Maximum route length calculator
Estimate the maximum Australian cable route length that fits an entered voltage-drop target, load current basis and project cable data.
allowable_Vdrop = Vn x target_% / 100; Lmax_mvam = allowable_Vdrop x 1000 / (mV/A/m x I); Lmax_impedance = allowable_Vdrop x 1000 / (F x I x (R x PF + X x sin(arccos(PF))))- The target is entered by the user and is not treated as a universal authority limit.
- The mV/A/m method uses project cable data directly.
- The impedance method uses entered resistance, reactance and power factor.
- The result is a planning length, not proof that the route can be installed.
| Variable | Meaning | Unit | Use |
|---|---|---|---|
| Vn | Nominal voltage | V | Single-phase or three-phase voltage basis entered by the user. |
| target_% | Voltage-drop target | % | Project review target used for the inverse calculation. |
| allowable_Vdrop | Allowable voltage drop | V | Voltage corresponding to the entered target. |
| I | Load current | A | Entered directly or calculated from kW or kVA. |
| mV/A/m | Voltage-drop data | mV/A/m | Project cable data for the selected cable context. |
| F | Phase factor | factor | Single-phase or three-phase voltage-drop factor. |
| R | Resistance | ohm/km | Entered conductor resistance for impedance mode. |
| X | Reactance | ohm/km | Entered conductor reactance for impedance mode. |
| PF | Power factor | decimal | Used for kW conversion and impedance mode. |
| Lmax | Maximum one-way route length | m | Estimated length at the entered target. |
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Maximum route length calculator technical guide
Estimate the maximum Australian cable route length that fits an entered voltage-drop target, load current basis and project cable data.
Use this calculator when the route is still being planned and the practical question is how far the cable can run before an entered voltage-drop target is reached. It is an inverse voltage-drop worksheet. It helps with layout decisions, switchboard location review and route comparison before the final route length is known.
The page is not a replacement for voltage-drop calculation on the final installed route. Once the route length is known, use the voltage-drop calculator to calculate the actual volts and percent drop.
Field Use Cases
| Work setting | Real question | Useful action from this page |
|---|---|---|
| Early route planning | How far can this circuit run before reaching the entered target? | Estimate a one-way route allowance before layout is fixed. |
| Switchboard location | Would moving the board materially improve route length margin? | Compare the maximum length with candidate board locations. |
| Cable candidate review | Does a candidate cable data value support the intended route? | Enter the candidate mV/A/m or R/X data and compare the result with route options. |
| Lighting or plant layout | Is a tight target likely to limit the route? | Use the result as a prompt to review distribution points and cable data. |
| Estimating note | What route allowance should be carried into a preliminary record? | Export current, target, cable data and maximum length together. |
The result is most useful before the route is final. If the project already has a measured route, use the voltage-drop calculator instead.
Data Checklist
| Value | Where it normally comes from | Stop if |
|---|---|---|
| Load current | Load schedule, equipment data, measured current or power conversion | The load is only a rough allowance and better data exists. |
| Supply arrangement | Project supply basis | The voltage does not match the single-phase or three-phase circuit. |
| Voltage-drop target | Project specification, design basis or reviewer instruction | The target is being treated as an automatic approval rule. |
| Cable data | Cable schedule, datasheet, manufacturer data or project source | The data does not match the cable and circuit context. |
| Power factor | Equipment data or design assumption | kW or impedance mode is used and PF is unknown. |
The calculator can convert kW or kVA to current, but a known design current is usually stronger. Keep the conversion basis with the export.
Method Comparison Matrix
| Method | What the calculator does | Best use | Main risk |
|---|---|---|---|
| mV/A/m inverse | Divides allowable voltage drop by mV/A/m and current. | Project cable data already gives mV/A/m for the selected context. | Using data from a different cable or circuit arrangement. |
| R/X inverse | Uses resistance, reactance, PF and phase factor. | Project source gives impedance values. | Mixing R, X and PF values from different assumptions. |
| Current input | Uses the entered current directly. | Reviewed current is available. | Treating a rough allowance as design current. |
| kW conversion | Converts real power to current with PF. | Equipment schedule lists kW and PF. | Forgetting that PF changes the current and length. |
| kVA conversion | Converts apparent power to current. | Apparent power is the available rating. | Confusing total kVA with per-phase values. |
The method should match the source values, not whichever entry gives the longest route.
Worked Records
| Situation | Inputs | Result | Record use |
|---|---|---|---|
| Single-phase final circuit | 32 A, 230 V, 2.8 mV/A/m, 5% target | 128.35 m maximum one-way route | Preliminary route allowance before final path measurement. |
| Three-phase plant | 25 kW, 400 V, PF 0.85, R 1.15 ohm/km, X 0.08 ohm/km, 3% target | 160.06 m maximum one-way route | Layout check for mechanical plant routing. |
| Lighting allowance | 20 A, 230 V, 4.4 mV/A/m, 3% target | 78.41 m maximum one-way route | Shows that a tighter target limits route length. |
The examples show why the result is a planning value. A revised current, target or cable data value changes the length immediately.
Review Workflow
- Identify the circuit or route being planned.
- Confirm the supply arrangement and voltage basis.
- Enter current directly where possible, or record the kW/kVA conversion basis.
- Enter the voltage-drop target used for this project stage.
- Enter mV/A/m or R/X cable data from the project source.
- Read allowable voltage drop and maximum length together.
- Compare the result with likely installed route length, including risers, deviations and termination allowances.
- If the planned route is longer than the estimate, review route, distribution point, target, current and cable data before changing drawings.
- When the route is known, run the voltage-drop calculator with the real one-way length.
Boundary With Voltage Drop And Cable Size
| Related task | Use this page? | Why |
|---|---|---|
| Actual voltage drop | No | Use voltage drop when the route length is known. |
| Cable-size candidate screen | No | Use cable size when current capacity and voltage-drop margin both matter. |
| Route length takeoff | No | This page estimates maximum allowable length; it does not measure the route. |
| Standards or DNSP decision | No | The entered target must be checked against current project and authority requirements. |
| Protection or fault review | No | Protection needs separate source and device data. |
The maximum length is a planning threshold. It is useful precisely because it tells you when a real route should be checked more carefully.
Australian Context
Australian low-voltage cable work normally uses 230 V single-phase and 400 V three-phase context, but project documents, equipment instructions and authority requirements can set different values. This calculator keeps the voltage and target editable so the record can match the job.
The calculator does not reproduce protected cable tables. It works from values entered by the user and keeps the source responsibility visible.
Minimum Export Record
| Record item | Why it matters |
|---|---|
| Length reference | Ties the estimate to a route, drawing or circuit. |
| Supply and voltage basis | Makes the target voltage allowance repeatable. |
| Load basis and current | Shows what current was used. |
| Cable data method | Identifies mV/A/m or R/X source basis. |
| Entered target | Separates project target from automatic approval. |
| Maximum length | Gives the planning threshold. |
| Reviewer | Identifies who prepared or checked the estimate. |
Stop Points
- The entered target has no project source or review basis.
- Cable data cannot be matched to the candidate cable context.
- Current is a rough allowance and a better source exists.
- The planned route is close to or longer than the maximum length.
- The result is being treated as final route approval.
- Cable size, derating, protection, installation method or authority requirements have not been reviewed.
The useful output is a route-planning threshold with its assumptions attached. It helps decide when to move from early layout to detailed cable review.
Single-phase final-circuit length check
A 32 A single-phase cable run is being planned before the route is final, so the reviewer works backwards from a 5% target.
- Length reference
- LENGTH-CHECK-1
- Supply arrangement
- Single phase
- Load basis
- 32 A
- Voltage
- 230 V
- Cable data
- 2.8 mV/A/m
- Target
- 5%
- Current used32 A is used for the length estimate.
- Allowable drop11.5 V is the entered target allowance.
- Maximum length128.35 m is the one-way route estimate for this basis.
Equivalent to 0.1283 km at the entered target and cable data basis.
The result gives a planning length for the entered target and cable data. If the installed route exceeds this, use the voltage-drop calculator with the real route length.
- 230 V single-phase Australian low-voltage context.
- mV/A/m data is entered from the project cable data source.
- The result is an inverse voltage-drop worksheet, not a cable selection.
Three-phase plant route allowance
A 25 kW three-phase plant load is checked with entered R/X cable data before routing is finalised.
- Length reference
- MECH-LENGTH-1
- Supply arrangement
- Three phase
- Load basis
- 25 kW
- Voltage
- 400 V
- Cable data
- 1.15 ohm/km R, 0.08 ohm/km X
- Target
- 3%
- Current used42.45 A is used for the length estimate.
- Allowable drop12 V is the entered target allowance.
- Maximum length160.06 m is the one-way route estimate for this basis.
Equivalent to 0.1601 km at the entered target and cable data basis.
The impedance method makes the power-factor and cable-data basis visible. The route allowance should be reviewed again when the real path is known.
- 400 V line-to-line Australian three-phase context.
- The kW load is converted using the entered power factor.
- Resistance and reactance are user-entered project data.
Low target lighting allowance
A lighting route is checked against a tighter entered target before the tray path is chosen.
- Length reference
- LIGHTING-LENGTH-1
- Supply arrangement
- Single phase
- Load basis
- 20 A
- Voltage
- 230 V
- Cable data
- 4.4 mV/A/m
- Target
- 3%
- Current used20 A is used for the length estimate.
- Allowable drop6.9 V is the entered target allowance.
- Maximum length78.41 m is the one-way route estimate for this basis.
Equivalent to 0.0784 km at the entered target and cable data basis.
The tighter target and higher voltage-drop data produce a shorter route allowance, so the route should be confirmed before the cable schedule is locked.
- The target is entered by the reviewer for this project stage.
- The current is an entered planning value.
- The result should be repeated if cable data changes.