What size wire do I need for a 100 amp circuit in NZ?

For a 100A single-phase circuit using copper cable installed in free air, a minimum of 35 mm² (rated at approximately 119A in free air per AS/NZS 3008.1.1) is typically required. If the cable is run in conduit or enclosed, you may need 50 mm² because the current rating drops with reduced ventilation. The exact size also depends on run length: longer runs increase voltage drop, which may require a larger cable to keep the drop within 5% of the supply voltage (about 11.5V for a 230V circuit). Always have a licensed electrician confirm sizing before installation.

How is voltage drop calculated for a cable run?

For a single-phase AC circuit, voltage drop is calculated as: V_drop = (2 × L × I × R) / 1000, where L is the one-way cable length in metres, I is the current in amps, and R is the conductor resistance in milliohms per metre (mΩ/m). The factor of 2 accounts for the active and neutral conductors. For three-phase circuits, V_drop = (√3 × L × I × R) / 1000. AS/NZS 3000 (the Wiring Rules) recommends keeping voltage drop to no more than 5% from the point of supply to the point of use.

Can I use aluminium cable for a 100 amp circuit?

Yes, aluminium cable can be used for 100A circuits, but it requires a larger cross-section than copper for the same current because aluminium has higher resistivity. An aluminium cable roughly one to two sizes larger than copper is needed to achieve the same current rating and voltage drop. For a 100A circuit where 35 mm² copper would suffice, approximately 50 to 70 mm² aluminium may be needed. Aluminium cables also require approved connectors to prevent corrosion at terminations. Consult your licensed electrician and check AS/NZS 3008.1.1 for the appropriate aluminium rating.

100 Amp Wire Size Calculator NZ

This calculator works out the minimum cable size needed to safely carry a 100 amp load in a New Zealand electrical installation, checking both current-carrying capacity and voltage drop against AS/NZS 3008.1.1 and AS/NZS 3000. Enter your design current, the one-way cable run length, supply voltage (230V single phase or 400V three phase), phase type, conductor material (copper or aluminium), installation method (free air, conduit, direct buried, or enclosed in a wall or ceiling cavity), and the maximum voltage drop you want to allow. It returns the minimum cable cross-section in mm² that satisfies both requirements, the resulting voltage drop in volts and as a percentage of supply voltage, the derated current rating for your installation method, and a comparison table showing how every standard size from 10 mm² to 240 mm² performs against your circuit. A verdict box explains in plain language which cable to use and whether the next size up gives extra headroom. Use it to check a long run or a restrictive installation method before committing to a job, or to compare copper against aluminium. Because ambient temperature, cable bundling, and your lines company's requirements can all change the required size, treat the result as an indicative guide only and have a licensed electrician confirm final cable sizing before installation.

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Updated Current ratings from AS/NZS 3008.1.1:2017. Wiring rules per AS/NZS 3000:2018.

1. Circuit Details

Design current (amps)

Cable run (one-way length) (metres)

Supply voltage

Phase type

2. Cable Options

Conductor material

Installation method

Max allowable voltage drop

Recommended Cable Size

Minimum Cable Size
35 mm2
Copper conductor

Voltage Drop

3.14 V

1.37% of 230V

Cable Current Rating

119 A

Rated capacity (A)

Derating Factor

100%

Installation method

Cable Size Comparison

Size (mm²)	Current Rating (A)	Voltage Drop (V)	Drop (%)	Status

Circuit Calculation

Design current100 A

One-way run length30 m

Supply voltage230 V

Phase typeSingle phase

Max allowable V drop11.5 V (5%)

Minimum cable size35 mm2

Selected Cable Detail

Conductor materialCopper

Installation methodFree air / open tray

Base current rating (free air)119 A

Derating factor100%

Derated current rating119 A

Resistivity (mΩ/m)0.524 mΩ/m

Voltage drop3.14 V (1.37%)

Result: Enter your circuit details above.

How to Size a Cable for a 100 Amp Circuit

Selecting the correct cable size for a 100A circuit involves two independent checks, both of which must be satisfied:

Current-carrying capacity: The cable must be rated to carry the design current continuously without overheating. Ratings are listed in AS/NZS 3008.1.1:2017 and vary by cable size, conductor material (copper or aluminium), and installation method. A cable run in a tightly enclosed space carries less current than the same cable in free air because it cannot dissipate heat as effectively.
Voltage drop: The resistance of the cable causes a voltage drop along its length. AS/NZS 3000 (the Wiring Rules) recommends that the total voltage drop from the point of supply to any socket outlet or fixed appliance does not exceed 5% of the nominal supply voltage (11.5V on a 230V circuit, or 20V on a 400V three-phase circuit). Longer runs or higher currents increase voltage drop and may require a larger cable than the minimum current rating alone would suggest.

You must satisfy both requirements; the larger of the two results governs cable selection.

Voltage Drop Formula

For a single-phase AC circuit:

V_drop = (2 x L x I x R) / 1000

Where L is the one-way cable length in metres, I is the current in amps, R is the conductor resistance in milliohms per metre (mΩ/m), and the factor 2 accounts for both the active and neutral conductors.

For a three-phase circuit:

V_drop = (1.732 x L x I x R) / 1000

The 1.732 factor (square root of 3) accounts for the three-phase geometry; only one conductor length (not two) is used because the return current flows through the other two phases.

Current Ratings for Common Cable Sizes (Copper, 75 degrees C)

Size (mm²)	Free Air (A)	In Conduit (A)	Direct Buried (A)	Resistance (mΩ/m)
16 mm²	73 A	59 A	72 A	1.15
25 mm²	98 A	80 A	95 A	0.727
35 mm²	119 A	96 A	115 A	0.524
50 mm²	142 A	115 A	138 A	0.387
70 mm²	180 A	146 A	174 A	0.268
95 mm²	215 A	175 A	209 A	0.193

Ratings are based on AS/NZS 3008.1.1:2017 for 75 degrees C insulation in an ambient temperature of 40 degrees C in air or 25 degrees C in ground. Aluminium conductors have approximately 60% of the current-carrying capacity of copper for the same cross-section.

Installation Method Derating

When a cable cannot dissipate heat freely, its current rating is reduced by a derating factor:

Free air or open tray: No derating (factor 1.00). The cable has maximum ventilation and radiates heat to open air.
Direct buried: Factor approximately 0.97 to 1.00 depending on soil thermal resistivity. The ground provides reasonable cooling for buried cables.
In conduit above ground: Factor approximately 0.80. Airflow in the conduit is restricted, raising conductor temperature.
Enclosed in wall or ceiling cavity: Factor approximately 0.70. Very limited heat dissipation. This is the most restrictive common installation method.

When multiple cables are run together (bundled or in the same conduit), additional derating applies for the number of current-carrying conductors. This calculator assumes a single circuit with no additional bundling derating.

Worked Example

A 100A single-phase supply, 30 m cable run (one-way), copper cable in free air, 230V, 5% max voltage drop:

Maximum allowable voltage drop: 5% x 230V = 11.5 V
Test 25 mm² copper (rated 98A in free air, derating 1.00): 98A < 100A. FAILS current rating.
Test 35 mm² copper (rated 119A in free air, derating 1.00, derated rating 119A): current OK (119A > 100A).
Resistance of 35 mm² copper: 0.524 mΩ/m.
Voltage drop: (2 x 30 x 100 x 0.524) / 1000 = 3.14 V (1.37%). Within 5% limit.
Result: 35 mm² copper satisfies both current (119A > 100A) and voltage drop (3.14V < 11.5V) criteria.

Important Notes

This calculator provides indicative guidance only. Cable sizing must be confirmed by a licensed electrician (registered with the Electrical Workers Registration Board) before any installation. Other factors that may require a larger cable include: ambient temperature above 40 degrees C, cables grouped together in bundles, fault level withstand requirements, cable insulation type, and local network operator requirements. For sub-mains and main switchboard connections, your lines company may have additional requirements.

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Sources and method: AS/NZS 3008.1.1:2017 Electrical installations - Selection of cables - Cables for alternating voltages up to and including 0.6/1 kV - Typical Australian and New Zealand conditions. AS/NZS 3000:2018 Wiring rules. Electrical Workers Registration Board (EWRB) New Zealand (ewrb.govt.nz).

This calculator is for guidance only and does not replace advice from a licensed electrician. Cable sizing must comply with AS/NZS 3000, AS/NZS 3008.1.1, and any requirements of your local network operator. Only a registered electrician may carry out electrical installation work in New Zealand.