Voltage drop calculator

Field use cases

Use this calculator when a cable run already has a load basis, a one-way route length and a project cable data source, and the reviewer needs a transparent AC voltage-drop estimate before moving into cable selection or installation documentation. Typical field uses include checking a proposed final subcircuit, comparing two route options, reviewing a submain allocation, or documenting the voltage-drop basis for a load schedule.

The result is a preliminary engineering estimate. It shows the voltage drop in volts, the percentage of the entered nominal voltage, the estimated receiving-end voltage and the maximum one-way route length implied by the entered target. It does not choose a conductor size, prove current-carrying capacity, decide grouping or derating, select protective devices, or make a final installation decision.

The default Australian supply context is 230 V single phase and 400 V three phase at 50 Hz. Treat those values as starting points for the project basis. For three-phase calculations, use the line-to-line voltage that belongs with the circuit being reviewed.

Data to confirm before checking a run

Confirm the load current before relying on the voltage-drop result. A known equipment or design current is usually stronger than a power conversion. If only kW or kVA is available, keep the conversion basis with the calculation record, including the selected supply arrangement and power factor where it is used.

Confirm the route length as the installed one-way cable path. Include vertical rises, bends, switchboard position, route deviations and termination allowance where they are part of the reviewed route. A straight drawing distance can understate the voltage drop if it omits the actual cable path.

Confirm the cable data source before choosing the method. The entered mV/A/m value or R/X impedance values should describe the same conductor, circuit arrangement, temperature basis, source document and installation context being reviewed. The calculator does not reproduce controlled cable tables, and it cannot correct a cable data value that belongs to a different cable or condition.

Method comparison matrix

Choose the method that matches the data available for the run. The method is a source judgement, not a preference for whichever entry is quicker.

How to read the result

Read volts, percentage and receiving-end voltage together. The voltage drop in volts is the calculated loss along the entered route. The percentage compares that loss with the entered nominal voltage. The receiving-end voltage shows the nominal voltage less the calculated drop. A percentage without the voltage basis, route length and load current is not enough information for another reviewer to repeat the check.

The maximum length result is an inverse calculation from the entered target using the same assumptions. It is useful for route review, but it is not a statement that the route can be installed without further checks. If current, cable data, power factor, installation basis or project target changes, the maximum length changes as well.

When the calculated percentage is above the entered target, review the route before relying on the result. The response might be a shorter route, different distribution point, revised load assumption, different cable data, larger candidate conductor, or a documented project decision. This calculator does not choose that response.

Worked Australian examples

Single-phase final subcircuit

A 32 A single-phase circuit is reviewed over a 40 m route. The project source gives voltage-drop data of 2.8 mV/A/m for the cable context being checked. The nominal voltage is 230 V and the project target entered for review is 5%.

The calculation is:

2.8 x 32 x 40 / 1000 = 3.58 V

The percentage drop is:

3.58 / 230 x 100 = 1.56%

The result is below the entered target. It can be carried forward as a voltage-drop estimate only with the 2.8 mV/A/m data source, the 32 A current basis and the 40 m route length attached. It does not decide conductor current capacity, grouping, installation method or protective-device suitability.

Three-phase equipment run

A balanced three-phase item is listed as 25 kW. The supply context is 400 V line-to-line, the power factor entered for the calculation is 0.85, and the route length is 60 m. The project data source gives conductor resistance of 1.15 ohm/km and reactance of 0.08 ohm/km.

The calculator first converts 25 kW into current using the three-phase relationship. The current used is about 42.45 A. It then applies the entered resistance and reactance values with the three-phase phase factor and power factor. The resulting voltage drop is about 4.50 V, or about 1.12% of 400 V.

This is an impedance-based estimate. Store it with the entered R, X, PF and length values. If equipment data later gives a different current, or the cable data changes with conductor selection, repeat the calculation.

Long lighting circuit review

A 20 A single-phase lighting circuit is being checked over a 90 m one-way route. The project source gives 4.4 mV/A/m for the cable context being considered, the nominal voltage is 230 V, and the project target entered for this review stage is 3%.

The calculation is:

4.4 x 20 x 90 / 1000 = 7.92 V

The percentage drop is:

7.92 / 230 x 100 = 3.44%

This result is above the entered 3% target. Treat it as a route-review trigger, not as a final defect label. Review why the route is long, whether the length includes real installation allowances, whether the cable data matches the candidate conductor, whether the load current is the right basis, and whether a different route, distribution point or conductor candidate should be considered. The maximum-length result is also useful here because it shows the same assumptions would need a route of about 78.4 m to sit at the entered target.

Minimum export record

Export only after the calculator has a valid result and a traceable run reference. The PDF should capture the calculation result; the project file should retain the cable-data source and any standards, DNSP, manufacturer or design-review basis.

Review workflow

1. Confirm the load source. Use known current where possible, or record the kW/kVA conversion basis.
2. Confirm the route length. Use the installed one-way cable path and note any provisional sections.
3. Select the cable data method. Match mV/A/m or R/X values to the project source being reviewed.
4. Enter the project target. Use the percentage set for this project stage or design review, not an assumed authority rule.
5. Review volts, percent and receiving-end voltage together. Check whether the route needs attention before the result is filed.
6. Carry the result into the next task only with the current, length, cable data source, method and target attached.

Boundary with cable size selection, submain allocation and inverter/battery routes

Voltage drop is one check in a wider review. A cable can have an acceptable voltage-drop estimate and still need review against another design requirement. A route can also need voltage-drop attention even when current capacity appears adequate.

The support utilities for percent conversion, maximum route length by target and mV/A/m entry stay inside this route because they depend on the same current, voltage, length and cable data assumptions. They should not be separated into smaller tools without a complete task promise and evidence review.

Australian standards and authority context

Australian electrical installation work must be checked against current standards, local authority or DNSP conditions, product instructions and project documentation. This calculator uses transparent arithmetic. It does not reproduce protected standard tables and does not claim that a result is a final installation decision. Where a standard, connection requirement or project specification sets a voltage-drop requirement, enter that requirement as the project target and keep the source with the calculation record.

AS/NZS cable selection and installation context can involve conductor material, insulation, installation method, ambient temperature, grouping, protective devices and other factors. Those factors are not solved by this page. They may affect the cable data entered into the calculator and may govern whether a cable is suitable for the installation.

Stop points

• Stop if the route length is still a straight-line drawing estimate rather than the installed one-way cable path.
• Stop if the cable data source cannot be matched to the conductor, circuit arrangement and installation basis being reviewed.
• Stop if the load current is a rough assumption and a reviewed design current or equipment current is available elsewhere.
• Stop if the entered voltage does not match the single-phase or three-phase supply context.
• Stop if the percentage result is being used on its own without volts, receiving-end voltage, load current, route length and cable data source.
• Stop if the result is being treated as cable selection, submain allocation acceptance, inverter AC route acceptance, battery DC cable assessment or final installation decision.
• Stop if current standards, DNSP requirements, manufacturer instructions or project documentation set conditions that have not been checked.

A useful calculation record states the run reference, load current, voltage basis, cable data source, project target and result. For example: "Run reference FINAL-CCT-1, 32 A, 230 V single phase, 40 m route, 2.8 mV/A/m project cable data, calculated drop 3.58 V or 1.56%." That record gives another reviewer enough information to understand the basis and repeat the calculation.