Voltage-drop formulas
Formula chart for AC voltage drop, voltage-drop percent, receiving-end voltage and route-length relationships.
Voltage drop relationships for cable-run records
Use these row summaries to read the calculation relationship before checking the value table.
mV/A/m method
Estimate voltage drop from sourced cable data.
Vd = mV/A/m x I x L / 1000Impedance method
Estimate from entered R and X values.
Vd = F x I x Lkm x (R x PF + X x sin(arccos(PF)))Voltage-drop percent
Compare voltage drop with the entered nominal voltage.
Vdrop% = Vd / Vn x 100Receiving-end voltage
Show nominal voltage less calculated drop.
Vreceiving = Vn - VdMaximum route length
Rearrange the mV/A/m method for a target voltage-drop allowance.
Lmax = Vd_target x 1000 / (mV/A/m x I)
- AC voltage-drop arithmetic using mV/A/m or entered R/X impedance, current, route length, phase factor and nominal voltage.
- Show voltage-drop formulas while keeping project-specific cable-run inputs in the voltage-drop calculators.
- AS/NZS 3000 and AS/NZS 3008 context noted for project review.
- Keep single-phase and three-phase basis visible, including whether the voltage comparison is line-to-neutral or line-to-line.
- mV/A/m, R and X values must come from a source that matches the cable arrangement and Australian project record.
Formula rows used by the voltage-drop calculators
Show voltage-drop formulas while keeping project-specific cable-run inputs in the voltage-drop calculators.
| Relationship | Formula | Use | Boundary |
|---|---|---|---|
| mV/A/m method | Vd = mV/A/m x I x L / 1000 | Estimate voltage drop from sourced cable data. | mV/A/m source must match the project cable context. |
| Impedance method | Vd = F x I x Lkm x (R x PF + X x sin(arccos(PF))) | Estimate from entered R and X values. | F, R, X and PF must share the same project phase basis. |
| Voltage-drop percent | Vdrop% = Vd / Vn x 100 | Compare voltage drop with the entered nominal voltage. | Percent review is not repeatable without voltage basis. |
| Receiving-end voltage | Vreceiving = Vn - Vd | Show nominal voltage less calculated drop. | Record whether Vn is line-to-neutral or line-to-line before project review. |
| Maximum route length | Lmax = Vd_target x 1000 / (mV/A/m x I) | Rearrange the mV/A/m method for a target voltage-drop allowance. | Vd_target is a project-entered allowance, not a cable selection decision. |
- The chart explains relationships; calculators own project-specific inputs and results.
Formula variables
Use these symbols and units when reading the chart rows.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| Vd | Calculated voltage drop. | V | Read as a worksheet estimate before project review. |
| mV/A/m | Sourced cable voltage-drop value. | mV/A/m | Use a value matched to the cable arrangement and source record. |
| I | Circuit current. | A | Use the current basis that belongs to the calculator record. |
| L | One-way route length. | m | Metric route length; do not use round-trip length unless the formula says so. |
| F | Phase factor for the impedance method. | Use the factor that matches single-phase or three-phase calculation basis. | |
| Lkm | One-way route length expressed in kilometres. | km | Used with per-kilometre R and X impedance values. |
| R | Cable resistance value from the selected source. | ohm/km | Must share the same data basis as X. |
| X | Cable reactance value from the selected source. | ohm/km | Must share the same data basis as R. |
| PF | Power factor as a decimal. | Used by the impedance method; record lagging or leading context separately. | |
| Vn | Nominal voltage basis. | V | Use line-to-neutral or line-to-line basis consistently. |
| Vreceiving | Estimated receiving-end voltage. | V | Nominal voltage less calculated voltage drop. |
| Lmax | Maximum one-way route length for a target voltage drop. | m | A rearranged estimate, not a final cable selection. |
| Vd_target | Target or allowance voltage drop. | V | A project-entered allowance, not a universal rule. |
Worked relationship example
This example shows how one set of entered values reads through the relationship.
230 V single-phase voltage-drop reading
A 230 V Australian circuit is reviewed with entered current, one-way route length and a sourced mV/A/m value.
- Nominal voltage
- 230 Vline-to-neutral basis
- Circuit current
- 32 A
- Route length
- 28 mone-way metric route
- Cable data
- 4.4 mV/A/millustrative user-entered value; source must match cable context
- Apply mV/A/m formulaVd = 4.4 x 32 x 28 / 1000 = 3.94 V.
- Convert to percentVdrop% = 3.94 / 230 x 100 = 1.7%.
Voltage-drop estimate3.94 V, about 1.7%
The result remains a worksheet estimate until cable source, installation conditions and project allowance are reviewed.
Use the example to review the formula relationship and keep the source record beside the calculator inputs.
- 230 V nominal line-to-neutral context.
- mV/A/m value is user-sourced and is not reproduced from controlled AS/NZS tables.
Where the voltage-drop chart belongs
Use the chart to understand the formula basis; calculators handle project-specific inputs, warnings and results.
| Item | Value |
|---|---|
| Purpose | Show voltage-drop formulas while keeping project-specific cable-run inputs in the voltage-drop calculators. |
| How to use the source | AC voltage-drop arithmetic using mV/A/m or entered R/X impedance, current, route length, phase factor and nominal voltage. |
| Standards and project context | AS/NZS 3000 and AS/NZS 3008 context noted for project review. |
| Where judgement is still needed | Use this chart to understand the relationship behind a calculation; do not treat it as a substitute for entered project values. |
| Before relying on it | Confirm current standards, local authority requirements, DNSP conditions, project documents and manufacturer data before relying on a decision. |
| Context | Applies to | Public boundary |
|---|---|---|
| AS/NZS 3000 and AS/NZS 3008 context | Voltage-drop formula reading and cable-data source records | The chart explains formula relationships and does not reproduce controlled cable table values or make a compliance ruling. |
| Voltage basis | Single-phase, three-phase, line-to-neutral and line-to-line comparisons | Keep the nominal voltage and phase basis with the calculator inputs. |
| Project review boundary | Voltage-drop formulas chart | Confirm current Australian standards, local authority requirements, DNSP conditions, project documents and manufacturer data before relying on a decision. |
Formula basis for voltage-drop relationships
Check the cited calculator source, cable-data basis, review date and Australian project boundary before carrying these voltage-drop relationships into a worksheet.
| Item | Value |
|---|---|
| Source | AUWiring voltage-drop calculator formula notes. |
| Source type | Released calculator formula source |
| Derivation basis | Derived from AUWiring calculator formulas and user-entered cable data; no AS/NZS cable table values are reproduced. |
| Last checked | 2026-07-13 |
| Review interval | Annual or when voltage-drop calculator formulas, cable-data practice or standards context changes. |
| Review trigger | Formula change, cable data source change, standards update or project feedback. |
| Version used | R02-2026-07-13 |
| Australian application | Australia; project, DNSP, authority and manufacturer requirements can override worksheet basis. |
Use a calculator for project values
Use the chart to read the formula relationship, then open a calculator when you need entered project values, warnings and result context.