Voltage Drop Calculator

Free AS/NZS 3008.1.1:2025 compliant voltage drop calculator for Australian electrical installations

How to use

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Electrical Load

Unit

Load

Amps

Power factor

Cable

Cable core

Conductor type

Insulation type

System Supply

Supply

Length of run

Voltage

Voltage drop threshold

Standard limit

The standard limits are totals from the point of supply, shared with the consumer mains and submains. (AS/NZS 3000 Cl. 3.6.2)

Fault level

Fault time

Installation Method

Derating Factor

Assistant

Overall derating factor

Ambient for operating temperature

Reference ambient per AS/NZS 3008.1.1:2025 (40 °C in air).

Protection

Enabled

Enable to check the fault loop impedance against a protective device.

Cable Size

ActiveManual

Circuits in parallel

NeutralManual–Circuits in parallel–

EarthManual–Circuits in parallel1

Spare capacity– A

Operating temperature– °C

Voltage drop– V (–%)

Three-phase SCC– A

Live to earth SCC– A

Fault loop impedance– Ω

Cable derated current1 A

Rated current– A

Cable resistance– Ω/km

Cable reactance– Ω/km

Min active conductor– mm²

Min earth conductor– mm²

Internal impedance– Ω

External impedance– Ω

How it's calculated

The AS/NZS 3008.1.1:2025 Method

The calculator above runs the full CableHero engine, the same one behind saved calculations and PDF reports. These are the checks it makes, with the numbers for the example it opens with.

1. Current-carrying capacity

AS/NZS 3008.1.1:2025 Clause 2.3.1, Section 3 tables; correction factors Clause 3.5

is the tabulated current-carrying capacity of the cable for its installation method (for example Table 3.15 for thermoplastic two-core cables), the design current and the product of the correction factors: grouping (Clause 3.5.2, Tables 3.33 to 3.43), air or soil temperature (Clause 3.5.3, Tables 3.44 and 3.45), depth of laying (Clause 3.5.4, Tables 3.46 and 3.47) and soil thermal resistivity (Clause 3.5.5, Table 3.48). The smallest size that passes, and then passes every check below, is selected.

2. Voltage drop

AS/NZS 3008.1.1:2025 Clauses 2.3.2, 4.3, 4.4 and 4.5

is the a.c. resistance at the conductor's operating temperature (Clause 4.4, read from Tables 4.1 to 4.13), the reactance, the load power factor and the route length in metres. is the three-phase value per Clause 4.3; a single-phase circuit has two conductors carrying the current, so its drop is times it (Clause 4.5). d.c. circuits use (Clause 4.7).

AS/NZS 3000:2018 Clause 3.6.2 limits the drop from the point of supply to any point to 5%, one total shared by the consumer mains, submains and final subcircuits.

3. Short circuit and fault loop

AS/NZS 3008.1.1:2025 Clause 5.3; AS/NZS 3000:2018 Clause 5.7.4 and Appendix B5.2.2

The active and earth conductors must withstand the prospective fault current for the clearing time ( from Table 5.1). is the resistance of the active and earth conductors over the route; dividing by 0.8 allows for the 20% of the loop impedance upstream of the circuit. must not exceed , where is the current that trips the device instantly (for example for a Type C circuit breaker), so the device clears an earth fault in time.

Worked example

Load of 32 A single phase at 230 V, 30 m of Thermoplastic (75 °C) copper multicore, enclosed wiring enclosure in air, cable chosen as 6 mm². Limit 5% drop.

6 mm²: .
at , , : .
. The 5% limit allows up to 48 m with this cable.
2.5 mm² earth: , ; with a protective device set, the calculator checks it against .

6 mm² gives a voltage drop of 3.13%.

References

Current-carrying capacity: AS/NZS 3008.1.1:2025 Clause 2.3.1; Table 3.15
Voltage drop: AS/NZS 3008.1.1:2025 Clauses 2.3.2, 4.3, 4.4 and 4.5; Table 4.7(A) (Rc); Table 4.1(B) (Xc)
Short-circuit temperature: AS/NZS 3008.1.1:2025 Clauses 2.3.3 and 5.3; Table 5.1 (K); Table 5.2 (limit)
Earth conductor size: AS/NZS 3000:2018 Table 5.1

How to Use This Voltage Drop Calculator

Enter the load, cable length, supply and installation above, and choose the cable size by hand (Manual, in the results) or let the calculator select it. It works out the voltage drop from the cable's AS/NZS 3008.1.1:2025 resistance and reactance at its operating temperature (three-phase as tabulated, single-phase 1.155 times it) and reports it in volts and as a percentage of the supply voltage, checked against your limit, together with the cable's derated current rating and fault loop impedance.

What is Voltage Drop?

Equipment malfunction: Motors may not start, or may run hot and inefficiently
Reduced light output: Lighting may appear dim or flicker
Increased energy consumption: Equipment draws more current to compensate
Overheating: Cables may overheat if undersized for the actual current draw
Non-compliance: Installations may not meet AS/NZS 3000:2018 requirements

Key Point: Voltage drop is calculated using the mV/A.m values tabulated in the a.c. tables of AS/NZS 3008.1.1:2025 (among Tables 4.14 to 4.31). The formula considers current, cable length, phase configuration, and cable characteristics.

The Voltage Drop Formula

Factors Affecting Voltage Drop

Cable Length

Voltage drop is directly proportional to length: doubling the run doubles the drop.

Current Magnitude

Higher currents produce proportionally higher voltage drop through the same cable.

Cable Size

Larger cross-sectional areas have lower impedance and therefore lower mV/A.m values.

Conductor Material

Copper has about 1.6 times the conductivity of aluminium, so aluminium cables of the same size drop more voltage.

Temperature

Conductor resistance rises with temperature, so hot cables drop slightly more voltage than the tabulated values suggest.

AS/NZS 3000:2018 Voltage Drop Requirements

Whole installation: maximum 5% from the point of supply to any point (Clause 3.6.2). Consumer mains, submains and final subcircuits share this total, so their percentages are added.
Dedicated substation: up to 7% where the point of supply is a substation on the premises dedicated to the installation (Exception 3)
Stand-alone systems: the voltage at equipment may fall up to 11% below nominal (Exception 4)
Extra-low voltage: maximum 10% (Clause 7.5.7)
Motor starting: transient starting currents are excluded from the check (Note 1)

How to Reduce Voltage Drop

Increase cable size - the most common fix, lowering the mV/A.m value
Shorten cable runs - relocate distribution boards closer to loads
Use copper instead of aluminium - better conductivity for the same size
Increase system voltage - three-phase supply reduces current for the same power
Split loads - run separate circuits so each carries less current
Use parallel cables - halves the effective impedance on large feeders

AS/NZS 3008.1.1:2025 Voltage Drop Values

Use these values for voltage drop calculations. Three-phase Vc in mV/A.m for thermoplastic (V-75) multicore cables with conductors at 75°C, worst-case power factor (Tables 4.17 and 4.22); single-phase is 1.155 times it (Clause 4.5).

Copper Conductors (mV/A.m)
Size (mm²)	Vc three-phase	Vc single-phase
1.5	28.6	33
2.5	15.6	18
4	9.71	11.2
6	6.49	7.49
10	3.86	4.46
16	2.43	2.81
25	1.54	1.78
35	1.11	1.28
50	0.827	0.955
70	0.581	0.671
95	0.428	0.494
120	0.348	0.402

Aluminium Conductors (mV/A.m)
Size (mm²)	Vc three-phase	Vc single-phase
16	4.04	4.66
25	2.54	2.93
35	1.84	2.12
50	1.36	1.57
70	0.947	1.09
95	0.69	0.797
120	0.551	0.636

AS/NZS 3008.1.1:2025 rates aluminium conductors from 16mm².

Voltage Drop Limits (AS/NZS 3000:2018)
Installation	Maximum Voltage Drop	Where it comes from
Low voltage installation on the network	5%	Clause 3.6.2: point of supply to any point, the total across consumer mains, submains and final subcircuits
Dedicated substation on the premises	7%	Clause 3.6.2, Exception 3
Stand-alone system	11%	Clause 3.6.2, Exception 4: below nominal supply voltage at the equipment
Extra-low voltage installation	10%	Clause 7.5.7
Motor starting	Not included	Clause 3.6.2, Note 1: transient starting currents are excluded

Important: The 5% is one total for the whole installation, so leave room for the consumer mains and submains when sizing a final subcircuit. Your distributor's service and installation rules may set further limits, such as on consumer mains.

Frequently Asked Questions

Voltage Drop Questions Answered

What is voltage drop?

Voltage drop is the reduction in voltage along a cable as current flows through its resistance and reactance. Every conductor has some impedance, so the voltage at the load end is always lower than at the supply end. Excessive voltage drop causes equipment to malfunction, lights to dim and motors to run hot.

What is the maximum allowable voltage drop in Australia?

AS/NZS 3000:2018 Clause 3.6.2 limits the voltage drop from the point of supply to any point in the installation to 5% of the nominal voltage, as one total for consumer mains, submains and final subcircuits. The limit is 7% where the point of supply is a substation on the premises dedicated to the installation, the voltage at equipment in a stand-alone system may fall up to 11% below nominal, and extra-low voltage installations are limited to 10% (Clause 7.5.7). Motor starting is excluded. Your distributor's service and installation rules may set further limits, such as on consumer mains.

How do I calculate voltage drop?

AS/NZS 3008.1.1:2025 tabulates the three-phase voltage drop Vc in mV/A.m for each cable. Three-phase voltage drop = (Vc × I × L) / 1000, where I is the current in amps and L is the route length in metres. For single-phase, multiply by 1.155 (2/√3, Clause 4.5), because the tabulated value already includes the √3. Divide by the system voltage and multiply by 100 for the percentage.

What factors affect voltage drop?

Cable length, current magnitude, cable size (cross-sectional area), conductor material (copper vs aluminium) and conductor temperature all affect voltage drop. Longer runs, higher currents, smaller cables, aluminium conductors and higher temperatures all increase voltage drop.

How do I reduce voltage drop?

Increase the cable size, shorten the cable run, use copper instead of aluminium, increase the system voltage (e.g. three-phase instead of single-phase), split the load across more circuits, or run cables in parallel.

What is mV/A.m and where do I find the values?

mV/A.m is the millivolts of voltage drop per amp of current per metre of route. AS/NZS 3008.1.1:2025 tabulates the three-phase value Vc in its a.c. tables (among Tables 4.14 to 4.31, which also hold the d.c. ones), by cable type, conductor material, conductor temperature and power factor, calculated from the cable's a.c. resistance and reactance (Tables 4.1 to 4.13). The reference table on this page lists common values for multicore cables at 75°C.

Reviewed by Ahmed Tayeh, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.

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How to use this calculator

Check a cable's voltage drop against the AS/NZS 3000:2018 limit, with the size fixed or chosen for you.

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