3 phase cable size calculator in mm

Undersizing a three-phase cable is one of those mistakes that looks fine on paper right up until something melts, trips, or — in the worst case — starts a fire. Yet oversize it too much and you're throwing money away on copper you didn't need. Getting that number right, in mm², before you buy or install anything, is exactly what this calculator is built for.

Three-phase power is the backbone of commercial and industrial electrical installations across Australia — air conditioning chillers, compressors, motors, pumps, CNC machines, welding sets. When any of these loads are being wired, the cable cross-section needs to account for load current, cable run length, voltage drop, installation environment, and conductor material. Miss any one of those and your sizing is wrong.

This tool handles all of that in one go. It applies the underlying principles of AS/NZS 3008.1 — the Australian and New Zealand standard for electrical cable sizing — and outputs a recommended cable size in mm² you can cross-check with your supplier or sparky before ordering.

What Goes Into a 3 Phase Cable Size Calculation — And What Most People Miss

The two constraints that always fight each other

Every three-phase cable sizing job has exactly two constraints working against each other: current-carrying capacity (also called ampacity) and voltage drop. Both have to be satisfied simultaneously — and the larger of the two required sizes wins.

Ampacity is the maximum current a cable can carry continuously without overheating its insulation. This depends on the conductor cross-section, the material (copper or aluminium), and the installation environment — a cable buried in dirt sheds heat differently to one clipped to a wall in a 45°C plant room.

Voltage drop is the loss of voltage along the cable due to its resistance. For a 415 V three-phase system, AS/NZS 3008 allows up to 5% total drop from the point of supply to the load — that's about 20.75 V. For sensitive equipment like VFDs or precision motors, you'd typically limit it to 2–3%. Long cable runs are where voltage drop becomes the dominant sizing factor, sometimes pushing you to a cable far larger than the current alone would ever require.

Derating — the factor that quietly doubles your cable size

Here's the part that catches a lot of people out. Ampacity tables in AS/NZS 3008 assume specific reference conditions. The moment you change the installation method or ambient temperature, you need to apply derating factors. A cable bunched with five others in a conduit inside a hot roof space might only carry 60–65% of its rated current. Ignore that, and you've undersized your cable on paper even before it's connected.

The calculator applies both an installation method factor and a temperature derating factor. You choose both — and if you're not sure, err on the conservative side. In practice, a seasoned sparky's rule is: when in doubt, go one size up. The cost difference between a 25 mm² and a 35 mm² copper cable is rarely worth the headache of a nuisance trip or an overheated enclosure.

The 3 phase cable size calculation formula

There are two formulas at play. The first finds the full-load current for a three-phase circuit:

I = P ÷ (√3 × V × PF)

Where I is current in amps, P is load power in watts, V is the line-to-line voltage (415 V in Australia), and PF is the power factor. The √3 (approximately 1.732) is the three-phase multiplier — it's what separates a three-phase calculation from single-phase.

The second formula calculates the minimum cable cross-section in mm² needed to keep voltage drop within the allowable limit:

A = (√3 × ρ × L × I) ÷ ΔV

Where A is the cross-sectional area in mm², ρ (rho) is resistivity of the conductor (0.0175 Ω·mm²/m for copper, 0.0282 Ω·mm²/m for aluminium), L is the one-way cable length in metres, and ΔV is the maximum allowable voltage drop in volts. The calculator then rounds up to the next standard cable size in the AS/NZS series: 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm² and above.

Worked example with real numbers

Say you're wiring a 22 kW three-phase air compressor in a warehouse in Queensland. Supply is 415 V, power factor is 0.85, the cable run is 80 metres through conduit, and the ambient temperature in the plant room hits 40°C. You want to keep voltage drop under 3%.

Step 1 — Full-load current: I = 22,000 ÷ (1.732 × 415 × 0.85) = 22,000 ÷ 610.2 = 36.1 A

Step 2 — Apply 40°C temperature derating (factor 0.87): Required derated capacity = 36.1 ÷ 0.87 = 41.5 A. A 10 mm² copper cable in conduit carries 55 A — so ampacity alone suggests 10 mm².

Step 3 — Voltage drop check: Max allowable drop = 3% × 415 = 12.45 V. A = (1.732 × 0.0175 × 80 × 36.1) ÷ 12.45 = 87.6 ÷ 12.45 = 7.04 mm². Round up to next standard size: 10 mm². Both constraints point to 10 mm² copper. But extend that run to 160 metres and the voltage drop calculation pushes you to 16 mm² — even though the current hasn't changed at all.

How to Enter Your Inputs and Read the Output

Getting your load power and power factor right

Enter your load's electrical input power in kilowatts. For motors, this is not the shaft output — it's the electrical demand. If a motor nameplate says 30 kW mechanical output at 92% efficiency, the electrical input is about 32.6 kW. Use the electrical input figure.

Power factor varies by load type. Resistive heating elements run close to 1.0. Induction motors typically sit between 0.7 and 0.9 under load. If the nameplate doesn't state it, 0.85 is a safe working assumption for most three-phase motor loads in Australian industrial design — but if your load is predominantly capacitive or highly inductive, check the datasheet.

For cable run length, measure the actual physical path of the cable — not the straight-line distance on a floor plan. Cable follows walls, drops through floor penetrations, and runs along cable trays. It's almost always longer than it looks. Add a 5–10% buffer if you're estimating.

Choosing the right installation method

The installation method directly affects the derating factor applied to the cable's rated ampacity. "In conduit / enclosed" is the standard for commercial fit-outs and factory wiring. "Bunched in air" applies when multiple circuits travel together on open cable tray. "Single in free air" is the best-case scenario — maximum cooling, highest ampacity. Underground direct buried is for sub-mains and site distribution cables laid in the ground without conduit protection.

A note on aluminium cables in Australian installations

Aluminium conductors are popular for large sub-main runs — typically 50 mm² and above — because the material cost per metre is substantially lower than copper at those sizes. The trade-off is higher resistivity, so you'll always need a larger cross-section than copper for the same job. Aluminium also requires specific lugs and connectors and should never be terminated directly into standard copper-rated terminals without compatible hardware. For most motor circuits under 35 mm², copper remains the standard choice across Australian industry.

A real scenario — kitchen exhaust motor in a Sydney commercial fit-out

A commercial kitchen in inner Sydney is getting a 7.5 kW three-phase exhaust fan motor installed. The cable runs 35 metres from the main switchboard through the ceiling space in conduit. The ceiling routinely reaches 40°C in summer. The motor datasheet shows a power factor of 0.82. The site electrician wants to hold voltage drop within 3%.

Full-load current works out to about 12.7 A. With the 40°C derating factor of 0.87, you need a cable rated for at least 14.6 A after derating. A 2.5 mm² copper cable in conduit carries 23 A — well above that. The voltage drop cross-section calculation gives roughly 2.0 mm², so the next standard size up is 2.5 mm². Both constraints converge on 2.5 mm² copper. Now extend that same run to 120 metres — say, out to a remote pump station on the same site — and voltage drop pushes you to 6 mm² despite the current being identical. That's the kind of result a quick kW-to-cable chart on someone's phone will completely miss.

What a kW to cable size chart doesn't show you

Quick-reference kW to cable size charts are built on fixed assumptions — usually a 30–50 metre run, a specific power factor, and a specific installation method. The moment your job deviates from those assumptions, the chart can point you to the wrong size. Use a proper calculation whenever the run is long, the load is large, or the installation environment is anything but standard.

FAQS ?

How do I calculate 3 phase cable size in mm?

You run two calculations in parallel. First, find full-load current using I = P ÷ (√3 × V × PF), then divide by the combined derating factor to get the current-adjusted minimum cable size. Second, find the minimum cross-section for voltage drop using A = (√3 × ρ × L × I) ÷ ΔV. Whichever gives the larger required cross-section is the one you size to — then round up to the next standard cable size in the AS/NZS 3008 series.

What size cable do I need for a 3 phase motor?

It depends on the motor's kW rating, power factor, cable run length, and installation environment — there's no single answer that covers all motors. A 5.5 kW motor on a 20-metre run in conduit might need 2.5 mm² copper, while the same motor on a 100-metre run would need 6 mm² purely to manage voltage drop. Always run the numbers for your specific job rather than relying on a generic motor cable size chart.

What is the maximum voltage drop allowed for 3 phase in Australia?

Under AS/NZS 3008 and the Wiring Rules (AS/NZS 3000), the maximum allowable voltage drop from the point of supply to any load point in an installation is 5% of nominal supply voltage. For a 415 V system, that's a maximum of 20.75 V. Many designers use 3% or less for motor loads and sensitive electronic equipment to avoid performance issues and nuisance tripping. For jurisdiction-specific guidance, check your state electrical safety regulator.

How do I convert mm² cable size to amps for 3 phase?

Look up the cable's ampacity from the AS/NZS 3008 table for the relevant installation method, then apply any derating factors for temperature and grouping. As a rough guide for copper cables in conduit at standard temperature: 2.5 mm² ≈ 23 A, 4 mm² ≈ 31 A, 6 mm² ≈ 40 A, 10 mm² ≈ 55 A, 16 mm² ≈ 73 A, 25 mm² ≈ 97 A, 35 mm² ≈ 119 A, 50 mm² ≈ 144 A. Always apply your derating factors on top of these reference figures — they're not the final answer on their own.

Is copper or aluminium cable better for 3 phase installations in Australia?

Copper is the standard choice for motor circuits and general three-phase wiring in Australia, particularly for cross-sections up to about 35 mm². It's easier to terminate, more flexible, and widely stocked by electrical wholesalers. Aluminium becomes cost-competitive at 50 mm² and above — typically for long sub-main runs where the volume of conductor material makes copper prohibitively expensive. Aluminium requires special termination hardware and careful jointing to prevent oxidation-related resistance buildup over time.

How do I calculate 3 phase power cable size from kW?

Start by converting kW to full-load amps: I = (kW × 1000) ÷ (√3 × 415 × PF). Apply derating factors to find the minimum required ampacity, then cross-reference the voltage drop formula to find the minimum cross-section for your cable length. The larger of the two minimum sizes — rounded up to the next standard mm² in the AS/NZS 3008 series — is your cable size. The calculator on this page automates all of that in one step.

Do I need a licensed electrician to size my 3 phase cable in Australia?

You need a licensed electrician or registered electrical contractor to carry out or supervise the actual installation work. That's a legal requirement under each state's electrical safety legislation — there are no exceptions for DIY three-phase work. The sizing calculation itself can be done by anyone as a planning step, but the physical installation, connection, and certificate of compliance must come from a licensed professional. See Safe Work Australia for electrical safety and licensing information.

Why does my 3 phase cable size change when I use different power factors?

Because power factor directly affects how much current your load draws for a given kW. A lower power factor means more current for the same real power — and more current means you need a larger cable. A 15 kW load at PF 0.7 draws about 29.9 A on 415 V three-phase, while the same 15 kW load at PF 0.95 draws only 22 A. That gap is significant when selecting cable cross-section, which is why getting the power factor right matters even at the rough-estimate stage.

Can I use this calculator for a 3 phase motor cable size?

Yes — it's well suited for three-phase motor circuits. Enter the motor's electrical input power (not shaft output), the rated power factor from the nameplate, and the measured cable run length. One thing worth knowing: starting currents for direct-on-line motors can be 5–7 times the full-load current, but AS/NZS 3008 cable sizing is based on full-load running current, not starting current. Switchgear and protection devices need to be selected separately to handle those inrush conditions.

This calculator gives you a solid, formula-based starting point for the vast majority of everyday three-phase cable sizing jobs in Australia. For complex installations — parallel cables, mixed installation methods along the same run, or very large loads above 300 kW — a full engineering calculation by a licensed electrical engineer is the right call. But for the jobs that land on most electricians' desks day to day, this tool will get you to the right number fast, without a spreadsheet in sight.