Calculate voltage drop for electrical cables per AS/NZS 3008
Enter your circuit parameters below to calculate voltage drop based on AS/NZS 3008.
Voltage drop is one of the most critical factors in electrical cable sizing. It represents the loss of electrical potential (voltage) that occurs as current flows through a cable due to the cable's inherent resistance and reactance. Understanding and properly calculating voltage drop is essential for compliant and efficient electrical installations.
When electrical current flows through a conductor, it encounters resistance. This resistance causes some of the electrical energy to be converted to heat, resulting in a decrease in voltage along the length of the cable. The voltage at the load end of a circuit is always lower than at the supply end - this difference is the voltage drop.
Excessive voltage drop can cause:
Key Point: Voltage drop is calculated using mV/A/m values from AS/NZS 3008 tables. The formula considers current, cable length, phase configuration, and cable characteristics.
Voltage drop is calculated using the mV/A/m (millivolts per amp per meter) values from AS/NZS 3008:
Single-phase: VD = (I × L × 2 × mV/A/m) / 1000
Three-phase: VD = (I × L × √3 × mV/A/m) / 1000
Percentage: VD% = (VD / System Voltage) × 100
Where: I = load current (A), L = cable length (m), mV/A/m = cable impedance from AS/NZS 3008
Several factors influence the amount of voltage drop in a circuit:
Voltage drop is directly proportional to cable length. Doubling the cable length doubles the voltage drop. This is why long cable runs often require larger cables than short runs carrying the same current.
Higher currents cause greater voltage drop. The relationship is linear - doubling the current doubles the voltage drop for the same cable.
Larger cables have lower resistance and therefore lower voltage drop. Increasing the cable cross-sectional area significantly reduces voltage drop.
Copper has lower resistance than aluminium for the same cross-sectional area. Aluminium cables typically need to be larger than copper cables to achieve the same voltage drop performance.
Cable resistance increases with temperature. Cables running hot will have higher voltage drop than the same cables at lower temperatures.
The Australian/New Zealand wiring rules (AS/NZS 3000) specify maximum allowable voltage drops:
These limits ensure that equipment receives adequate voltage for proper operation and that installations operate safely and efficiently.
If your calculation shows excessive voltage drop, consider these solutions:
Voltage Limits (AS/NZS 3000)
Consumer mains: 2%
Total to outlet: 5%
Recommended max: 3%
Common Causes of High VD
Undersized cables
Long cable runs
High load currents
Aluminium vs copper
Solutions
Increase cable size
Shorten cable runs
Use copper conductors
Split loads across circuits
Use these values for voltage drop calculations. Based on AS/NZS 3008.1.1 for cables enclosed in conduit at 50Hz.
| Size (mm²) | mV/A/m (0.8 PF) |
|---|---|
| 1.5 | 29 |
| 2.5 | 18 |
| 4 | 11 |
| 6 | 7.3 |
| 10 | 4.4 |
| 16 | 2.8 |
| 25 | 1.8 |
| 35 | 1.3 |
| 50 | 0.93 |
| 70 | 0.63 |
| 95 | 0.46 |
| 120 | 0.36 |
| Size (mm²) | mV/A/m (0.8 PF) |
|---|---|
| 16 | 4.6 |
| 25 | 2.9 |
| 35 | 2.1 |
| 50 | 1.55 |
| 70 | 1.1 |
| 95 | 0.78 |
| 120 | 0.61 |
| Circuit Type | Maximum Voltage Drop | Notes |
|---|---|---|
| Consumer mains | 2% | From point of supply to main switchboard |
| Submains | 5% total | Combined with consumer mains |
| Final subcircuits | 5% total | From point of supply to any outlet |
| Motor circuits | 5% running / 3% starting | Starting voltage drop may need separate assessment |
Professional Features
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Frequently Asked Questions
Voltage drop is the reduction in voltage that occurs as electrical current flows through a cable due to the cable's resistance and reactance. The longer the cable and the higher the current, the greater the voltage drop. It's calculated using the mV/A/m values from AS/NZS 3008 and must be kept within limits specified by AS/NZS 3000 to ensure equipment operates correctly.
According to AS/NZS 3000, the maximum voltage drop from the point of supply to any outlet should not exceed 5%. For consumer mains (from point of supply to main switchboard), the limit is typically 2%. These limits ensure equipment receives adequate voltage for proper operation. Best practice is to design for 3% or less where possible.
Voltage drop is calculated using: For single-phase: VD = (I × L × 2 × mV/A/m) / 1000. For three-phase: VD = (I × L × √3 × mV/A/m) / 1000. Where I is the current in amps, L is the cable length in meters, and mV/A/m is the cable impedance value from AS/NZS 3008 tables. The percentage is: VD% = (VD / System Voltage) × 100. Use our calculator above for quick calculations.
The main factors are: Cable length - longer cables have more drop; Current magnitude - higher current means more drop; Cable size - smaller cables have higher resistance; Conductor material - aluminium has higher resistance than copper; Temperature - higher temperatures increase resistance; and Power factor - affects the reactive component of voltage drop.
To reduce voltage drop: Increase cable size - the most common solution as larger cables have lower resistance; Shorten cable runs - relocate equipment or switchboards where possible; Use copper instead of aluminium - copper has lower resistance per unit area; Increase system voltage - three-phase has lower drop than single-phase; Split loads - distribute across multiple circuits.
mV/A/m stands for millivolts per amp per meter. It's a value that represents the voltage drop characteristics of a cable based on its size, material (copper or aluminium), and installation method. These values are found in the tables of AS/NZS 3008.1.1. Lower mV/A/m values mean less voltage drop per unit length. Our calculator uses these standard values automatically.
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