Voltage Drop Calculator

Parameters

Every field on the form, what it means, the range the form accepts, and the traps worth knowing before you trust the number.

Phase configurationsingle phase, three phase, or DC

Selects the multiplier applied to the run length. Single phase and DC use a factor of 2, because the current travels out along the active and back along the neutral or negative. Three phase uses 1.732, the square root of 3, because the return path is shared between phases. Changing this selector also resets the nominal voltage to 230, 400 or 48 volts.

Nominal voltagevolts

The declared supply voltage the drop is measured against. It is set automatically by the phase selector: 230 volts for single phase, 400 volts line to line for three phase, and 48 volts for DC. The percentage result is the drop divided by this number, so a three phase result is expressed against the line-to-line voltage, not the phase voltage.

Load currentamps, 0.1 to 500

The design current of the circuit, the current the cable actually carries. Use the calculated load current, not the protective device rating. Sizing a 20 amp breaker onto a circuit that draws 8 amps and entering 20 amps here will overstate the drop by a factor of two and a half.

Cable run lengthmetres, 1 to 2000

The one-way route length from the source to the load. Do not double it for the return conductor. The calculator applies the return path itself through the phase multiplier, so entering the total there-and-back length will double the answer. Use the routed length including vertical drops and risers, not the straight-line distance on the plan.

Cable sizesquare millimetres, 1.5 to 300

The cross-sectional area of one active conductor. Standard sizes only: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 and 300. This calculator validates a size you have already chosen. If you want the calculator to pick the size for you, use the cable sizing calculator instead.

Maximum voltage droppercent, 0.1 to 10, default 5

Found under advanced options. The compliance target the result is graded against. AS/NZS 3000:2018 Clause 3.6 uses 5 percent for the whole installation. Tighten it to 3 percent or less when you are budgeting a submain and need to leave headroom for the final subcircuits downstream.

Conductor materialcopper or aluminium

Selectable under advanced options. Aluminium has a higher resistance for the same cross-sectional area, so the same run will show a larger drop.

Assumptions and limits

This is a resistive voltage drop check. Knowing exactly what it leaves out is the difference between using it well and being caught out by it.

What the calculator assumes

• Resistance only, no reactance. The calculation is load current times resistance times length times the phase factor, divided by 1000. Inductive reactance is not included at all. For conductors of 35 square millimetres and above, and for single-core cables installed spaced, reactance is a real share of the total impedance, so the answer here will read low.
• No power factor term. The engine defines a power factor field but never uses it. Every load is treated as though the drop is purely resistive. A motor at 0.8 power factor on a large conductor will drop more in reality than this figure suggests.
• Copper resistance at 75 degrees Celsius. One fixed resistance value per size, regardless of insulation type or how lightly the cable is loaded. A cable running well below its rating sits cooler and has lower resistance, so the calculator is conservative in that direction.
• One cable, one segment. No parallel conductors, no grouping, no ambient temperature correction, no harmonics, no neutral current in an unbalanced three phase system.
• Two checks are run. Drop as a percentage against your target, and voltage at the load against 94 percent of nominal. The percentage is rounded to one decimal place before it is compared, so a true 5.04 percent displays as 5.0 percent and reads as a pass. Treat anything within about a tenth of a percent of the limit as needing a second look.

What this should not be used for

• Final design sign-off on any conductor of 35 square millimetres or larger, where the missing reactance term matters.
• Solar photovoltaic string and inverter circuits, where the concern is voltage rise rather than drop. Use the voltage rise calculator.
• Motor starting voltage dip, which is a transient at locked-rotor current, not a steady-state drop.
• Whole-of-installation budgeting on its own. The 5 percent allowance runs from the point of supply to the most remote point, so mains, submains and final subcircuits all share it. This tool checks one segment at a time.
• Aluminium conductors, which the web form cannot select.

The resistance values behind this calculator are indicative figures for the conductor sizes listed. They have not been transcribed from a licensed copy of AS/NZS 3008.1.1 and have not been validated by a chartered professional engineer. Check every result against the current edition of AS/NZS 3008.1.1 and AS/NZS 3000, and have the responsible person for the installation sign off the final design.

Worked examples

Three runs through the same arithmetic the calculator performs, so you can check the tool against a hand calculation and see where the answer comes from.

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.

Key concepts

• The voltage drop formula. The general form is voltage drop equals I x L x (R x cos phi + X x sin phi), where I is load current in amps, L is the route length, R and X are the per-metre resistance and reactance, and cos phi is the load power factor. By default this calculator uses the resistive form, I x R x L, with a factor of 2 for the return path on single phase and DC, or 1.732 for three phase. Tick Include reactance under advanced options and it applies the full expression with your load power factor.
• 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. Tick Include reactance under advanced options for those runs, then set the load power factor and the cable construction. Single-core conductors sit further apart than multi-core, which raises reactance further.
• 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%.