NZ Cable Sizer — AS/NZS 3008:2017
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.
The three checks an electrician must satisfy
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.
Why installation method changes the answer
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.
Why oversizing is safe and undersizing isn't
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).
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Free Electrical Calculators
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- Maximum Demand Calculator - AS/NZS 3000 Section 2
- Conductor Sizing Calculator - Cable sizing calculations
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