What is the maximum allowable voltage drop in Australia?

AS/NZS 3000:2018 Clause 3.6.2 limits the total voltage drop from the point of supply to the point of utilisation to 5% of the nominal voltage: 11.5 V on a 230 V single-phase circuit, 20 V on a 400 V three-phase circuit. The 5% is one limit for the whole path, shared between the consumer mains, submains and final subcircuits combined, which is why final subcircuits commonly target a 2.5% allowance.

How is voltage drop calculated for AC cables?

Voltage drop is calculated using Vd = (K × I × L × Z) / 1000. K is 2 on single-phase, because the current returns through the neutral, and √3 on three-phase. I is the load current in amps, L is the one-way run length in metres, and Z is the cable impedance in ohms per kilometre from AS/NZS 3008.1.1, taken at the cable operating temperature.

Does cable temperature affect voltage drop?

Yes: conductor resistance rises with temperature. AS/NZS 3008.1.1 tables list impedance at the maximum operating temperature for each insulation type (75 °C for V-75/PVC, 90 °C for X-90/XLPE). The ScaleSet calculator applies the correct value automatically.

What is the difference between voltage drop and voltage rise?

Voltage drop is the reduction in voltage as current flows from the source to the load (most installations). Voltage rise is the increase in voltage as current flows from a distributed generator (typically a rooftop solar inverter) back to the point of supply. AS/NZS 4777.1 Clause 3.3.3 limits voltage rise to 2% along the whole path from the point of supply to the inverter a.c. terminals (not just the inverter-supply cable). The ScaleSet voltage rise calculator covers that case.

How do I calculate voltage drop in a three-phase cable?

For balanced three-phase circuits Vd = √3 × I × L × Zc / 1000, with L the one-way length in metres and Zc the cable impedance in ohms per kilometre from the AS/NZS 3008.1.1:2025 Section 4 tables. By default the calculator uses the worst-case impedance Zc = √(Rc² + Xc²); turn Worst Case PF off and it uses Zc = Rc·cosφ + Xc·sinφ from the power factor you enter. Either way the resistance is taken at the cable operating temperature.

Why do my voltage drop results differ from a 20 °C calculation?

AS/NZS 3008.1.1:2025 publishes AC resistance at the cable operating temperature: 75 °C for V-90 PVC, 90 °C for X-90 XLPE. A 20 °C value (sometimes used in textbook examples) under-estimates real-world drop by 18–28 % for a fully loaded circuit because conductor resistance rises with temperature.

How do I calculate the voltage drop of a busbar trunking or rising main?

AS/NZS 61439.6 Annex AA gives u = k × √3 × (R cos φ + X sin φ) × IB × L for a three-phase run, with R and X the mean resistance and reactance per metre the busbar trunking manufacturer declares at rated current and 35 °C ambient (Table 102). Below 100 A the reactance is deemed negligible. The factor k accounts for where the load sits: 1 when it is concentrated at the far end, (n + 1) / 2n when it is spread evenly over n tap-off units, so a riser feeding four equal floors sees five-eighths of the end-load drop. Choose busbar trunking as the conductor in this calculator and enter those figures; the AS/NZS 3000 5% limit still applies to the total.

When do I need to include cable reactance Xc?

You do not need to decide: this calculator always includes reactance, and a cable with no reactance value in the tables is treated as missing data rather than zero. As a rule of thumb, reactance is negligible on small cables (AS/NZS 3000 Appendix B4.3 allows it to be ignored at or below 35 mm² where the conductors run close together) and becomes significant on large cables and at low power factor.

Does AS/NZS 3008.1.1:2025 change the voltage drop methodology from 2017?

The voltage drop method is unchanged. The 2025 revision renumbered the impedance tables into the Section 4 series (4.1 to 4.10), updated a number of entries (notably aluminium AC resistance for some sizes) tightened the temperature correction factors and added entries for high-temperature 110 °C insulations. The ScaleSet calculator uses the 2025 table values.

Does the calculator handle multiple cables in parallel?

Not on this page: the voltage drop calculator sizes one cable at a time. For parallel runs use the Cable Selection calculator, which has a Parallel Runs stepper and divides the design current between the runs. AS/NZS 3000 Clause 3.6.3 says the voltage drop of a parallel set is the drop in one conductor carrying the circuit current, divided by the number of conductors in parallel.

How far can I run a 2.5 mm² cable at 20 A before voltage drop is a problem?

On 230 V single-phase, 2.5 mm² copper V-90 drops 18.02 mV per ampere-metre from the AS/NZS 3008.1.1:2025 tables, so at the full 20 A it reaches 5% (11.5 V) at 31 m one way and a 2.5% final-subcircuit budget at 15 m. For a socket-outlet or lighting circuit with the load spread along it, Clause 3.6.2 Exception 1 lets you check at half the breaker rating, which doubles both figures to 63 m and 31 m. On 400 V three-phase the lengths double again because the per-phase drop is √3 × Zc rather than 2 × Zc.

Can I use half the breaker rating for a voltage drop calculation?

Yes, for one case. AS/NZS 3000:2018 Clause 3.6.2 Exception 1 (added by Amendment 2) allows half the current rating of the protective device to be used for a final subcircuit with a distributed load such as socket-outlets or lighting. It does not apply to a single fixed load, a submain or consumer mains: those are checked at the connected load, the maximum demand or the device rating, whichever is lowest.

Does the 5% voltage drop include the distributor's network?

No. Clause 3.6.2 measures from the point of supply of the low voltage installation to any point in that installation, so it is the customer's cables only. The distributor keeps its own supply voltage within the AS 60038 range at the point of supply; the 5% is the allowance the installation may take off whatever arrives there.

What voltage drop is allowed with an on-site substation?

7%. AS/NZS 3000:2018 Clause 3.6.2 Exception 3 raises the limit from 5% to 7% where the point of supply is the low voltage terminals of a substation located on the premises and dedicated to the installation. Set the limit field to 7 on this page to check against it, and record the exception on the drawing.

Is motor starting voltage drop covered by the 5% rule?

No. Note 1 to Clause 3.6.2 excludes motor starting, solenoid closing and similar transient currents from the 5% limit. Check the cable at the motor full load current here, and treat the starting dip as a motor performance question: the starter type, the motor's minimum starting voltage and any supply authority limit on starting current under AS/NZS 3000 Clause 4.13 and the distributor's service rules.

Is aluminium cable acceptable for voltage drop?

Yes, at about 1.6 times the copper cross-section for the same drop. On a 160 A, 60 m three-phase run at 0.9 power factor, 70 mm² copper X-90 drops 1.36%, 95 mm² aluminium drops 1.60% and 120 mm² aluminium drops 1.29%. Select aluminium as the conductor and the calculator reads Table 4.7(B) instead of 4.7(A) and caps the operating temperature at 90 °C.

What is the simplified voltage drop method in AS/NZS 3000 Appendix C?

Table C8 tabulates ampere-metres per 1% voltage drop for twelve copper PVC cable sizes, single-phase at 230 V and three-phase at 400 V, so a run can be checked by dividing current × length by the table figure. It is a quick check for standard cables at unity power factor; it has no aluminium, XLPE or power factor columns, and it was tabulated against an earlier AS/NZS 3008.1.1 edition. This calculator reproduces Table C8 to within 1% up to 35 mm² on the 2025 tables and is the method to use for anything the table does not cover.

What happens if the voltage drop is over 5%?

Equipment may not receive its rated voltage, motors run hotter and draw more current, lighting dims and electronic loads can reset, and the installation does not comply with Clause 3.6.2. The fixes, in the order they are usually cheapest: shorten the route, move allowance from another section of the budget, go up one conductor size on the longest section, or move the distribution board closer to the load. On a three-phase site, rebalancing a single-phase load across phases also helps because the neutral current falls.

ScaleSet

Voltage Drop Calculator: AS/NZS 3008.1.1 Cable Voltage Drop for Australia

Free, browser-based voltage drop calculator built for Australian and New Zealand electricians, designers and electrical engineers. Calculates cable voltage drop straight from the AS/NZS 3008.1.1:2025 impedance tables (Tables 4.1–4.10) using the Clause 4.4 method (Vc = √3 × (Rc·cosφ + Xc·sinφ) for balanced three-phase, 2 × (Rc·cosφ + Xc·sinφ) for single-phase, and the resistance-only form for DC) with AC resistance taken at the correct conductor operating temperature (75 °C for V-90 PVC, 90 °C for X-90 XLPE, 110 °C for high-temperature insulations) so the result reflects a fully loaded cable, not a 20 °C textbook value. Supports single-phase (230 V) and three-phase (400 V) circuits, copper and aluminium conductors, multi-core and single-core constructions, the full AS/NZS 3008.1.1 installation method library, the 1–630 mm² cable size range, load power factor and parallel cables per phase. Returns the voltage drop in volts and as a percentage of nominal voltage, a pass/fail check against the AS/NZS 3000:2018 Clause 3.6.2 5% limit (11.5 V on 230 V single-phase, 20 V on 400 V three-phase), and a branded PDF report citing the clause, table and resistance value used: ready for the design submission record.

Overview

Voltage drop is what decides whether a cable that can carry the current is actually big enough. AS/NZS 3000 Clause 3.6 caps the total drop at 5% from the point of supply to the furthest point, which is about 11.5 V on a 230 V single-phase supply and 20 V on 400 V three-phase. This voltage drop calculator works from the AS/NZS 3008.1.1:2025 impedance tables using the Clause 4.4 method, with AC resistance taken at the conductor operating temperature rather than a 20 °C textbook value, so the answer reflects a fully loaded cable.

Key facts

AS/NZS 3000 Clause 3.6 limits total voltage drop to 5% from the point of supply to the furthest point: 11.5 V on a 230 V supply, 20 V on 400 V.
Voltage drop is calculated from AS/NZS 3008.1.1 conductor resistance and reactance at operating temperature.
Vc = K × Z, with K = 2 for single-phase circuits and √3 for balanced three-phase circuits.
The drop is apportioned across consumer mains, submains and final subcircuits; the sum must stay within 5%.
A busbar trunking (busway) run has no AS/NZS 3008 table: switch the conductor to busbar trunking and enter the manufacturer's resistance and reactance per metre (AS/NZS 61439.6 Table 102). The Annex AA load-distribution factor k then scales the drop: 1 for a load at the end of the run, (n + 1) / 2n for a load spread evenly over n tap-offs.

Who this page is for

Electrical engineers, designers, estimators and licensed electricians checking cable voltage performance for consumer mains, submains and final subcircuits against the AS/NZS 3000 Clause 3.6 5% voltage drop budget on residential, commercial and industrial installations across Australia and New Zealand.

Relevant standards

AS/NZS 3008.1.1:2025 (Selection of Cables: Clause 4.4 voltage drop method, Tables 4.1–4.10 conductor R and X impedance values)
AS/NZS 3000:2018 (Wiring Rules: Clause 3.6.2 maximum 5% voltage drop from the point of supply to the point of utilisation)
AS 60038 (Standard Voltages: 230 V / 400 V nominal voltage the 5% limit is expressed against)

What this tool helps with

Applies the AS/NZS 3008.1.1:2025 Clause 4.4 voltage drop method: Vc = √3 × (Rc·cosφ + Xc·sinφ) for balanced three-phase, 2 × (Rc·cosφ + Xc·sinφ) for single-phase, and the resistance-only form for DC circuits.
Checks the result against the AS/NZS 3000:2018 Clause 3.6.2 limit: 5% of nominal voltage from the point of supply to any point of utilisation (11.5 V on 230 V single-phase, 20 V on 400 V three-phase) with a clear pass/fail pill.
Reads AC resistance Rc and reactance Xc directly from the AS/NZS 3008.1.1:2025 impedance tables (Tables 4.1–4.10) at the conductor operating temperature, not a 20 °C value that under-estimates real drop by 18–28%.
Temperature-correct by insulation type (75 °C for V-90 PVC, 90 °C for X-90 XLPE and 110 °C for high-temperature cross-linked cables) selected automatically from the chosen cable.
Single-phase (230 V) and three-phase (400 V) circuits, copper and aluminium conductors, multi-core and single-core constructions across the standard 1 mm² to 630 mm² cable size range.
Full AS/NZS 3008.1.1 installation method library and single-core arrangement (trefoil, flat-touching, flat-spaced) feeding the reactance, so the calculated drop matches the real install.
Load power factor and parallel-cables-per-phase support: the calculator divides per-cable current and applies the parallel impedance per AS/NZS 3008.1.1 Clause 4.4.
Branded PDF voltage drop report showing the voltage drop in volts and percent, the governing clause and table, the resistance value and operating temperature used: ready for the design submission and verification record.
Built and reviewed by a Chartered Professional Engineer (CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE Aus): see the Verification page for the testing and review process.

How to calculate voltage drop under AS/NZS 3008.1.1:2025

Enter the load current: Enter the design current Ib in amperes: the worst-case continuous current the cable will carry (after applying diversity for max-demand calculations).
Set the circuit type: Pick single-phase 230 V, three-phase 400 V balanced, or DC. The calculator applies factor 2 × L for single-phase / DC and √3 × L for balanced three-phase per AS/NZS 3008.1.1 Clause 4.4.
Enter the cable run length: Enter the one-way circuit length in metres. The factor of 2 or √3 in the formula accounts for the return path automatically: do not double the length manually.
Pick the cable conductor and insulation: Choose copper or aluminium, conductor cross-sectional area, and insulation (V-90 PVC, X-90 XLPE, X-90-HT). The calculator reads AC resistance and reactance from the AS/NZS 3008.1.1:2025 Section 4 tables (4.1 to 4.10) at the matching operating temperature (75 °C, 90 °C or 110 °C).
Enter the load power factor: For motor loads use the nameplate cosφ; for typical lighting / electronic loads with PFC use 0.95–0.99. Power factor enters the formula as Rc·cosφ + Xc·sinφ.
Review the result and check against the 5% Clause 3.6 limit: The calculator displays the voltage drop in volts and as a percentage of the nominal voltage, and flags any result above the AS/NZS 3000:2018 Clause 3.6 5% limit. Export the branded PDF for the design submission record.

Complete guide to voltage drop calculation under AS/NZS 3008.1.1:2025

What does AS/NZS 3000:2018 say about voltage drop?

Clause 3.6.2 of AS/NZS 3000:2018 limits the total voltage drop between the point of supply and any point of utilisation to 5% of the nominal voltage when supplied at the nominal voltage. This is the single design limit most Australian and New Zealand electrical installations work to: 230 V × 5% = 11.5 V maximum drop on a single-phase circuit, or 400 V × 5% = 20 V on a three-phase circuit.

The 5% allowance is a global ceiling: it includes consumer mains, submains and final subcircuits combined. Most consulting practice budgets 2% to consumer mains, 1% to submains and 2% to final subcircuits, but the split is a design choice as long as the total stays below 5%.

The exact voltage drop formula from AS/NZS 3008.1.1:2025

AS/NZS 3008.1.1:2025 Clause 4.4 gives the voltage drop per ampere per metre as Vc = √3 × (Rc·cosφ + Xc·sinφ) for three-phase circuits or Vc = 2 × (Rc·cosφ + Xc·sinφ) for single-phase. Rc and Xc are the AC resistance and reactance in mΩ/m taken from the AS/NZS 3008.1.1:2025 impedance tables (Tables 4.1–4.10), evaluated at the cable operating temperature.

For DC circuits the reactance term drops out entirely and Vc = 2 × Rc with Rc taken at the operating temperature. For LV three-phase balanced circuits with cosφ near unity the formula collapses to Vd ≈ √3 × I × L × Rc: the form most engineers use as a sanity check.

Why operating temperature matters

AC resistance in the AS/NZS 3008.1.1:2025 Section 4 tables (4.1 to 4.10) is published at 75 °C for V-90 PVC cables, 90 °C for X-90 XLPE cables and 110 °C for high-temperature cross-linked types. Voltage drop calculated at 20 °C ambient resistance under-estimates real-world drop by 18–28 % for a fully loaded V-90 circuit.

The ScaleSet voltage drop calculator picks the correct operating temperature from the cable insulation type automatically and applies the matching Section 4 table entry. The PDF report shows the resistance value, the temperature and the clause used so the calculation can be re-traced.

Single-phase, three-phase and DC: when each applies

Use the single-phase formula (factor 2 × L) for any 230 V single-phase circuit and for any 400 V three-phase circuit operating with an unbalanced load that returns through the neutral. Use the three-phase formula (factor √3 × L) for balanced three-phase circuits (motors, three-phase final subcircuits with balanced lighting).

For DC circuits (solar string DC, EV charger DC link, battery banks) use 2 × L and ignore the reactance term. AS/NZS 4777.1 (Clause 3.3.3) sets a separate 2% maximum voltage rise from the point of supply to the inverter a.c. terminals on the AC side of grid-connected inverters; the ScaleSet voltage rise calculator handles that case separately.

Worked example 1: a 20 A single-phase final subcircuit

A 20 A socket-outlet circuit runs 25 m from the distribution board in 2.5 mm² copper V-90 twin and earth, clipped direct. From AS/NZS 3008.1.1:2025 Table 4.7(A) the AC resistance of 2.5 mm² copper at 75 °C is 9.01 Ω/km and from Table 4.1(B) the reactance is 0.102 Ω/km, so the worst-case impedance is Zc = √(9.01² + 0.102²) = 9.01 Ω/km and the single-phase voltage drop per ampere-metre is Vc = 2 × 9.01 = 18.02 mV/A·m.

Vd = Vc × I × L / 1000 = 18.02 × 20 × 25 / 1000 = 9.01 V, which is 3.92% of 230 V. That passes the 5% installation limit on its own, but it leaves less than 1.1% for the consumer mains and submain ahead of it, so on a real job it fails a 2.5% final-subcircuit budget. Going to 4 mm² (Vc = 11.22 mV/A·m) brings the same run down to 5.61 V, or 2.44%, which is why 4 mm² is the usual answer for a long 20 A circuit.

If the load is socket-outlets or lighting spread along the circuit, AS/NZS 3000:2018 Clause 3.6.2 Exception 1 lets the check use half the protective device rating: at 10 A the 2.5 mm² run drops 4.51 V (1.96%) and passes the same 2.5% budget without upsizing.

Worked example 2: a 63 A three-phase submain at 0.85 power factor

A 63 A submain to a distribution board runs 40 m in 16 mm² four-core copper X-90 on a perforated tray. Table 4.7(A) gives 1.40 Ω/km at 90 °C and Table 4.1(B) gives 0.0805 Ω/km. With a mixed motor and lighting load at 0.85 lagging, the Clause 4.4 form is Vc = √3 × (Rc cos φ + Xc sin φ) = √3 × (1.40 × 0.85 + 0.0805 × 0.527) = 2.13 mV/A·m.

Vd = 2.13 × 63 × 40 / 1000 = 5.38 V, or 1.34% of 400 V. Checking the same cable at worst-case power factor (Zc = √(Rc² + Xc²), the default on this page) gives 2.43 mV/A·m and 6.12 V, or 1.53%. The gap between the two is the reactance term, and it is small on a 16 mm² cable; on a 240 mm² cable at 0.8 power factor it is most of the answer, which is why the calculator never drops Xc.

A 1.3% to 1.5% submain leaves 3.5% for the consumer mains and the final subcircuits downstream, which is comfortable. The same run in 25 mm² would sit at 0.87%; that is the size to pick if the board is going to feed long final subcircuits of its own.

Maximum cable run length for voltage drop

Maximum one-way route length in metres for copper V-90 multicore cable on the AS/NZS 3008.1.1:2025 Table 4.7(A) and 4.1(B) impedance at 75 °C, worst-case power factor, at the full protective device rating. The 5% column is the whole installation limit; the 2.5% column is the usual final-subcircuit budget. Longer runs need the next size up, or the Exception 1 half-rating check for distributed loads.
Cable and breaker	Vc 1-phase (mV/A·m)	230 V at 2.5%	230 V at 5%	Vc 3-phase (mV/A·m)	400 V at 2.5%	400 V at 5%
2.5 mm² on 16 A	18.02	19 m	39 m	15.61	40 m	80 m
2.5 mm² on 20 A	18.02	15 m	31 m	15.61	32 m	64 m
4 mm² on 25 A	11.22	20 m	41 m	9.72	41 m	82 m
4 mm² on 32 A	11.22	16 m	32 m	9.72	32 m	64 m
6 mm² on 32 A	7.50	23 m	47 m	6.50	48 m	96 m
6 mm² on 40 A	7.50	19 m	38 m	6.50	38 m	76 m
10 mm² on 50 A	4.46	25 m	51 m	3.87	51 m	103 m
16 mm² on 63 A	2.81	32 m	64 m	2.43	65 m	130 m
25 mm² on 80 A	1.78	40 m	80 m	1.54	81 m	162 m
35 mm² on 100 A	1.29	44 m	89 m	1.11	90 m	180 m

Splitting the 5% between consumer mains, submains and final subcircuits

A typical voltage drop budget for a commercial installation. AS/NZS 3000 fixes only the 5% total in Clause 3.6.2; how it is shared is the designer's choice, and a building with a long consumer mains run will move allowance out of the final subcircuits to pay for it.
Section	Typical allowance	On 230 V	On 400 V	Why
Consumer mains	1.0% to 2.0%	2.3 V to 4.6 V	4 V to 8 V	Carries the whole maximum demand; the cheapest place to spend copper because one upsize fixes every circuit downstream.
Submains	1.0% to 1.5%	2.3 V to 3.5 V	4 V to 6 V	Sized on the board's maximum demand after diversity, never on the sum of its breakers.
Final subcircuits	2.0% to 2.5%	4.6 V to 5.8 V	8 V to 10 V	The long thin runs. Exception 1 allows half the breaker rating for distributed socket and lighting loads.
Total	5.0% maximum	11.5 V	20 V	Clause 3.6.2. Rises to 7% where the point of supply is the LV terminals of a dedicated on-site substation (Exception 3).

Aluminium versus copper for voltage drop

Aluminium has about 1.6 times the resistivity of copper, so an aluminium conductor needs roughly 1.6 times the cross-section for the same voltage drop. On a 160 A, 60 m three-phase submain at 0.9 power factor, 70 mm² copper X-90 drops 5.43 V (1.36%). The nearest aluminium sizes are 95 mm² at 6.39 V (1.60%) and 120 mm² at 5.16 V (1.29%), from Table 4.7(B). Aluminium is usually still the cheaper cable at 95 mm² and above, but the larger conductor, the larger conduit and the bimetallic terminations all have to be carried through the rest of the design.

The calculator reads the aluminium tables directly: pick aluminium as the conductor and it switches from Table 4.7(A) to Table 4.7(B) for resistance, and limits the operating temperature to 90 °C, which is the highest temperature the aluminium tables are published at.

What the 5% limit does not apply to

Clause 3.6.2 has a note and four exceptions that change the check. Note 1 excludes motor starting, solenoid closing and similar transient currents from the 5% limit: the transient dip is a motor-performance question (AS/NZS 3000 Clause 4.13 and the motor manufacturer's minimum starting voltage), not a wiring-rules limit, so the calculator is run at the motor's full load current, not its locked-rotor current. Exception 2 excludes high voltage and extra-low voltage circuits, which have their own rules in Clauses 7.6 and 7.5.

Exception 3 raises the limit to 7% where the point of supply is the LV terminals of a substation on the premises and dedicated to the installation, because the distributor's share of the drop has been taken out of the picture. Exception 4 lets a stand-alone system to Clause 7.3 run to a total of 11% below nominal at the equipment terminals, source regulation and cable drop together. Both are a design decision to record, not a default: the calculator checks 5% unless the limit field is changed.

Key terms

Voltage drop (Vd): The reduction in voltage along a cable under load. AS/NZS 3000 Clause 3.6 limits the total from the point of supply to the furthest point of the installation to 5% of nominal voltage.
Vc (mV/A·m): The voltage drop per ampere of load current per metre of route length, in millivolts, derived from the cable's resistance and reactance in the AS/NZS 3008.1.1 tables.
Operating temperature: The conductor temperature used to read resistance from AS/NZS 3008.1.1; a loaded conductor at 75 or 90 degrees Celsius has a higher resistance than at 20 degrees Celsius, so using the cold value understates the drop.
Route length: The one-way cable run length used in the voltage drop formula. The single-phase factor K = 2 already accounts for the return conductor.

Reviewed by

Wisam Tozah: Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). See how these calculations are verified. LinkedIn. Updated 25 September 2026.

Frequently asked questions

What is the maximum allowable voltage drop in Australia?: AS/NZS 3000:2018 Clause 3.6.2 limits the total voltage drop from the point of supply to the point of utilisation to 5% of the nominal voltage: 11.5 V on a 230 V single-phase circuit, 20 V on a 400 V three-phase circuit. The 5% is one limit for the whole path, shared between the consumer mains, submains and final subcircuits combined, which is why final subcircuits commonly target a 2.5% allowance.
How is voltage drop calculated for AC cables?: Voltage drop is calculated using Vd = (K × I × L × Z) / 1000. K is 2 on single-phase, because the current returns through the neutral, and √3 on three-phase. I is the load current in amps, L is the one-way run length in metres, and Z is the cable impedance in ohms per kilometre from AS/NZS 3008.1.1, taken at the cable operating temperature.
Does cable temperature affect voltage drop?: Yes: conductor resistance rises with temperature. AS/NZS 3008.1.1 tables list impedance at the maximum operating temperature for each insulation type (75 °C for V-75/PVC, 90 °C for X-90/XLPE). The ScaleSet calculator applies the correct value automatically.
What is the difference between voltage drop and voltage rise?: Voltage drop is the reduction in voltage as current flows from the source to the load (most installations). Voltage rise is the increase in voltage as current flows from a distributed generator (typically a rooftop solar inverter) back to the point of supply. AS/NZS 4777.1 Clause 3.3.3 limits voltage rise to 2% along the whole path from the point of supply to the inverter a.c. terminals (not just the inverter-supply cable). The ScaleSet voltage rise calculator covers that case.
How do I calculate voltage drop in a three-phase cable?: For balanced three-phase circuits Vd = √3 × I × L × Zc / 1000, with L the one-way length in metres and Zc the cable impedance in ohms per kilometre from the AS/NZS 3008.1.1:2025 Section 4 tables. By default the calculator uses the worst-case impedance Zc = √(Rc² + Xc²); turn Worst Case PF off and it uses Zc = Rc·cosφ + Xc·sinφ from the power factor you enter. Either way the resistance is taken at the cable operating temperature.
Why do my voltage drop results differ from a 20 °C calculation?: AS/NZS 3008.1.1:2025 publishes AC resistance at the cable operating temperature: 75 °C for V-90 PVC, 90 °C for X-90 XLPE. A 20 °C value (sometimes used in textbook examples) under-estimates real-world drop by 18–28 % for a fully loaded circuit because conductor resistance rises with temperature.
How do I calculate the voltage drop of a busbar trunking or rising main?: AS/NZS 61439.6 Annex AA gives u = k × √3 × (R cos φ + X sin φ) × IB × L for a three-phase run, with R and X the mean resistance and reactance per metre the busbar trunking manufacturer declares at rated current and 35 °C ambient (Table 102). Below 100 A the reactance is deemed negligible. The factor k accounts for where the load sits: 1 when it is concentrated at the far end, (n + 1) / 2n when it is spread evenly over n tap-off units, so a riser feeding four equal floors sees five-eighths of the end-load drop. Choose busbar trunking as the conductor in this calculator and enter those figures; the AS/NZS 3000 5% limit still applies to the total.
When do I need to include cable reactance Xc?: You do not need to decide: this calculator always includes reactance, and a cable with no reactance value in the tables is treated as missing data rather than zero. As a rule of thumb, reactance is negligible on small cables (AS/NZS 3000 Appendix B4.3 allows it to be ignored at or below 35 mm² where the conductors run close together) and becomes significant on large cables and at low power factor.
Does AS/NZS 3008.1.1:2025 change the voltage drop methodology from 2017?: The voltage drop method is unchanged. The 2025 revision renumbered the impedance tables into the Section 4 series (4.1 to 4.10), updated a number of entries (notably aluminium AC resistance for some sizes) tightened the temperature correction factors and added entries for high-temperature 110 °C insulations. The ScaleSet calculator uses the 2025 table values.
Does the calculator handle multiple cables in parallel?: Not on this page: the voltage drop calculator sizes one cable at a time. For parallel runs use the Cable Selection calculator, which has a Parallel Runs stepper and divides the design current between the runs. AS/NZS 3000 Clause 3.6.3 says the voltage drop of a parallel set is the drop in one conductor carrying the circuit current, divided by the number of conductors in parallel.
How far can I run a 2.5 mm² cable at 20 A before voltage drop is a problem?: On 230 V single-phase, 2.5 mm² copper V-90 drops 18.02 mV per ampere-metre from the AS/NZS 3008.1.1:2025 tables, so at the full 20 A it reaches 5% (11.5 V) at 31 m one way and a 2.5% final-subcircuit budget at 15 m. For a socket-outlet or lighting circuit with the load spread along it, Clause 3.6.2 Exception 1 lets you check at half the breaker rating, which doubles both figures to 63 m and 31 m. On 400 V three-phase the lengths double again because the per-phase drop is √3 × Zc rather than 2 × Zc.
Can I use half the breaker rating for a voltage drop calculation?: Yes, for one case. AS/NZS 3000:2018 Clause 3.6.2 Exception 1 (added by Amendment 2) allows half the current rating of the protective device to be used for a final subcircuit with a distributed load such as socket-outlets or lighting. It does not apply to a single fixed load, a submain or consumer mains: those are checked at the connected load, the maximum demand or the device rating, whichever is lowest.
Does the 5% voltage drop include the distributor's network?: No. Clause 3.6.2 measures from the point of supply of the low voltage installation to any point in that installation, so it is the customer's cables only. The distributor keeps its own supply voltage within the AS 60038 range at the point of supply; the 5% is the allowance the installation may take off whatever arrives there.
What voltage drop is allowed with an on-site substation?: 7%. AS/NZS 3000:2018 Clause 3.6.2 Exception 3 raises the limit from 5% to 7% where the point of supply is the low voltage terminals of a substation located on the premises and dedicated to the installation. Set the limit field to 7 on this page to check against it, and record the exception on the drawing.
Is motor starting voltage drop covered by the 5% rule?: No. Note 1 to Clause 3.6.2 excludes motor starting, solenoid closing and similar transient currents from the 5% limit. Check the cable at the motor full load current here, and treat the starting dip as a motor performance question: the starter type, the motor's minimum starting voltage and any supply authority limit on starting current under AS/NZS 3000 Clause 4.13 and the distributor's service rules.
Is aluminium cable acceptable for voltage drop?: Yes, at about 1.6 times the copper cross-section for the same drop. On a 160 A, 60 m three-phase run at 0.9 power factor, 70 mm² copper X-90 drops 1.36%, 95 mm² aluminium drops 1.60% and 120 mm² aluminium drops 1.29%. Select aluminium as the conductor and the calculator reads Table 4.7(B) instead of 4.7(A) and caps the operating temperature at 90 °C.
What is the simplified voltage drop method in AS/NZS 3000 Appendix C?: Table C8 tabulates ampere-metres per 1% voltage drop for twelve copper PVC cable sizes, single-phase at 230 V and three-phase at 400 V, so a run can be checked by dividing current × length by the table figure. It is a quick check for standard cables at unity power factor; it has no aluminium, XLPE or power factor columns, and it was tabulated against an earlier AS/NZS 3008.1.1 edition. This calculator reproduces Table C8 to within 1% up to 35 mm² on the 2025 tables and is the method to use for anything the table does not cover.
What happens if the voltage drop is over 5%?: Equipment may not receive its rated voltage, motors run hotter and draw more current, lighting dims and electronic loads can reset, and the installation does not comply with Clause 3.6.2. The fixes, in the order they are usually cheapest: shorten the route, move allowance from another section of the budget, go up one conductor size on the longest section, or move the distribution board closer to the load. On a three-phase site, rebalancing a single-phase load across phases also helps because the neutral current falls.