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AS/NZS 3008 · Australia & New Zealand

Voltage Drop Calculator

Size your conductors with confidence. Check your voltage drop, find the smallest cable that passes, or see how far a run can go — every result cross-checked against current rating.

✓Single, three-phase & DC ✓PVC, XLPE & fire-rated ✓Ampacity cross-check
Step 01
Circuit Inputs
Solve for
Enter cable size and run length — the drop is solved live.
System
Voltage drop (load)
Standard load direction — drop measured at the point of use.
Supply voltage 230 V nominal
V
Load
= 20.0 A design current
Conductor material
Cable size — solved
2.5 mm²
Maximum run length — solved
—
Max allowable drop
AS/NZS 3000 — point of supply to the far point of use.
Cores
Conductors per phase
Parallel sets are conductors joined at both ends — two sets halve the effective impedance.
Calculation method
Accurate uses R·cosφ + X·sinφ with the entered power factor — closer to real installed behaviour.
Origin → main switchboard → sub-board → final circuit. Each segment carries its own length, cable size and current — the drops add up against one budget.
Cumulative drop 3.24%
7.45 V across 2 segments · budget 5%
0%budget 5%10%
Voltage drop
Compliant
1.76 %
4.04 V dropped · 226.0 V at the load
0%limit 5%10%
✓ Passes voltage drop and current rating (33 A rating vs 20.0 A design).
Conductor R
3.700 Ω/km
Reactance X
0.0860 Ω/km
mV / A·m
6.73
Current rating
33 A
Design current
20.0 A
Total run
60 m
↓ Passes with headroom. 4 mm² would also comply on both drop and current rating.
Adjacent Sizes
SizeDrop VDrop %LimitRating A
2.5 mm²9.634.19%✓20
4 mm²6.012.61%✓26
6 mm²4.041.76%✓33
10 mm²2.411.05%✓45
16 mm²1.530.67%✓60
Gold row is the smallest size that passes. Ratings are indicative for the selected construction.
On The Tools

Where Voltage Drop Actually Bites

Electrician installing a long sub-main cable in a backyard trench
Long sub-mains

Where drop shows up first — distance multiplies every milliohm.

Electrician measuring voltage at the load with a digital multimeter
At the load

Measure at the load, not the board — that's the real number.

Cut ends of different electrical cable sizes — 2.5, 6, 16 and 35 square millimetre
Size up early

Cheap at rough-in, expensive once the walls close.

AS/NZS 3008 Reference

How the Calculation Works

Single-phase AC
Vd = 2 · I · L · (R·cosφ + X·sinφ) ÷ 1000
Active and neutral both carry the current, so the factor is 2 × the one-way length.
Three-phase AC
Vd = √3 · I · L · (R·cosφ + X·sinφ) ÷ 1000
Line-to-line drop on a balanced load — √3 replaces the factor of 2.
DC circuits
Vd = 2 · I · L · R ÷ 1000
No reactance at DC. ELV systems reach the limit fast — 5% of 12 V is 0.6 V.
Load conversions to amps
1-phase kWI = kW × 1000 ÷ (V × pf)
3-phase kWI = kW × 1000 ÷ (√3 × V × pf)
kVAI = kVA × 1000 ÷ V  (÷ √3 × V for three-phase)
HorsepowerkW = hp × 0.746, then use the kW formula
Horsepower is converted at 746 W per hp (motor output). Add motor efficiency separately if you are sizing from shaft power.
Allowable limits (AS/NZS 3000)
5%Point of supply to the far point of use — the AS/NZS 3000 maximum.
2%Consumer mains guideline, leaving budget for the sub-circuits.
3%Final sub-circuit guideline, best-practice split of the 5%.
7%Allowed where supply comes from a private transformer within the installation.
Standards & Assumptions
✓Formulae and conductor values follow AS/NZS 3008.1.1 for AU / NZ installations.
✓Resistance is scaled from 75 °C to the selected insulation temperature.
✓Reactance differs for single-core and multi-core construction.
✓Parallel sets divide the effective impedance by the number of sets.
✓Current ratings are indicative for the selected construction — derate for grouping, ambient and installation method.
✓Length is one-way; the 2 / √3 factor accounts for the full circuit.

Indicative values for planning. Confirm against the full AS/NZS 3008 tables and manufacturer data before installation.

Sub-main cable terminations landed on a main switch at a switchboard
Rule of thumb

If a run is over 30 metres, assume voltage drop decides the cable size — not the breaker.

Below that, ampacity almost always governs. Past it, the 5% budget runs out first and every extra metre costs you. Check both, then buy the larger of the two.

Worked Examples

Run The Numbers Yourself

230 V single-phase · 20 A · 40 m one way · 6 mm² copper multi-core V-90 · pf 0.9 · 5% limit
Vd = 2 × 20 × 40 × (3.70 × 0.9 + 0.086 × 0.436) ÷ 1000 = 5.39 V → 5.39 ÷ 230 = 2.34%
Passes — 2.34% against a 5% budget, and 6 mm² is rated 33 A.
400 V three-phase · 22 kW at pf 0.86 (= 36.9 A) · 100 m one way · 16 mm² copper · 5% limit
I = 22 000 ÷ (√3 × 400 × 0.86) = 36.9 A
Vd = √3 × 36.9 × 100 × (1.38 × 0.86 + 0.079 × 0.510) ÷ 1000 = 7.85 V → 1.96%
Passes comfortably — and 16 mm² carries 55 A on a three-phase circuit.
12 V DC · 30 A · 8 m one way · 16 mm² copper · 5% limit (= 0.6 V)
Vd = 2 × 30 × 8 × 1.38 ÷ 1000 = 0.66 V → 0.66 ÷ 12 = 5.52%
Fails — at 12 V the 5% budget is only 0.6 V. Step up to 25 mm² or shorten the run.
Dan Di Martino, founder of Spark Innovation Group
“One size up costs a few hundred dollars at rough-in. Getting it wrong costs a re-pull… and in 21 years of Sydney installs, I've never once had a callback about a cable that was too big.”
Dan Di Martino
Founder & Licensed ElectricianSpark Innovation Group
Reference Library

Voltage Drop, Explained Properly

Everything behind the numbers above — the AS/NZS 3008 formulae, the legal limits, where the conductor data comes from, and the questions we get asked most on site. Open what you need.

AS/NZS 3008.1.1 gives two ways to arrive at the same answer: the impedance method, where you multiply current, length and the conductor's resistance and reactance; and the tabulated mV/A·m method, where the standard has already done that multiplication for you. This calculator uses the impedance method so the power factor and conductor temperature you enter are actually applied, then reports the equivalent mV/A·m figure so you can cross-check it against the printed tables.

Single-phase AC (active + neutral)
Vd = 2 · I · L · (R·cos φ + X·sin φ) ÷ 1000
2 — current flows out and back, so the loop is twice the run length
I — design load current in amps (Ib), not the breaker size
L — one-way route length in metres, measured along the cable
R — conductor resistance in Ω/km at operating temperature
X — conductor reactance in Ω/km (matters above ~25 mm²)
cos φ — power factor; sin φ = √(1 − cos²φ)
Three-phase AC (line to line, balanced)
Vd = √3 · I · L · (R·cos φ + X·sin φ) ÷ 1000
Same variables. √3 (1.732) replaces the factor of 2 because a balanced three-phase load has no return current in the neutral — the drop is measured between phases. Divide the phase-to-phase result by √3 if you need the phase-to-neutral drop at a single-phase load fed off that board. For a run with cables in parallel, divide the result by the number of sets per phase.
DC circuits (solar, battery, ELV)
Vd = 2 · I · L · R ÷ 1000
Reactance and power factor disappear at DC — only resistance remains. Low-voltage DC is unforgiving: 3% of a 12 V system is just 0.36 V, which a 20 A load loses in a few metres of 4 mm². For PV strings and battery interconnects, size on voltage drop first and ampacity second.
Converting the answer to a percentage

Vd% = Vd ÷ Vnominal × 100. Use the nominal voltage of the system you are calculating — 230 V for single-phase, 400 V for three-phase line-to-line. Mixing 400 V drop against 230 V nominal is the single most common error we see in submitted calculations.

The mV/A·m shortcut

The tables in AS/NZS 3008.1.1 list a millivolt drop per amp per metre for each size and installation arrangement. Multiply it by your current and route length, divide by 1000, and you have volts: Vd = mV/A·m × I × L ÷ 1000. It is quick, but the printed figure assumes a fixed power factor and conductor temperature, so it will not match a corrected impedance calculation exactly.

Every conductor has resistance. Push current through it and some of the supply voltage is consumed getting to the load instead of doing work there — that loss is voltage drop. A 230 V outlet at the board can be a 213 V outlet at the end of a long sub-main, and the appliance only ever sees 213 V.

It is not just a compliance number. The lost voltage becomes heat in the cable, so you pay for it on every kilowatt-hour, and the equipment at the far end compensates by drawing more current — which increases the drop again. Motors are the clearest example: torque falls with the square of the voltage, so a 10% drop costs roughly 19% of starting torque and the motor runs hotter to deliver the same shaft power.

EquipmentWhat excessive drop looks like
Induction motors, pumps, compressorsHard starting, nuisance overload trips, higher running current, shortened winding life
LED lighting & driversVisible dimming along a run, flicker, mismatched colour between the first and last fitting
EV chargersCharger derates or faults out; continuous full-load current makes drop worse than any other domestic circuit
Resistive heating, ovens, HWSSlow heat-up — output falls with the square of voltage, so 5% low is about 10% less heat
Grid-connect solar invertersVoltage rise instead of drop — inverter throttles or disconnects on over-voltage during peak export
Switchboards & sub-boardsReduced fault level at the far end, which can push protective device clearing times out of compliance

The practical consequence is that voltage drop, not current-carrying capacity, is what determines cable size on most long runs. A 6 mm² cable may be perfectly rated for 34 A, but at 60 metres it will fail the 5% limit long before it gets warm.

AS/NZS 3000 (the Wiring Rules) sets the ceiling: 5% from the point of supply to any point of use. That is a total budget for the whole path — consumer mains, sub-mains and final sub-circuit combined — not 5% per section. Where the installation is fed from its own transformer inside the site, the allowance increases to 7% measured from the transformer's LV terminals.

AllowanceApplies toAt 230 V / 400 V
5%Point of supply to the far point of use — the hard limit11.5 V / 20.0 V
2%Working target for consumer mains, leaving room downstream4.6 V / 8.0 V
3%Working target for a final sub-circuit; also the common spec for lighting6.9 V / 12.0 V
7%Installations supplied from a private transformer within the site16.1 V / 28.0 V
How to split the 5% budget

A workable division for a house or small commercial job is 1–2% on the consumer mains, 1% on sub-mains to a sub-board, and 2–3% on the final sub-circuit. Fix the mains allowance first — it is the section you can least afford to re-pull later. Use the cascaded-run mode in the calculator above to add the segments up properly rather than checking each one in isolation.

Two things sit outside the Wiring Rules but still bind you. Distributors publish their own service and connection rules — some require the mains drop to stay within 1% or specify a minimum consumer mains size regardless of calculation. And for grid-connect solar, AS/NZS 4777.1 limits the total drop from the inverter to the point of supply to 1%, because on export the same impedance produces a voltage rise at the inverter terminals.

The R and X figures in this calculator follow AS/NZS 3008.1.1 for annealed copper and aluminium conductors at their normal operating temperature — 75 °C for V-75 PVC and 90 °C for XLPE. Resistance rises with temperature at about 0.393% per °C for copper, so a cable running hot drops more voltage than the same cable running cool. That is why the insulation selection in the advanced panel changes the answer.

Copper sizeR at 75 °C (Ω/km)X (Ω/km)Reactance share
2.5 mm²8.870.121Negligible
6 mm²3.700.112Negligible
16 mm²1.380.102Minor
35 mm²0.6270.0967Starts to count
95 mm²0.2320.0918Significant
240 mm²0.09850.0879Comparable to R
Indicative multicore values. Single-core cables in flat or trefoil formation have different reactance — check the arrangement column in the printed tables for large installations.

Notice the pattern: resistance falls almost in proportion to cross-sectional area, but reactance barely changes. Below about 25 mm² the resistive term dominates and power factor hardly affects the result. Above it, reactance becomes a real share of the impedance — which is why going up two sizes on a large three-phase sub-main returns less improvement than you would expect, and why paralleling two smaller cables is sometimes the better answer.

Aluminium is roughly 1.6 times the resistance of copper for the same area, so an aluminium conductor needs about two sizes up to match a copper one on voltage drop. It is still common on larger consumer mains because the cost and weight savings outweigh the extra size.

A compliant cable has to satisfy four tests, and voltage drop is only one of them. Sizing on drop alone is how undersized mains get installed on short runs and oversized cable gets bought on long ones.

TEST 1
Current-carrying capacity
The derated rating must be at least the protective device rating. AS/NZS 3008 Tables 4–21 give the base figures by installation method.
TEST 2
Voltage drop
Within the 5% total budget at design load — the calculation this page performs.
TEST 3
Short-circuit withstand
The conductor must survive the prospective fault current for as long as the protective device takes to clear it.
TEST 4
Earth fault loop impedance
Loop impedance must be low enough to trip the device within the required time — long runs often fail here first.
Typical derating factors that reduce the rating
Ambient air 40 °C / 45 °C / 50 °C (V-75 PVC)0.94 / 0.87 / 0.79
2 / 3 / 4 circuits grouped and enclosed0.80 / 0.70 / 0.65
Touching thermal insulation on one side0.75
Completely surrounded by thermal insulation0.50
Buried direct, thermal resistivity above 1.2 K·m/W0.90 or lower
Factors multiply. A cable in a roof space at 45 °C grouped with two others can lose more than a third of its rating before you start on voltage drop.

The current-rating line in the results above applies a single set of assumptions as a sanity check. It tells you when the size that passes voltage drop is clearly under-rated for the load — it does not replace a full Table 4 selection against your actual installation method, grouping and ambient temperature.

01
Choose what you are solving for
Voltage drop when the cable is already chosen, minimum cable size when it is not, or maximum run length when you are working out how far you can reach with what is on the truck.
02
Enter the design current, not the breaker
Use the actual maximum demand (Ib). If you only know the load in kW, kVA or hp, switch the unit and the calculator converts it for you using the power factor you set. Sizing on a 32 A breaker when the load is 18 A wastes copper.
03
Measure the route, then add slack
Length is the path the cable takes — up walls, along joists, around obstructions — not the straight line on the plan. Add tails at both ends. Ten metres of forgotten route is a full size on a long sub-main.
04
Set the limit to your budget, not the maximum
If this is a sub-main feeding a board, give it 1–2% and leave the rest for the sub-circuits. Only use the full 5% when this is the entire path from the point of supply.
05
Use cascaded runs for multi-segment paths
Mains to main switchboard, sub-main to sub-board, then the final circuit — add them as segments so the cumulative total is checked against the 5% ceiling instead of each leg passing on its own.
06
Check the adjacent sizes before you commit
The comparison table shows the size either side. If you are sitting at 4.8% on a 5% limit, take the next size up — real installations run hotter and longer than the plan.
07
Save the result with the job
Print the one-page record or copy the share link — it captures every input, so the calculation can be reproduced or handed to a certifier months later.
Is 5% voltage drop the limit for the whole installation or each circuit?
The whole path. AS/NZS 3000 measures 5% from the point of supply to the far point of use, so the consumer mains, any sub-mains and the final sub-circuit share that single budget. Each section passing on its own means nothing if the total exceeds 5%.
Do I calculate voltage drop on the breaker size or the actual load?
The design current — the maximum demand the circuit will actually carry. Current-carrying capacity is checked against the protective device rating; voltage drop is checked against the load. Using the breaker rating for drop is conservative but routinely leads to a cable one or two sizes larger than necessary.
Does length mean one way or the full loop?
One way. Enter the route length from source to load and the formula applies the loop factor for you — 2 for single-phase and DC, √3 for balanced three-phase. Doubling the length yourself double-counts the return path.
Why does my three-phase answer differ from an online calculator?
Usually the reference voltage. A three-phase result is a line-to-line drop and must be compared with 400 V; divide by √3 if you want the phase-to-neutral figure at 230 V. The other common cause is power factor — a calculator fixed at unity ignores the reactive term and reads low on large cables.
What power factor should I use if I do not know the load?
0.8 is the conventional worst case for mixed and motor load, and it is what most inspectors expect to see. Resistive loads such as hot water and heating are effectively 1.0. Modern LED lighting and switch-mode supplies are typically 0.9 or better. If you are unsure, calculate at 0.8 — the result is the safe side.
Can I use two smaller cables instead of one big one?
Yes. Two identical cables in parallel per phase halve both resistance and reactance, so they halve the voltage drop, and they are often easier to terminate and pull than a single large conductor. They must be the same size, type and length, and each set needs to be accounted for in the grouping derating.
Why does voltage rise matter for solar?
On export, current flows from the inverter towards the grid, so the same cable impedance raises the voltage at the inverter terminals instead of lowering it. If the rise pushes the terminal voltage above the inverter's limit it throttles output or disconnects. AS/NZS 4777.1 caps the total drop from inverter to point of supply at 1%, which usually means a bigger cable than the ampacity alone would suggest.
Does aluminium change the calculation?
Only through the resistance value. Aluminium has about 1.6 times the resistance of copper for the same cross-section, so expect to go up roughly two standard sizes to match a copper cable on drop. It is common and perfectly compliant on larger consumer mains, but termination technique matters far more than it does with copper.
Is a calculated result enough for compliance?
A calculation is the design step, not the verification step. AS/NZS 3000 also requires the installation to be tested and verified once installed, and distributors may impose their own limits on the mains. Keep the calculation with the job file, then confirm the real voltage at the load under full load.

Disclaimer: This calculator is designed in accordance with AS/NZS 3008.1.1 — Electrical Installations: Selection of Cables and typical Australian installation conditions. While every effort has been made to ensure accuracy using official formulae and data, Spark Innovation Group accepts no liability for design or compliance decisions based on these calculations.