Cable size calculator

Field use cases

Use this calculator when a candidate metric cable size has already been nominated and the reviewer needs to decide whether it is worth carrying forward to the next stage of Australian cable selection review. The candidate might come from a preliminary schedule, an estimator's allowance, a previous project, a manufacturer cable data sheet, a design drawing mark-up or a licensed standard source checked by the project team.

Typical field uses include checking a final subcircuit before it is added to a load schedule, screening a submain candidate before allocating voltage drop across the installation, reviewing a mechanical equipment run with a long route, or checking whether a proposed cable from an earlier drawing revision still has enough margin for the latest load basis. In each case the result is a screen of the candidate data entered by the user. It is not a replacement for the final cable-selection record.

The calculator is useful because it keeps two common rejection points visible at the same time: current capacity and voltage drop. A candidate can look acceptable on voltage drop but still need current-capacity review. It can also have current-capacity margin and still exceed the entered voltage-drop target because the route is long, the voltage basis is low, the candidate cable data is not favourable, or the project target is tight.

Minimum export record

Keep the export concise. Source documents for current capacity, cable data, derating, installation method, protection and AS/NZS 3008 review should remain in the project file rather than being compressed into the calculator output.

The default context is Australian low voltage at 230 V single phase or 400 V three phase, 50 Hz, with metric cable-size labels. Treat those values as the project starting point. If the project documents, DNSP requirements, equipment instructions or authority conditions set a different basis, use the project basis and keep that source with the calculation record.

Candidate data to confirm

Confirm the design current before treating a status as useful. A measured, nameplate or reviewed design current is usually stronger than a rough load allowance. If the current is calculated from kW or kVA, keep the conversion basis with the record, including phase arrangement, nominal voltage and power factor where power factor is used.

Confirm the candidate current capacity as a sourced project value. The entered value should come from the current project cable schedule, manufacturer data, a licensed or current selection source, or reviewed design documentation. It must match the conductor material, insulation, installation method, ambient basis, grouping assumptions and any derating already allowed for in that source. The calculator does not select this value for the user.

Confirm the candidate cable voltage-drop data before choosing mV/A/m or impedance mode. The entered mV/A/m value, or the entered resistance and reactance values, must belong to the same candidate cable, conductor arrangement, source document and operating basis being screened. A voltage-drop value from a different conductor material, temperature basis or circuit arrangement can give a misleading screen even when the arithmetic is correct.

Confirm the route length as the installed one-way cable path. Include switchboard position, vertical risers, route deviations, tray or conduit changes and termination allowance where they form part of the reviewed route. A drawing shortcut can understate the voltage drop and may hide the reason a candidate should not progress.

Screening matrix

The status is driven by two comparisons. The first compares design current with the entered candidate current capacity. The second compares calculated voltage drop with the entered voltage-drop target. Both comparisons must be read with their source data attached.

Capacity margin is the entered current capacity minus the design current. A positive value means the screen has current-capacity margin; a negative value means the entered design current is already above the candidate capacity value. Capacity utilisation shows the same relationship as a percentage. High utilisation is a prompt to check the source assumptions, especially when installation conditions, ambient temperature, grouping, protective-device rating or future load allowance are still unsettled.

Voltage-drop margin is the entered target minus the calculated percentage drop. A positive value means this screen is within the target. A negative value means the calculated drop is above the target. The receiving-end voltage is the entered nominal voltage less the calculated drop in volts. It is useful for documentation, but it must not be separated from the load current, route length, voltage basis, cable data source and target used to produce it.

Worked Australian examples

Workshop final subcircuit candidate

A 24 A single-phase workshop final subcircuit is screened at 230 V over a 35 m one-way route. The candidate is labelled 4 mm2. The reviewer enters a candidate current capacity of 32 A from the project source and cable voltage-drop data of 5.6 mV/A/m. The entered voltage-drop target is 5%.

The current-capacity margin is:

32 - 24 = 8 A

The utilisation is:

24 / 32 x 100 = 75%

The voltage-drop estimate is:

5.6 x 24 x 35 / 1000 = 4.70 V

The voltage-drop percentage is:

4.70 / 230 x 100 = 2.04%

The candidate has margin against both entered checks, so it can be carried forward for further review. The calculation record still needs the 32 A current-capacity source, the 5.6 mV/A/m cable data source, the route basis and the downstream selection checks.

Remote three-phase pump candidate

An 18 kW balanced three-phase pump load is screened at 400 V line-to-line. The route length is 180 m, power factor is entered as 0.86, and the candidate current capacity entered from the project source is 45 A. The reviewer uses impedance mode with 1.15 ohm/km resistance and 0.08 ohm/km reactance. The entered voltage-drop target is 2%.

The calculator first converts the 18 kW load to a design current of about 30.21 A. Current-capacity margin is therefore about 14.79 A, so the current-capacity comparison has margin for this screen.

The long route is limiting. Using the entered resistance, reactance and power factor, the calculated voltage drop is about 9.70 V, or about 2.42% of 400 V. That is above the entered 2% target, so the status is voltage-drop review. The next action is to review the route, cable data source, distribution point, voltage-drop allocation and candidate size before relying on the candidate.

Distribution board spare-way candidate

A 32 A single-phase circuit is checked at 230 V over an 18 m route. The candidate is labelled 4 mm2, but the current-capacity value entered from the schedule is 28 A. The reviewer enters 3.2 mV/A/m as the cable data and a 5% voltage-drop target.

The current-capacity margin is:

28 - 32 = -4 A

The utilisation is:

32 / 28 x 100 = 114.3%

The voltage-drop estimate is:

3.2 x 32 x 18 / 1000 = 1.84 V

The voltage-drop percentage is:

1.84 / 230 x 100 = 0.80%

The route is short enough that voltage drop is within the entered target, but the design current is above the entered candidate current capacity. The useful result is current-capacity review, not a route-length change. Confirm the capacity source, installation assumptions, load basis and candidate size before progressing.

Review workflow

1. Identify the circuit and candidate. Match the circuit reference and metric cable-size label to the drawing, schedule, field note or design record being checked.
2. Confirm the load basis. Use a reviewed design current where possible, or record the kW or kVA conversion basis with voltage, phase arrangement and power factor.
3. Enter the candidate current capacity from the project source. Do not invent a capacity value to make the candidate fit the load.
4. Enter cable voltage-drop data from the source that matches the candidate. Use mV/A/m when that value is available for the reviewed context, or impedance mode when resistance and reactance are the available source data.
5. Confirm the route length. Use the installed one-way cable route rather than a drawing shortcut.
6. Enter the voltage-drop target for this review stage. Keep the project source for the target with the calculation record.
7. Read the status and margins together. A carry-forward result still needs table selection, derating basis, protection review, fault withstand and project documentation. A review result should name the limiting comparison before the candidate is changed.
8. Repeat the screen if any source value changes. A revised load, route length, candidate capacity, cable data value or project target can change the status.

Boundary with AS/NZS 3008 table selection, protection and submain allocation

This page is a candidate screen. It does not reproduce AS/NZS 3008 tables and does not decide the final cable. AS/NZS 3008 table selection can involve conductor material, insulation, installation method, ambient temperature, grouping, circuit arrangement, operating temperature assumptions and other project-specific conditions. Those factors govern the current-capacity value and cable data that the user enters here; they are not automatically selected by this calculator.

Protection review is a separate task. A candidate that carries forward from this screen still needs protective-device rating, disconnection requirements, fault-loop or short-circuit conditions, thermal withstand, installation constraints and manufacturer instructions checked by the project workflow. The candidate current capacity comparison does not select a breaker or fuse, and it does not prove device coordination.

Submain allocation is also outside this automatic result. The screen can check the voltage drop for a candidate submain or final subcircuit, but it does not decide how total voltage drop is allocated across consumer mains, submains and final circuits. Allocation should follow the project design basis, current Australian requirements, DNSP conditions where relevant and the documented distribution strategy.

Stop points

• Stop if the candidate current-capacity value cannot be traced to current project data, manufacturer data, a licensed source or reviewed documentation.
• Stop if the current-capacity source does not match the conductor material, installation method, ambient basis, grouping or derating basis being reviewed.
• Stop if the cable voltage-drop data cannot be matched to the candidate cable, source document and circuit arrangement.
• Stop if the design current is only a rough allowance and a reviewed equipment current, load schedule current or measured current is available elsewhere.
• Stop if the route length is a straight-line drawing distance rather than the installed one-way cable path.
• Stop if capacity utilisation is high and installation conditions, grouping, ambient temperature, protective-device rating or future load allowance are unresolved.
• Stop if voltage-drop margin is negative and the route, target, cable data source or distribution strategy has not been reviewed.
• Stop if the result is being used as an AS/NZS 3008 table-selection substitute, a protective-device decision, a submain allocation rule or a final installation decision.
• Stop if current standards, DNSP requirements, manufacturer instructions or project documentation set conditions that have not been checked.

A useful record states the circuit reference, candidate size, design current basis, entered current capacity and source, route length, cable data source, voltage-drop target and screening status. For example: "FINAL-CIRCUIT-1, 4 mm2 candidate, 24 A design current, 32 A entered current capacity, 35 m route, 5.6 mV/A/m project cable data, 4.70 V drop or 2.04%, candidate carried forward for further review." That record lets another reviewer see what was screened and what still needs separate confirmation.