Add each part of the path from the point of supply to the load. Each stage's volt drop is worked out with the same method as the Mains & Submain Cable Selection calculator, then added together.
Multicore 1 to 10 mm² is rated as PVC (75°C); 16 mm² and up as XLPE (90°C). For typical New Zealand installation conditions only. Based on 30°C ambient air temperature, 15°C ambient soil temperature and 1.2°C.m/W thermal resistivity.
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.
One 5%, shared by every cable on the way
AS/NZS 3000 Clause 3.6.2 allows 5% volt drop from the point of supply to any point in the installation. That limit covers the whole path: the consumer mains, any submains and the final subcircuit. Checking each cable on its own against 5%, or even against a 2.5% share, can miss an installation that is over the limit in total.
This calculator lets you add each part of the path, works out the volt drop on each one, and adds them up so you can see the real total and how much of the 5% is left. Each stage is worked out with the same method as the Mains & Submain Cable Selection calculator, including the conductor's operating temperature at your load.
Worked example
House, sleepout submain and a socket circuit
A 60A consumer mains on 16 mm² multicore buried direct for 20 m, a 32A submain on 6 mm² buried direct for 30 m to a sleepout, and a 20A socket circuit on 2.5 mm² run 25 m through the ceiling (distributed load, so worked at half the breaker rating).
The mains comes to 1.25%, the submain 2.68% and the socket circuit 1.77%, a total of 5.70%, which is over the 5% limit even though no single cable looks unreasonable. Changing only the submain to 10 mm² brings the total down to 4.58%.
Frequently asked questions
Is the 5% volt drop limit per cable or for the whole installation?
For the whole installation. AS/NZS 3000 Clause 3.6.2 limits volt drop to 5% between the point of supply and any point in the installation, so the drop on the consumer mains, every submain and the final subcircuit all add up against that one 5%.
Where is the point of supply?
AS/NZS 3000 Clause 1.4.95 defines it as the junction of the consumer mains with the conductors of the electricity distribution system (or the output terminals of a generating system on the premises). It is not the main switchboard, so the consumer mains count towards the 5%.
What current should I use for a socket or lighting circuit?
For final subcircuits with distributed load, such as socket-outlets or lighting, Clause 3.6.2 Exception 1 allows half the rating of the protective device. Tick the distributed load option on the final subcircuit and enter the breaker rating.
When can the limit be 7%?
Clause 3.6.2 Exception 3 allows up to 7% where the point of supply is the low voltage terminals of a substation located on the premises and dedicated to the installation.
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