Build a preliminary circuit scenario

How the resistance and voltage drop are estimated

The calculator begins with nominal conductor resistivity at 20 C: 0.017241 ohm mm2/m for copper and 0.028264 for aluminium. It divides by cross-sectional area and converts to ohms per kilometre, then applies a linear temperature correction using 0.00393 per C for copper or 0.00403 for aluminium.

Real cable resistance depends on conductor construction, stranding, compaction, manufacturing tolerance, joints and actual temperature. Manufacturer or AS/NZS 3008 design data should replace this approximation when available.

For DC, power factor and reactance are ignored. The result is divided by nominal voltage to show a percentage and subtracted from supply voltage to show an estimated load voltage. It is a steady-state arithmetic model, not a power-quality study.

What the suggested cable area really means

The page tests common nominal conductor areas from 1.5 to 240 mm2 using the same material, temperature, reactance, length, current and parallel-run assumptions. It reports the first area whose calculated drop does not exceed the entered project percentage.

That output is explicitly “drop-only”. It does not establish current-carrying capacity, conductor insulation, installation method, grouping, ambient temperature, thermal insulation, harmonics, neutral loading, short-circuit withstand, fault-loop impedance, protective-device operation, earthing, mechanical strength or termination suitability.

A smaller area can pass voltage drop yet overheat. A larger area can pass both current and drop but be incompatible with equipment terminals or protection. Cable selection is a coordinated design task, not a single-formula lookup.

Use design inputs that match the actual circuit

Voltage drop accumulates from the point of supply through mains, submains and final subcircuits. A final circuit cannot consume the whole project allowance if upstream conductors already use part of it.

Parallel runs need more than dividing resistance

The arithmetic divides impedance and loss by the entered number of identical parallel conductors. That assumes equal length, material, area, routing, termination and current sharing. Real imbalance can concentrate heat and current.

Parallel conductor arrangements are subject to wiring rules and engineering requirements. Protective devices, grouping, magnetic effects, neutral configuration and termination capacity must be coordinated. Do not create parallel fixed wiring as a DIY response to a high drop result.

When cable reactance or inductive effects matter, physical arrangement influences impedance. Use the manufacturer or standard table for the actual arrangement, not the default example.

Read conductor loss as a scenario, not an electricity bill forecast

Resistive loss is estimated as current squared x resistance x length, using two conductors for single-phase/DC or three phase conductors for the balanced three-phase case, divided by parallel runs. It excludes reactive energy, neutral current, joints and variable load.

The energy output multiplies that full-load loss by entered operating hours. Most loads do not remain at design current continuously, so use a defensible load profile. Motors, chargers and heating equipment have different duty patterns.

Energy cost multiplies loss kWh by the entered tariff. Demand charges, time-of-use periods, export, power-factor charges and future prices are excluded. The output is useful for comparing conductor scenarios on the same assumptions.

A larger conductor may reduce losses but cost more and use more material. Life-cycle selection needs purchase, installation, energy, maintenance and end-use requirements, plus safety and compliance first.

Why excessive voltage drop matters

Low voltage at equipment can reduce heating performance, affect motor starting torque, increase current in some loads, cause control drop-out or produce nuisance operation. Sensitive electronics may have their own input range.

Starting and transient drop can be much larger than the steady design-current result. Long motor circuits, pumps, compressors and welders may need specific starting studies. This calculator does not model inrush, harmonics, flicker or generator regulation.

For solar, batteries, EV charging and other power-electronic systems, direction of power flow and allowable voltage rise can matter. Use dedicated design methods and network requirements rather than changing a sign in this form.

A safer Australian design workflow

Give the licensed contractor load details, route, supply, equipment, future expansion and operating pattern. The designer determines the applicable edition of standards, jurisdiction rules and network requirements.

Review cable data, current capacity, derating, voltage drop, protection, fault performance, earthing, isolation and mechanical installation together. Document assumptions and selected products.

After installation, required inspection, testing and certification must be completed before or when connecting as required. Keep certificates, circuit schedules and test records. If lights dim, equipment trips, cables smell hot or shocks or tingles occur, stop using the affected system and seek urgent professional help.

Worked comparison: the same cable in three circuit models

Take the default 32 A, 35 m, 6 mm2 copper scenario at 75 C. The calculator first estimates conductor resistance from resistivity and temperature. In single-phase AC it includes the outgoing and return path, then combines resistance and entered reactance using power factor.

If the system is changed to balanced three-phase while other inputs stay fixed, the geometric multiplier changes from 2 to square root of 3. The nominal voltage will normally also be reviewed, commonly from a line-to-neutral figure to the appropriate line-to-line figure for the load. Do not compare percentages while leaving the wrong voltage in place.

On DC, power factor becomes 1 and reactance becomes zero. The two-conductor route remains. That may suit a simple resistive screening model, but battery and solar systems introduce equipment voltage windows, protection, polarity, isolation and voltage-rise questions that need their dedicated rules.

Now double the route length. Voltage drop and conductor energy loss double under the steady assumptions. Double the current and drop doubles, while resistive loss rises by the square of current. Those relationships explain why long, heavily loaded circuits demand early coordination rather than a late cable substitution.

Design calculations do not replace inspection and testing

Calculated voltage at the load assumes the supply voltage, current, route and conductor properties are as entered. Installed connections, protective devices, joints, supply variation and load behaviour can change the real result. Testing verifies the completed installation under the applicable procedure.

A certificate is not generated by this page. In Queensland, for example, the electrical contractor performing installation work must provide the required certificate of testing and compliance when the affected installation is connected. Other jurisdictions have their own licensing, certificate and notification systems.

Record the calculation revision, cable product, route, installation method, protective device, test results and final circuit identification together. If a later extension is added, the upstream voltage-drop allowance and conductor loading must be reassessed rather than assuming the original spare margin remains available.

Never probe live fixed wiring to confirm this estimate unless you are authorised, competent and following required safe systems of work. Consumers should use licensed professionals and report shocks or tingles promptly to the appropriate emergency or network channel.