• Exposure to Sunlight,

• Thermal Insulation,

• Conduit Installation,

• Multiple Cables Together &

• High Ambient Temperature.

3 Keys to Cable Sizing

1). Current Carrying Capacity

The cable must be safely handle the expected load current without excessive heat.

It is based on material, size, insulation & installation method and follow AS/NZS 3008 capacity tables

Load current < Cable capacity

2). Voltage Drop

The cable resistance causes voltage loss over distance affecting equipment performance.

It is based on resistivity, current, distance & cross-sectional area and typically 2-5% depending on load type

Actual drop must be < Maximum allowed

3). Short Circuit Protection

The cable should work with protective devices. It is based on cable impedance and fault clearing time and follows manufacturer specifications.

Cable Components

Conductor

The metal that carries current (copper / aluminium).

• Copper: Better conductor & smaller sizes needed.

• Aluminium: Good conductor & larger sizes needed.

Insulation

Plastic covering that protects the conductor.

• PVC: Common & lower temperature rating.

• XLPE: Better & higher temperature rating.

Sheath

Outer covering that protects the whole cable.

• PVC: General purpose

• Other types: Special conditions (UV, water, etc.)

Core Configuration

• Single-core: One conductor per cable

• Multi-core: Multiple conductors in one cable

Calculation 

Voltage Drop Calculation

The voltage drop in a cable depends on:

How much current flows through it (I).

How long the cable is (L).

What material it is made of (ρ).

How thick it is (CSA).

Formula

V_D = (ρ × I × L) / CSA

Voltage Drop = (Resistivity x Current x Length) / Cross-sectional Area

Example

Copper cable (ρ = 0.0186)

50A current

50m length

25mm² cable

V_D = (0.0186 x 50 x 50) / 25 = 1.86V

To convert to percentage

V_D% = (1.86 / 400V) x 100 = 0.465% (Good)

If we had used 16mm²

V_D = (0.0186 x 50 x 50) / 16 = 2.91V

V_D% = (2.91 / 400V) x 100 = 0.73% (Still acceptable)

If we had used 10mm²

V_D = (0.0186 x 50 x 50) / 10 = 4.65V

V_D% = (4.65 / 400V) x 100 = 1.16% (Exceeds 5% in single phase & acceptable in 3-phase)

Current Capacity Calculation

Current capacity depends on:

• Material (copper conducts better than aluminium)

• Insulation type (higher temp rating = higher capacity)

• Installation method (air cooling better than conduit)

• Cable size (larger = more current)

Current capacity values come from the AS/NZS 3008 tables developed through testing. 

These tables account for:

• Heat dissipation rate

• Ambient temperature

• Thermal insulation effects

• Safety margins

Example

A 25mm² copper multi-core cable:

In air: 73A capacity

In conduit: ~62A (15% reduction)

In thermal insulation: ~50A (30% reduction)

CSA Calculation

To find the minimum cable size required:

Step-1: Determine limiting factor

Voltage drop limit usually is limiting for longer runs.

Current capacity usually is limiting for shorter runs.

Step-2: Calculate minimum CSA

For voltage drop

CSA = (ρ x I x L) / (V x V_D%)

Cross-sectional Area = (Resistivity x Current x Length) / (Voltage x Voltage Drop %)

For current capacity

CSA selected from tables > Load current

Step-3: Select Standard Size

Round up to next standard size.

Step-4: Verify Both Limits

Check that final size meets the both voltage drop and current capacity requirements.

AS/NZS 3008 Cable Current Rating Tables

AS/NZS 3008.1.1 is the Australian/New Zealand standard used to determine current-carrying capacity, voltage drop and cable selection for LV and MV cables. 

It provides detailed tables based on:

• Conductor material (Copper / Aluminium)

• Insulation type (PVC, XLPE, EPR)

• Installation method (air, conduit, buried, tray)

• Number of cores

• Ambient temperature

• Grouping factor

Below are the most commonly used AS/NZS 3008 current rating tables.

1). Copper Cable Current Rating – XLPE Insulation (90°C)

Single-Core Copper Cable in Air (Reference Method C)

Size (mm²)	Current Rating (A)
1.5	23
2.5	31
4	42
6	54
10	75
16	100
25	134
35	162
50	195
70	245
95	292
120	335
150	380
185	437
240	515
300	594
400	689
500	783

2). Copper Cable Current Rating – XLPE Insulation (Buried Direct)

Size (mm²)	Current Rating (A)
1.5	26
2.5	35
4	47
6	60
10	83
16	110
25	144
35	174
50	209
70	262
95	312
120	357
150	406
185	467
240	550
300	636
400	738
500	840

3). Multi-Core Copper Cable Current Rating – XLPE (3 Core)

Size (mm²)	Current Rating (A)
1.5	19
2.5	26
4	35
6	45
10	62
16	83
25	108
35	131
50	158
70	200
95	239
120	274
150	312
185	358
240	423
300	489
400	568

4). Aluminium Cable Current Rating – XLPE (Single Core in Air)

Size (mm²)	Current Rating (A)
16	76
25	100
35	121
50	146
70	183
95	218
120	250
150	284
185	327
240	385
300	443
400	515
500	585

5). AS/NZS 3008 Voltage Drop Values (Copper & 3-Phase)

Size mm²	mV/A/m
1.5	29
2.5	18
4	11
6	7.3
10	4.4
16	2.8
25	1.75
35	1.25
50	0.93
70	0.68
95	0.49
120	0.39
150	0.32
185	0.26
240	0.20

6). Installation Reference Methods (AS/NZS 3008)

Method	Description
A	Enclosed in conduit in wall
B	Enclosed in conduit on wall
C	Clipped direct or on tray
D	Underground direct buried
E	In air on ladder/tray

7). Temperature Correction Factors

Temperature	Factor
25°C	1.03
30°C	1.00
35°C	0.96
40°C	0.91
45°C	0.87
50°C	0.82

8). Grouping Correction Factor

Number of Cables	Factor
1	1.00
2	0.80
3	0.70
4	0.65
5	0.60
6	0.57