Voltage Drop in Australian Installations Explained

Voltage drop is the reduction in voltage along a cable as current flows through it. Every cable has resistance (and reactance for larger sizes), and that resistance converts some of the supply voltage into heat rather than delivering it to the load. AS/NZS 3000:2018 Clause 3.6 caps voltage drop at 5 percent of the nominal supply voltage. This guide explains the formula, walks through single-phase and three-phase calculations, and shows where the Voltage Drop Calculator fits into the workflow.

What is voltage drop and why it matters

Every conductor has electrical resistance. When current flows through a cable, some voltage is lost as heat across that resistance. The longer the cable and the higher the current, the more voltage is lost. At the load end, the voltage is lower than at the supply end.

This matters because equipment is designed to operate within a specific voltage range. A motor running at 210 V instead of 230 V draws more current to deliver the same shaft power, overheats, and fails earlier. LED drivers and switchmode power supplies may malfunction or produce visible flicker. Heating elements take longer to reach temperature. The 5 percent limit in AS/NZS 3000 exists to keep all connected equipment within its rated operating range.

The AS/NZS 3000 Clause 3.6 limit

AS/NZS 3000:2018 Clause 3.6 states that the voltage drop from the point of supply to the most remote point of the installation must not exceed 5 percent of the nominal supply voltage. For a standard 230 V single-phase supply that means a maximum drop of 11.5 V. For a 400 V three-phase supply the limit is 20 V line-to-line.

Some supply authorities and specific equipment standards require tighter limits. Medical facilities, data centres, and high-bay LED lighting installations commonly use a 3 percent target. Always check the project specification before defaulting to 5 percent.

The voltage drop formula

The general formula for voltage drop per phase is:

Vd = Ib x L x (R x cos(phi) + X x sin(phi))

where Ib is the design current in amps, L is the one-way route length in metres divided by 1000 (because R and X are in milliohms per metre), R is the cable per-metre resistance from AS/NZS 3008.1.1, X is the per-metre reactance, and cos phi is the load power factor.

For three-phase circuits, the line-to-line voltage drop is 1.732 times the per-phase drop. The Voltage Drop Calculator applies this factor automatically when you select three-phase.

Single-phase vs three-phase calculation

For single-phase circuits, the formula gives the full circuit voltage drop directly. The R and X values from AS/NZS 3008.1.1 already account for the go-and-return path in the impedance per metre.

For three-phase balanced circuits, the formula gives the per-phase drop. Multiply by 1.732 to get the line-to-line drop, which is what you compare to the 5 percent of 400 V limit. For unbalanced three-phase loads, calculate the worst-case phase individually.

Using per-metre resistance and reactance tables

AS/NZS 3008.1.1:2025 Tables 30, 31, and 32 provide R and X values in milliohms per metre for different cable sizes, constructions, and operating temperatures. Table 30 covers PVC insulated cables, Table 31 covers XLPE, and Table 32 covers mineral-insulated cables.

For cables under 25 mm squared, reactance (X) is negligible and the formula simplifies to Vd = Ib times L times R times cos phi. For cables 35 mm squared and larger, X becomes a meaningful share of the impedance and must be included. Ignoring reactance on large cables underestimates voltage drop.

How cable size, length, and current interact

Voltage drop is proportional to both current and length. A circuit carrying 20 A over 50 metres drops the same voltage as a circuit carrying 40 A over 25 metres (assuming the same cable size). This means long, lightly loaded circuits can still exceed the voltage drop limit.

Increasing cable size reduces resistance per metre, which reduces voltage drop. The Cable Sizing Calculator checks voltage drop alongside current carrying capacity and earth fault loop impedance, returning the smallest standard size that passes all three constraints.

Temperature effect on conductor resistance

Conductor resistance increases with temperature. The R values in AS/NZS 3008.1.1 are typically given at 75 degrees Celsius for PVC cables (the maximum continuous operating temperature). If the cable operates hotter (e.g. in an unventilated roof space in summer, near a boiler, or in a tightly grouped cable tray), the actual resistance is higher and the real voltage drop exceeds the table prediction.

For accurate results on hot-environment circuits, use the temperature-corrected R value: R_hot = R_table times (1 + alpha times (T_actual minus T_table)), where alpha is the temperature coefficient of the conductor material (0.00393 for copper, 0.00403 for aluminium).

Worked example: residential 32 A circuit, 50 m run

A residential cooktop circuit runs 50 metres from the switchboard on single-phase 230 V. Design current is 32 A, power factor 0.95. Cable is 6 mm squared copper PVC twin and earth.

From AS/NZS 3008.1.1 Table 30, R for 6 mm squared copper at 75 degrees is approximately 3.7 milliohms per metre. X is approximately 0.1 milliohms per metre (negligible for this size).

Vd = 32 x 50 / 1000 x (3.7 x 0.95 + 0.1 x 0.31)
Vd = 1.6 x (3.515 + 0.031)
Vd = 5.67 V

5.67 V is 2.5 percent of 230 V, well under the 5 percent limit. The 6 mm squared cable passes.

Worked example: commercial 100 A three-phase feeder

A commercial subboard feeder carries 100 A balanced three-phase at 400 V, power factor 0.85, on 35 mm squared copper XLPE four-core. Route length is 80 metres.

From AS/NZS 3008.1.1 Table 31, R for 35 mm squared copper XLPE at 90 degrees is approximately 0.65 milliohms per metre. X is approximately 0.09 milliohms per metre.

Vd per phase = 100 x 80 / 1000 x (0.65 x 0.85 + 0.09 x 0.53)
Vd per phase = 8.0 x (0.5525 + 0.0477)
Vd per phase = 4.80 V

Vd line-to-line = 4.80 x 1.732 = 8.31 V

8.31 V is 2.1 percent of 400 V, well under the 5 percent limit. If the route were 200 metres instead of 80, the drop would be 20.8 V (5.2 percent), which fails, and the cable would need to be upsized to 50 mm squared.

Common mistakes

• Using straight-line distance instead of route length. Cables follow walls, ceilings, and cable trays. The actual route is always longer than the floor plan distance.
• Ignoring reactance on large cables. For cables 35 mm squared and larger, reactance adds 5 to 15 percent to the impedance. Ignoring it underestimates voltage drop.
• Using DC resistance tables. AC resistance includes skin effect and proximity effect, which increase resistance above the DC value for large conductors.
• Forgetting the return path. The R values in AS/NZS 3008.1.1 already include the go-and-return path for single-phase calculations. Do not double the length.
• Not checking at full load. Voltage drop must be checked at the design current, not the typical operating current. The 5 percent limit applies under full load conditions.

Where the calculator fits in

The ElecCalc Voltage Drop Calculator applies the formula automatically for any cable size, length, and load. Enter the circuit parameters and it returns the voltage drop in volts and as a percentage, with a pass or fail compliance check against AS/NZS 3000 Clause 3.6. For full cable selection including current carrying capacity and earth fault loop impedance, use the Cable Sizing Calculator which runs all three checks in parallel.