Voltage-drop formulas
Formula chart for AC voltage drop, voltage-drop percent, receiving-end voltage and route-length relationships.
Voltage-drop formulas explains formula chart relationships for Australian cable and voltage records. Show voltage-drop formulas while keeping project-specific cable-run inputs in the voltage-drop calculators. Read the variables, formula notes and Voltage drop handoff before using the relationship with entered site values, product data, DNSP conditions or competent review.
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%.
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.