Electrical System Design
Size low voltage cables to AS/NZS 3008.1.1 and AS/NZS 3000 across a whole project, with derating, fault loop impedance, short circuit withstand, discrimination and a single-line diagram.
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Walkthrough
Review the short circuit withstand of the cable, the protective earth and neutral conductor sizes, and the arc flash estimate.
Switch to automatic sizing and the tool picks the smallest cable that meets every check, naming the constraint that set the size.
Edit the cable types and protective devices available to the project. Library changes are saved with the project.
Export the project as a JSON file for backup, peer review or sharing, and import it again later.
Open, import or start a new project.
Overview
Cable sizing in an Australian or New Zealand installation is rarely a single calculation. A run that passes voltage drop can fail its current rating once ambient temperature and grouping are applied, a cable that carries its load can fail to clear an earth fault within the AS/NZS 3000 disconnection time, and a correctly rated conductor can be damaged by the fault energy let through before the protective device opens. Finding one of those failures at inspection means rework.
The Cable Sizing and Voltage Drop tool puts these checks in one place. Current ratings, derating factors, resistance, reactance and short circuit constants come from the AS/NZS 3008.1.1:2009 tables, and disconnection times, earth and neutral conductor sizing and RCD coordination follow AS/NZS 3000:2018. Each result shows the table or clause it came from, and an automatic mode finds the smallest cable that meets every constraint at once.
It works as a project rather than a single calculation. A project holds consumer mains, submains, final subcircuits and DC circuits, which you can draw on a single-line diagram or add directly for a quick check. A cable schedule lists every cable with its result, a discrimination view grades each pair of protective devices, and projects save automatically, sync to your account when you are signed in, and export as JSON.
Current carrying capacity, resistance, reactance and short circuit constants come from the AS/NZS 3008.1.1:2009 tables for copper and aluminium cables in common constructions and insulation types. Voltage drop is reported in volts and as a percentage, with the voltage at the load and the cable loss.
Apply the AS/NZS 3008.1.1 rating factors for air and soil ambient temperature, burial depth, soil thermal resistivity and grouping, with the right table selected for the cable and installation. Each factor and the combined derating are shown with their table reference.
Enter the source as an impedance or a fault level, and the tool sums the loop impedance through the upstream circuits. It compares the result with the maximum allowed for the protective device and the AS/NZS 3000 disconnection time, and reports the prospective earth fault current.
The cable is checked against the adiabatic limit I²t ≤ K²S² from AS/NZS 3008.1.1. The protective earth conductor is sized to AS/NZS 3000 with an adiabatic cross-check, and the neutral is sized for imbalance and harmonic content.
In automatic mode the tool steps through the standard sizes and picks the smallest cable that meets voltage drop, derated current rating, disconnection time, short circuit withstand and the mechanical minimum for the circuit. It names the constraint that set the size.
Draw the distribution on a single-line diagram using standard symbols, and each connection becomes a sized cable circuit. The cable schedule lists every cable in the project with its result, and an editable library holds cable types and protective devices.
The discrimination view checks each upstream and downstream device pair against their magnetic trip bands at the downstream fault current, along with RCD coordination and cable withstand. A log-log time-current chart shows both trip envelopes and the fault level.
Check inverter back-feed voltage rise against AS/NZS 4777.1, the cable voltage drop while a motor starts, and the neutral current on an unbalanced three phase load. A simplified arc flash estimate gives a design stage figure, with a reminder that a full study is needed before labelling.
Inputs and outputs
Inputs 9
Outputs 10
Use cases
Current ratings, derating factors, resistance, reactance and short circuit constants come from AS/NZS 3008.1.1:2009, which has since been superseded by the 2017 and 2025 editions; the tables are being checked against the 2025 edition. Disconnection times, earth and neutral conductor sizing, mechanical minimums and RCD coordination follow AS/NZS 3000:2018. Discrimination uses the AS/NZS 60898-1 magnetic trip bands, and inverter voltage rise follows the 2 % limit of AS/NZS 4777.1:2016.
It applies the AS/NZS 3008.1.1 rating factors to the base current rating: ambient temperature in air or soil, burial depth, soil thermal resistivity and grouping. The correct table and column are chosen for the cable construction and installation method, and each factor is shown with its table reference.
It adds the source impedance to the impedance of each cable in the path, and compares the total with the maximum the protective device allows for the AS/NZS 3000 disconnection time. That is 0.4 s for final subcircuits supplying socket-outlets up to 63 A or hand-held or portable equipment, and 5 s for other circuits. The same fault current feeds the short circuit withstand check.
It steps through the standard cable sizes and returns the smallest that meets voltage drop, derated current rating, disconnection time, short circuit withstand and the mechanical minimum for the circuit. The constraint that set the size is named, so you know which check governed the result.
DC circuits are sized for current rating and voltage drop as a single loop, and the tool checks inverter back-feed voltage rise against AS/NZS 4777.1. It does not yet apply the AS/NZS 5033 PV array current and voltage rating checks, so confirm those separately.
It is a simplified design stage estimate based on the IEEE 1584 empirical model, giving incident energy, the arc flash boundary and a PPE category. It is not a substitute for a full arc flash study, which should be done before PPE labels are issued.
Projects save automatically in your browser, and are also backed up to your account when you are signed in. A project can be exported as a JSON file and imported again, which suits backup, peer review or passing a design to another engineer.
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