WireWrite's NZ cable sizer applies the three checks every electrician must satisfy before specifying a conductor: current-carrying capacity, voltage drop, and short-circuit thermal withstand. The calculator is free, requires no login, and works offline as a PWA after first load.
1. Current-carrying capacity (Iz ≥ Ib). The conductor's de-rated current rating must equal or exceed the design current of the circuit. AS/NZS 3008.1.1:2017 supplies the base rating; installation method, grouping with other circuits, and ambient temperature all reduce that figure via correction factors. The calculator applies these for you when you pick an installation method.
2. Voltage drop. Voltage drop across the conductor is calculated from Vdrop = (I × L × mV/A) / 1000, where mV/A is the conductor's tabulated voltage drop per amp per metre. AS/NZS 3000 Clause 3.6.2 sets the maximum permitted drop across consumer mains, sub-mains and final sub-circuits combined — for most NZ installations this is a 5 % limit referenced to nominal supply voltage (230 V single phase).
3. Adiabatic short-circuit check. The conductor must survive a prospective fault current for the time taken by the upstream protective device to clear the fault. The adiabatic equation k²S² ≥ I²t (where k is the conductor material constant, S the cross-sectional area in mm², I the fault current and t the disconnection time) checks this in the background.
The same 2.5 mm² cable has a different current-carrying capacity depending on how it's installed. Clipped direct to a surface runs cooler than buried in thermal insulation or grouped in a conduit with other circuits. The calculator's installation-method picker applies the AS/NZS 3008.1.1 correction factors automatically so you don't have to chase them manually for each circuit.
An oversized cable wastes copper but is safe. An undersized cable overheats under continuous load, degrades its insulation, and can ignite. The calculator returns the smallest legal size for the inputs you provide — round up one nominal size if your installation has correction factors not covered (for example, grouped in a hot ceiling space).
NZ cable size calculator: 2.5mm² for 16/20A power, 4mm² for 25-32A, 6mm² for EV chargers. Free, AS/NZS 3008 compliant with voltage drop check.