What wire size do I need for 20 amps in NZ?

For a 20 A circuit in NZ (230 V single-phase), a 2.5 mm² copper conductor is the standard minimum for most fixed wiring runs up to around 20 metres. For longer runs or where voltage drop must stay within 3%, you may need 4 mm². Always confirm with a licensed electrician, as installation method, insulation type, grouping, and ambient temperature all affect the permitted current-carrying capacity under AS/NZS 3000.

How is minimum wire size calculated?

Wire size is determined by two independent checks. First, the conductor must have sufficient current-carrying capacity (ampacity) to carry the load continuously without overheating. This depends on the conductor cross-section, insulation rating, installation method, and grouping factor. Second, the voltage drop along the cable run must not exceed the permitted limit (typically 3% for final sub-circuits under AS/NZS 3000). The voltage drop formula for a single-phase AC circuit is: V_drop = (2 x L x rho x I) / A, where L is the one-way cable length in metres, rho is the resistivity of copper (approximately 0.0175 ohm.mm²/m at 20°C), I is the current in amps, and A is the conductor cross-section in mm². The larger of the two minimum sizes governs.

Do I need a licensed electrician to install wiring in NZ?

Yes. In New Zealand, all fixed electrical wiring work must be carried out by, or under the direct supervision of, a registered electrician licensed under the Electricity Act 1992. This includes replacing cable, adding circuits, and connecting to the switchboard. Using the wrong wire size or carrying out unlicensed electrical work is dangerous and may void your home insurance. This calculator is a planning and learning tool, not a substitute for professional advice.

Amp to Wire Size Calculator

This calculator works out the minimum cable size you need for an electrical circuit in New Zealand, based on the two checks that determine safe wire size: how much current the conductor can carry without overheating, and how much voltage it loses over the length of the run. Enter your current load in amps, choose your system voltage (230 V single-phase, 400 V three-phase, or 24 V or 12 V DC for low-voltage and automotive work), select single-phase or three-phase, then enter the one-way cable length in metres, your maximum permitted voltage drop (3% for a standard sub-circuit, 5% for sub-mains, or a tighter or looser limit for other setups), and whether you are using copper or aluminium conductor. The calculator returns the recommended minimum cross-section in mm², the equivalent AWG size, the actual voltage drop percentage for your run, and which factor governed the result, ampacity or voltage drop. It also shows a comparison table of standard cable sizes against your load, plus a breakdown of the voltage drop formula and current capacity check behind the recommendation. Use it to plan a circuit before you call an electrician, or to sanity check a quote. This follows AS/NZS 3000 Wiring Rules and gives indicative guidance only; installation method, cable grouping, ambient temperature and insulation type all affect real-world sizing, so always have fixed wiring installed by a licensed electrician.

Calculate.co.nz is proud to be partnered with realtor.co.nz, a trusted resource for navigating the New Zealand property market. Their Helpful Articles section offers clear, well-structured insights across buying, selling, and building, making complex real estate topics more accessible. With a focus on up-to-date guidance and practical knowledge, they empower Kiwis to move forward with clarity and confidence in a constantly evolving property landscape.

Updated June 2026 Based on AS/NZS 3000:2018 Wiring Rules. Copper conductor resistivity 0.0175 ohm.mm²/m at 20°C.

1. Circuit Details

Current load (amps)

System voltage

Circuit type

2. Cable Run

One-way cable length (metres)

Maximum permitted voltage drop

Conductor material

Recommended Wire Size

Minimum Size
-
mm² conductor

AWG Equivalent

US/international reference

Voltage Drop

At recommended size

Governing Factor

What sized the cable

Standard Cable Sizes for Your Load

Cable Size (mm²)	AWG Approx.	Max Ampacity (A)	Voltage Drop (%)	Status

Voltage Drop Calculation

Formula used-

Current (I)-

One-way length (L)-

Resistivity (rho)-

Conductor size (A)-

Voltage drop (V)-

Voltage drop (%)-

Current Capacity Check

Required current-

Recommended size-

Ampacity at rec. size-

Safety headroom-

Size required for V-drop-

Size required for ampacity-

Governing requirement-

Result: Enter your circuit details above.

How to Size a Wire or Cable

Selecting the correct conductor cross-section for an electrical circuit involves two independent checks, and the larger result governs:

Current-carrying capacity (ampacity): The conductor must be large enough to carry the load without overheating. AS/NZS 3000 Table C1 specifies the maximum continuous current for each cable size, based on insulation type, installation method (in conduit, clipped direct, in free air), and grouping. This calculator uses typical in-conduit values for 75°C insulation as a conservative baseline.
Voltage drop: The resistance of the cable causes a voltage drop along its length, reducing the voltage available at the load. AS/NZS 3000 limits voltage drop to 3% of supply voltage for final sub-circuits (the last circuit from the board to the outlet or appliance) and 5% for sub-mains (the feed from the main switchboard to a distribution board). Low-voltage DC systems often use higher limits (up to 10%) because the starting voltage is already low.

The Voltage Drop Formula

For single-phase AC and DC circuits, the voltage drop formula is:

V_drop = (2 x L x rho x I) / A

Where:

V_drop = voltage drop in volts
L = one-way cable length in metres (current travels both the active and neutral conductors, so the factor of 2 accounts for the return path)
rho = resistivity of the conductor material (copper: 0.0175 ohm.mm²/m; aluminium: 0.028 ohm.mm²/m) at 20°C
I = current in amps
A = conductor cross-sectional area in mm²

For three-phase circuits, the factor of 2 is replaced by the square root of 3 (approximately 1.732), because each phase conductor only carries current one way and returns via the neutral or other phases. Rearranging for minimum area: A = (2 x L x rho x I) / V_drop_max.

Standard Cable Sizes in NZ

mm²	AWG Approx.	Typical Max. Current (in conduit, 75°C)	Typical Use
1.0	18	13 A	Lighting circuits (light-duty)
1.5	16	17 A	Standard lighting circuits
2.5	14	23 A	Standard power (GPO) circuits
4	12	31 A	Heavy power, range circuits
6	10	40 A	Sub-mains, large appliances
10	8	54 A	Sub-mains, EV chargers
16	6	73 A	Main switchboard feeds
25	4	95 A	Main switchboard feeds
35	2	119 A	Service mains
50	1	145 A	Service mains

These ampacity values are conservative estimates for copper conductors in conduit at an ambient temperature of 30°C with 75°C insulation. Cables in free air or clipped direct to a surface can often carry more current. Cables installed in bunches or in insulation must be de-rated. Always confirm with your electrician and refer to AS/NZS 3000 Table C1 for your specific installation conditions.

Worked Example

A 20 A circuit at 230 V single-phase with a 15 m one-way cable run and a 3% maximum voltage drop limit:

Maximum permitted voltage drop: 3% of 230 V = 6.9 V
Minimum cross-section for voltage drop: A = (2 x 15 x 0.0175 x 20) / 6.9 = 10.5 / 6.9 = 1.52 mm²
Next standard size above 1.52 mm² is 2.5 mm²
Ampacity check: 2.5 mm² carries 23 A, which is above the required 20 A. Pass.
Voltage drop at 2.5 mm²: V_drop = (2 x 15 x 0.0175 x 20) / 2.5 = 4.2 V, or 1.83% of 230 V. Within the 3% limit. Pass.
Recommended size: 2.5 mm² (both the voltage-drop minimum of 1.52 mm² and the 20 A ampacity requirement land on 2.5 mm², so both factors govern equally).

This matches the default output of the calculator above.

Related Calculators

Essential Calculators hub
Electrical Power Calculator (P = VI): calculate watts from voltage and current.
Ohm's Law Calculator: find voltage, current, or resistance from any two values.
Amp Hour Battery Life Calculator: estimate how long a battery will last at a given load.
Appliance Running Cost Calculator: calculate the electricity cost of running appliances.

Sources and method: Standards Australia / Standards New Zealand AS/NZS 3000:2018 Wiring Rules (Electrical Installations), Table C1 current-carrying capacities. Copper resistivity at 20°C: 0.0175 ohm.mm²/m (IEC 60228). Aluminium resistivity at 20°C: 0.028 ohm.mm²/m. AWG to mm² conversions per ASTM B258.

This calculator provides indicative guidance only. Actual conductor sizing must account for installation method, grouping, ambient temperature, insulation rating, and other factors specified in AS/NZS 3000. All fixed electrical wiring in New Zealand must be installed by or under the supervision of a registered electrician. This tool is not a substitute for professional electrical advice.