Cl 3.6.2 of AS/NZS 3000:2018 sets the voltage drop allowed in an installation. It is one clause, and it decides how far most circuits can run.
What the clause governs
Cl 3.6.2 governs the drop between the point of supply and any point of use. It covers the whole chain: consumer mains, submains and final subcircuits together. The drop is a percentage of the nominal supply voltage.
Two points follow from that wording. The limit applies to the sum of the legs, not to each leg on its own. The limit also applies at the far end of the last cable, not at the board.
Note 3 to the same clause deals with the opposite case, voltage rise from an inverter. It refers that case to AS/NZS 4777.1. AmpSize does not hold AS/NZS 4777.1. The voltage rise tool therefore uses 2 % as its own default and says so. Read the current edition of AS/NZS 4777.1 for the figure that applies.
The Electricity (Safety) Regulations 2010 reg 61 also bears on supply voltage. This page cites the regulation by number only.
Which tools read the clause
Three calculators use this limit.
- The voltage drop tool takes the limit as the
limitPercentinput. It reports the drop, the percentage and the longest run that still meets the limit. - The cable size tool applies the same limit to every candidate conductor. A size that carries the current can still fail here, and on long runs it often does.
- The voltage rise tool applies a separate limit for inverter cables, described above.
The inputs
One final subcircuit in a house.
- Supply: 230 V single phase
- Design current: 25 A
- Route length: 35 m
- Power factor: 0.9
- Cable: 6 mm² multicore copper, PVC insulation, one cable
- Limit: 5 %
The steps
The engine reads the conductor data first. Resistance is 3.75 Ω/km and reactance is 0.097 Ω/km. It combines the two at power factor 0.9 and gets 6.834 mV/A·m. Cl 4.5 of AS/NZS 3008.1.2:2017 sets that combination.
The rest is multiplication. 6.834 mV/A·m across 25 A and 35 m gives a drop of 5.98 V. Against a 230 V supply that is 2.6 %.
The result
The circuit passes. The drop is 2.6 % against the 5 % limit.
The engine also reports the maximum length at the limit: 67.31 m. That is the figure to plan with. It tells you how much route is left before the clause stops the design.
Splitting the budget
A single circuit rarely owns the whole 5 %. A consumer main, a submain and a final subcircuit each take a share, and the shares add up.
Give each leg its own limit before you size it. A common split gives 1 % to the consumer mains and 1 % to the submain. The final subcircuit keeps 3 %. Those shares are a design choice, not a figure from the standard.
Set limitPercent to the share for the leg you are sizing. The 67.31 m figure above assumes the
final subcircuit owns all 5 %. At a 3 % share the same cable runs a shorter distance.
Equipment can set a tighter floor again. Motors, LED drivers and control gear state their own input voltage range. Where that range is narrower than the clause allows, the equipment figure governs the design.
This page is a design aid. Verify every value against the current edition of the standard.