Ac cable size calculator

Picking the wrong cable size in an electrical installation isn't just an efficiency problem — it's a safety hazard. Undersized cables run hot, cause voltage drop, and in worst-case scenarios create a fire risk. Oversized cables are expensive and often physically difficult to terminate. Getting it right means understanding two separate limits: current-carrying capacity and voltage drop.

In Australia, all fixed electrical installations must comply with AS/NZS 3008.1, which specifies how to select cables based on current capacity and installation method. This calculator applies those principles — working out the minimum conductor size for your load, then checking whether the voltage drop over your cable run stays within the 5% limit set by AS/NZS 3000 (the Wiring Rules).

This is a planning and educational tool. All fixed electrical work in Australia must be carried out or supervised by a licensed electrician.

Two Things That Control Cable Size — and Why Both Matter

Current-carrying capacity — the thermal limit

Every cable has a maximum current it can carry continuously without overheating. This is determined by the conductor cross-section, the insulation type, and critically, the installation method. A 2.5 mm² copper cable clipped in open air can carry around 26 A. That same cable run inside conduit drops to about 22 A because the conduit restricts heat dissipation. Add more cables in the same conduit and you need to apply additional derating factors.

The design current for a single-phase circuit is:

I = P ÷ (V × PF)

For a three-phase circuit:

I = P ÷ (√3 × V × PF)

Where P is power in watts, V is voltage (line-to-neutral for single phase, line-to-line for 3-phase), and PF is power factor.

Worked example — single phase 3.6 kW load at 230V

A 3,600 W single-phase load at 230 V with a power factor of 0.9:

1. Current = 3,600 ÷ (230 × 0.9) = 3,600 ÷ 207 = 17.4 A
2. In conduit, a 2.5 mm² copper cable handles 22 A — so 2.5 mm² satisfies the current requirement

The voltage drop check — where longer runs bite you

Even if your cable is thermally adequate, a long run means the resistance of the cable causes a voltage drop along its length. AS/NZS 3000 limits this to 5% of supply voltage for most final subcircuits. On a 230 V circuit, that's a maximum of 11.5 V drop.

The voltage drop formula for single-phase (using millivolt per ampere per metre values from AS/NZS 3008 Table 30):

VD = (mV/A/m × I × L × 2) ÷ 1000

The factor of 2 accounts for the return conductor — current flows out and back, so the effective cable length is doubled. For three-phase circuits, the factor of 2 is replaced by √3, or more precisely the mV/A/m values from the three-phase column of AS/NZS 3008 are used without the doubling.

Continuing the example — how far can that 2.5mm² cable run?

2.5 mm² copper has an mV/A/m resistance of approximately 14.8 mV/A/m. With 17.4 A over a one-way run of 25 m:

1. VD = (14.8 × 17.4 × 25 × 2) ÷ 1000 = 12,876 ÷ 1000 = 12.88 V
2. As a percentage: 12.88 ÷ 230 × 100 = 5.6%
3. That's over the 5% limit — you'd need to step up to 4 mm² copper

This is exactly the kind of situation the calculator catches automatically. A cable that's thermally fine can still fail the voltage drop test, and the fix is always to go up a size.

Real Australian Scenarios Where This Calculation Matters

Sizing a cable for a reverse-cycle air conditioner

A 7.1 kW reverse-cycle split system running on single phase draws roughly 32 A at full load (7,100 ÷ 230 ÷ 0.95 ≈ 32.5 A). In conduit, that needs at least a 6 mm² copper cable for current capacity. But if the outdoor unit is 30 m from the switchboard, check the voltage drop — you may need 10 mm². If you're sizing the actual air conditioner rather than its wiring, the AC size calculator helps you work out the right kW rating for your room first.

3 phase cable size calculator — workshop subboard

A small workshop running a 15 kW 3-phase motor at 400 V with a 0.85 power factor: I = 15,000 ÷ (1.732 × 400 × 0.85) = 15,000 ÷ 588.5 = 25.5 A. In conduit, a 6 mm² copper cable covers 37 A — so thermally fine. But on a 50 m run, voltage drop needs checking before you lock in that size.

Underground electrical cable sizing in Australia

Underground cables in conduit follow the same current capacity rules as surface-mounted conduit, but with the addition of soil thermal resistivity as a derating factor for longer cable routes. For most suburban Australian soils, a standard installation at 500 mm depth in conduit can be treated the same as above-ground conduit for calculation purposes, though you should confirm with your local network operator for any supply authority cables.

Tips that catch expensive mistakes before they happen

• Always use the design current — not the nameplate rating — when sizing. Motor starting currents can be 5–7 times the running current, so motor circuits use a separate sizing method with a demand factor.
• For long cable runs, always check voltage drop before purchasing cable. Going from 2.5 mm² to 4 mm² on a roll of cable is much cheaper than re-pulling an entire run.
• Multiple cables in the same conduit need derating — the calculator above assumes standard single-circuit derating for conduit. If you're running more than two circuits in the same conduit, apply the grouping factors from AS/NZS 3008 Table 22.
• Always confirm sizing with a licensed electrician before proceeding with any fixed installation. This tool is for planning and learning, not a substitute for professional design.

Check the AC size calculator if you need to work out the cooling capacity needed before you start sizing the electrical circuit for your air conditioner installation.

FAQs

What cable size do I need for a 20 amp circuit in Australia?

For a 20 A single-phase circuit in conduit, a 2.5 mm² copper cable is the minimum current capacity (rated at 22 A in conduit). However, you must also check voltage drop for your specific cable run length — if the run is long, 4 mm² may be needed even though 2.5 mm² satisfies the thermal requirement.

What is the maximum allowable voltage drop in Australia?

AS/NZS 3000 (the Australian Wiring Rules) specifies a maximum voltage drop of 5% from the point of supply to any point of utilisation for most final subcircuits. On a 230 V supply, that's 11.5 V. Some circuits — such as those supplying sensitive equipment — may require a tighter limit. Check the relevant standard or consult a licensed electrician for specific requirements.

How do I calculate voltage drop in an Australian cable?

Use the formula: VD = (mV/A/m × I × L × 2) ÷ 1000 for single-phase circuits, where mV/A/m is the millivolt drop per ampere per metre from AS/NZS 3008, I is the current in amps, and L is the one-way cable run in metres. The result is the voltage drop in volts. Divide by the supply voltage and multiply by 100 to get the percentage.

Is aluminium cable acceptable for residential wiring in Australia?

Aluminium conductors are commonly used for service mains and larger commercial or industrial cables in Australia, but are rarely used for internal domestic wiring. Aluminium requires larger conductor sizes than copper for the same current capacity, and special termination practices to prevent oxidation at connection points. For most residential subcircuits, copper remains the standard.

Does installation method affect what cable size I need?

Yes, significantly. A cable clipped in open air can dissipate heat more easily and can carry more current than the same cable run inside conduit or buried in thermal insulation. AS/NZS 3008 provides current capacity tables for each installation method. Always use the correct table for your actual installation — assuming open-air ratings for a cable inside conduit is a common and potentially dangerous error.

What does AS/NZS 3008 say about copper cable sizing?

AS/NZS 3008.1.1 covers cable selection for Australian and New Zealand low-voltage installations. It provides current-carrying capacity tables by conductor size, installation method, and ambient temperature, along with mV/A/m values for voltage drop calculations. It's the primary reference standard for all licensed electricians sizing cables for fixed electrical work in Australia.

Can I use a cable size calculator for a 3-phase installation?

Yes, but the current calculation changes. Three-phase current = Power ÷ (√3 × Line voltage × Power factor). The voltage drop calculation also differs — the factor of 2 used for single-phase (to account for the return conductor) is replaced by the three-phase mV/A/m values from AS/NZS 3008. The calculator above handles both single-phase and 3-phase correctly.