Voltage Drop Calculator

Voltage Drop Guide for AS/NZS 3000:2018

Voltage drop is the reduction in voltage along a cable run caused by the resistance and reactance of the conductor. Every cable carrying current will drop some voltage between its supply end and its load end. If the voltage at the load is too low, equipment may malfunction, motors may overheat or stall, and lighting may dim or flicker. AS/NZS 3000:2018 Clause 3.6 sets a maximum allowable voltage drop of 5% from the point of supply to the most remote point of the installation. This calculator determines whether a given cable size, length, and load current combination stays within that limit, and it can also back-solve the minimum cable size needed to meet a target voltage drop percentage.

Key concepts

• The voltage drop formula. Voltage drop in volts equals I x L x (R x cos phi + X x sin phi), where I is load current in amps, L is the one-way cable route length in metres, R and X are the per-metre resistance and reactance from AS/NZS 3008.1.1 tables, and cos phi is the load power factor. For three-phase circuits, a correction factor of 1.732 is applied. The calculator pulls R and X values automatically once you select a cable size and installation method.
• When voltage drop governs cable size. For short cable runs (under approximately 30 metres), the current carrying capacity of the cable almost always determines the minimum size. For longer runs, particularly on circuits with high load currents, voltage drop becomes the binding constraint and may force a cable one or two sizes larger than what current rating alone would require. Knowing which constraint governs saves material cost on short runs and prevents compliance failures on long runs.
• Reactance in larger cables. For cables 35 mm squared and above, the inductive reactance (X) becomes a meaningful component of the total impedance. Ignoring reactance on large cables underestimates voltage drop and may lead to a cable selection that fails compliance at full load. The calculator includes reactance from AS/NZS 3008.1.1 Tables 30 to 32 for all cable sizes.
• The 5% limit is cumulative. The 5% limit applies from the point of supply (typically the meter or main switchboard) to the most remote outlet on the installation. This means the voltage drop on each segment of the wiring system (mains, submains, final subcircuits) must be considered together. A submain consuming 3% of the budget leaves only 2% for the final subcircuit.

Common scenarios

• Running a long final subcircuit to a shed or granny flat. A 2.5 mm squared cable supplying a 20 A circuit over a 50-metre run will likely exceed the 5% voltage drop limit. The electrician enters the cable size, run length, and load current to check compliance. If the drop is too high, the calculator determines whether upsizing to 4 mm squared or 6 mm squared brings the installation within limits. This is one of the most common voltage drop problems in residential work.
• Sizing a submain from the main switchboard to a sub-distribution board. In a commercial fit-out, the submain may run 80 to 120 metres from the main switchboard to a remote distribution board. The electrician needs to size the submain so that the voltage drop on the submain itself leaves enough voltage drop budget for the final subcircuits downstream. The calculator helps balance the submain and subcircuit voltage drop allocation.
• Checking voltage drop on a motor circuit with low power factor. Motors typically operate at power factors between 0.75 and 0.90, which increases the reactive component of voltage drop compared to a resistive load. The electrician enters the motor FLC and power factor to see the actual voltage drop, which will be higher than a unity power factor load on the same cable. This is especially important for long motor runs where the combined effect of distance and low power factor can push the voltage drop well beyond 5%.