Whether you're signing off a new install or auditing an existing one, a compliant final subcircuit in NZ comes down to three things: the cable can carry the current the breaker lets through, the breaker is rated for the load, and voltage drop across the whole run stays under the Clause 3.6.2 limit. The calculator above bundles those checks into a single pass/fail so you can sanity-check a circuit in seconds.
The first two checks come straight out of AS/NZS 3000 Clause 2.5.3.1, which sets the coordination between a conductor and its overload protective device. The third is Clause 3.6.2, which you already know if you've used the voltage drop calculator. Together, they are most of the electrical design safety net for a standard domestic or light commercial subcircuit.
AS/NZS 3000 Clause 2.5.3.1 gives two conditions a protective device must satisfy to protect its conductor against overload. The headline rule is Equation 2.1:
IB ≤ IN ≤ IZ
Where:
Read left to right, the equation is saying: the load must not exceed the breaker rating, and the breaker rating must not exceed what the cable can safely carry. The calculator runs both halves of that inequality and flags which half fails if either does.
There's a second condition, Equation 2.2 (I₂ ≤ 1.45 × IZ), where I₂ is the current that ensures the protective device actually trips in conventional time. For standard MCBs this works out to 1.45 × IN, and Note 1 to the clause confirms that if you satisfy Equation 2.1 for a circuit breaker, Equation 2.2 is automatically satisfied. For HRC fuses it's different — see the FAQ below.
Even a perfectly coordinated circuit can fail at the far end if the run is long enough. Clause 3.6.2 caps total voltage drop from the point of supply to any point in the installation at 5% of the nominal voltage. The calculator computes drop across the run and flags it against the 5% limit.
It also shows a 3% check as a non-critical advisory — because lighting circuits above 3% can visibly flicker or shift colour temperature, and it's common design practice to aim tighter for lighting. That 3% figure is not a regulatory limit in NZ, just a quality target. Only the 5% line is binding.
That the protective device coordinates with the conductor so that the circuit can be overloaded without damaging the cable. The headline condition is IB ≤ IN ≤ IZ — the load current must not exceed the breaker rating, and the breaker rating must not exceed the cable's current-carrying capacity.
IB is the design current (the circuit's expected load). IN is the nominal current of the protective device (the breaker rating). IZ is the cable's continuous current-carrying capacity from AS/NZS 3008.1, with any derating for installation method, grouping and ambient temperature applied.
Yes. For HRC fuses, Note 1 to Clause 2.5.3.1 states the effective rule becomes IB ≤ IN ≤ 0.9 × IZ (Equation 2.3). Standard circuit breakers are fine with just Equation 2.1 — satisfy that and Equation 2.2 is automatically satisfied.
The regulatory limit in Clause 3.6.2 is 5%, which is what this calculator fails on. The 3% advisory is for lighting circuits where voltage fluctuations can cause visible flicker and colour shift — it's a widely used design target but not a code requirement in NZ.
NZ circuit check: instant pass/fail compliance verification. Breaker, cable size, voltage drop all in one. AS/NZS 3000 compliant.