NZ Voltage Drop Calculator — AS/NZS 3000

What voltage drop actually means (and why it bites)

Voltage drop is the voltage you lose between the point of supply and the far end of the cable. Push 32 A through 50 m of 2.5 mm² and by the time you reach the load, a meaningful chunk of your 230 V is gone — often enough to push you out of compliance and make the gear on the end misbehave.

It's not a safety check like RCDs or earthing — it's a performance and compliance check. Fail it and gear runs hot, motors stall, LEDs flicker, and EV chargers throttle themselves down.

The NZ rule — AS/NZS 3000 Clause 3.6.2

AS/NZS 3000:2018 sets a single total voltage drop limit for low voltage installations: the drop between the point of supply and any point in the installation must not exceed 5% of the nominal voltage at the point of supply. That's it. One number, and it applies to every circuit type — power, cooking, lighting, whatever.

At 230 V single-phase that works out to:

• 5% = 11.5 V maximum total drop

Two quick exceptions to keep in the back pocket:

• Where the point of supply is the LV terminals of an on-site substation dedicated to the installation, the allowance rises to 7%.
• Stand-alone systems (off-grid, per Clause 7.3) are exempt from the 5% rule and instead work to a combined source + cable drop budget.

So what about the "3% for lighting" rule?

You'll see it everywhere — trade texts, training material, equipment brochures, even the disclaimer on this page's calculator. It's a sensible design rule of thumb to keep lamp colour and output stable on long runs, and the calculator flags it as a secondary check. But it is not a limit imposed by AS/NZS 3000 or AS/NZS 3008.1 in New Zealand. The only regulatory number is 5%. If you're designing for a client who cares about lighting performance, aim for 3% because it's good practice — not because the standard requires it.

The formula

The simplified method from AS/NZS 3008.1.2 Section 4 uses the mV/A.m value for your cable — the voltage drop you'd get per amp per metre of circuit length:

Vd (volts) = (L × I × Vc) ÷ 1000

Where:

• L = route length in metres (one-way)
• I = circuit current in amps
• Vc = mV/A.m from the AS/NZS 3008.1.2 tables for your cable size, type, and installation method

For three-phase the formula uses phase-to-phase voltage (400 V in NZ) as the reference, and single-phase values are related to three-phase values by a 1.155 multiplier where the tables are three-phase-based. The calculator above handles both cases.

Worked example — EV charger to the carport

A 32 A single-phase EV charger, 40 m from the switchboard, 6 mm² multicore copper (default installation method, 75°C operating temperature). Using the standard reference value of 7.3 mV/A.m for 6 mm² under these conditions:

• L = 40 m, I = 32 A, Vc = 7.3
• Vd = (40 × 32 × 7.3) ÷ 1000 ≈ 9.3 V
• As a percentage of 230 V: about 4.1% — comfortably inside the 5% limit

Drop one size to 4 mm² (Vc ≈ 11 under the same conditions) and the drop climbs to around 14.1 V / 6.1% — fails 5%. That's the textbook case where the calculator pushes you up a cable size, and why 6 mm² is the NZ de facto minimum for 32 A continuous loads on anything but the shortest runs.

Where you'll trip up in real NZ installs

• Long driveway feeds to sheds and sleep-outs — 80 m+ runs burn through voltage fast, and 6 mm² often isn't enough.
• EV chargers — 32 A continuous loads magnify small drops, and charger manufacturers sometimes publish their own minimum supply voltage requirements on top of AS/NZS 3000.
• Upstream drops eat into the budget — the 5% is total from the point of supply. If the consumer mains already take 1.5% and the submain takes another 1%, the final subcircuit only has 2.5% left before it breaches the overall limit.
• Long commercial lighting runs — while not a code cap in NZ, lighting flicker and colour-temperature shift at 4-5% drop is noticeable, which is why designers aim for 3% even when not required.
• Derating and grouping — the published Vc values are for specific cable types, installation methods and operating temperatures. Real installs with bunched cables or high ambients can run hotter than the tables assume.

FAQ

What is the NZ voltage drop limit?

5% of the nominal voltage at the point of supply, measured to any point in the installation. Set by Clause 3.6.2 of AS/NZS 3000:2018. The 5% is the total across consumer mains, submains and the final sub-circuit combined.

Is there a separate 3% limit for lighting circuits in NZ?

No. AS/NZS 3000:2018 does not impose a 3% lighting limit — the 5% total applies to every circuit. The 3% figure is a common design target for lighting to avoid visible flicker and colour shift, but it's a rule of thumb, not a regulatory cap.

Does voltage drop work the same on three-phase?

The 5% limit applies the same way, but the formula uses a root-3 factor and phase-to-phase voltage (400 V in NZ) as the reference.

Is 5% the only allowance — ever?

Mostly, yes. One exception: where the point of supply is the LV terminals of an on-site substation dedicated to the installation, the allowance rises to 7%. Stand-alone (off-grid) systems work under Clause 7.3 with a different budget.

Related calculators

• Cable Sizer — pick the right cable for any circuit, with voltage drop built in
• Circuit Check — verify an existing circuit passes AS/NZS 3000
• Maximum Demand — size your main switch and submain

NZ voltage drop calculator. AS/NZS 3000 Clause 3.6.2 flat 5% limit. Free, formula display, region-specific limits.