Voltage Drop Calculator
Check any cable run's volt drop against the allowance you set.
Three phase here assumes a balanced load. For an unbalanced three-phase mains or submain, use the Mains & Submain Cable Selection calculator, which checks the busiest phase against 230 V.
One-way run length, not the return path.
The volt drop you're allowing for this run. The whole installation must stay within 5% from the point of supply (Clause 3.6.2), so mains, submain and final subcircuit drops all add up against that one 5%.
For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
What voltage drop actually means on site
Every metre of conductor has a bit of resistance, and current flowing through that resistance loses a bit of voltage along the way. By the time it reaches the load, the appliance isn't seeing the full 230V or 400V you started with at the switchboard - it's seeing that minus whatever the cable ate on the way. Volt drop is just that loss, expressed as a voltage or a percentage of nominal supply.
It matters because too much drop means motors run hotter and less efficiently, lights dim and flicker, and electronic gear can behave oddly or throw faults under load. That's why AS/NZS 3000 sets a limit on it.
5% is for the whole installation, not each cable
AS/NZS 3000 Clause 3.6.2 allows 5% volt drop from the point of supply to any point in the installation. The point of supply is where the consumer mains join the electricity distribution system (Clause 1.4.95), not your main switchboard. So the drop on the consumer mains, any submain and the final subcircuit all add together against that single 5%.
A common way to design for this is to give roughly 2.5% to the mains and submains and 2.5% to the final subcircuit. That split is design practice, not a separate rule, so this calculator lets you set the allowance for each run yourself and checks against it. A cable that passes 5% on its own can still push the installation over 5% once the rest of the path is added.
In practice, volt drop is the thing that decides cable size on long runs far more often than current-carrying capacity does. A circuit to a shed 40m from the house will usually fail on volt drop long before it fails on ampacity, which is why sparkies size up on distance even when the actual load is small.
Worked examples
Garage submain, 35m run
A single-phase submain on 6mm² copper feeding a detached garage 35m from the main board, sized for 32A. On its own it comes in under 5%, but it fails a 2.5% allowance. Add the drop on the mains and on the garage's own circuits, and the installation as a whole can end up past the 5% limit. Setting a realistic allowance for this run is what catches it.
Heat pump circuit, short run
A single-phase heat pump circuit, 12m from the switchboard, drawing 16A on 2.5mm². Short runs like this almost always pass comfortably on volt drop - the cable size here is set by current-carrying capacity and breaker coordination, not volt drop, which is a useful sanity check when you're deciding where to spend your cable budget.
Three-phase workshop submain, 60m run
A three-phase submain to a workshop 60m from the house, carrying 32A per phase. Three-phase circuits generally handle distance better than single-phase for the same current, but 60m is far enough that it's worth checking rather than assuming - particularly if the workshop has its own switchboard and you're deciding what size feeder to run to it.
Frequently asked questions
What's an acceptable voltage drop in NZ?
AS/NZS 3000 Clause 3.6.2 limits volt drop to 5% of nominal voltage between the point of supply and any point in the installation. That 5% covers the whole path, so the drop on the consumer mains, any submain and the final subcircuit all add together. A common way to design for this is roughly 2.5% for mains and submains and 2.5% for the final subcircuit, which is why this calculator starts at 2.5%. Set the percentage to whatever share of the 5% this run is allowed.
Are there exceptions to the 5% limit?
Clause 3.6.2 has a few. For final subcircuits with distributed load, such as socket-outlet or lighting circuits, you can calculate volt drop using half the protective device rating (Exception 1). Where the point of supply is the low voltage terminals of a substation on the premises and dedicated to the installation, the limit can be increased to 7% (Exception 3). Motor starting and similar short transient currents aren't counted (Note 1).
Do I use the single-phase or three-phase mV/A/m value?
Cable tables are usually published as three-phase mV/A/m values. For a single-phase circuit, that value gets adjusted by a conversion factor to reflect the actual current path. This calculator handles that conversion for you when you select single phase, so you just enter the circuit as it actually is.
Does length mean one-way or there-and-back?
One-way run length - the distance from the switchboard to the load. Don't double it yourself; the calculator and the underlying method already account for the return conductor.
Why does the same cable pass on one job and fail on another?
Volt drop is a function of current, length and cable size together, not any one of those alone. A 2.5mm² circuit might comfortably pass at 10m but fail at 40m for the same load, which is exactly why a long run to a shed or a garage often needs a size up even when the load current is modest.
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