Cable Sizing Walkthrough for AS/NZS 3008.1.1:2025

Cable sizing in Australia is governed by AS/NZS 3008.1.1:2025 (the 4th edition, published December 2025) and AS/NZS 3000:2018 (the Wiring Rules). Every cable must pass three checks. It must safely carry the full load current after derating for ambient temperature, grouping, and depth. The voltage drop from source to load must stay within the AS/NZS 3000 limit of 5 percent. And the earth fault loop impedance must let the protective device disconnect within Table 5.1 times. This guide walks through each check, the 2025 edition changes, and two worked examples. By the end you will know how to pick the smallest standard cable size that passes everything, and where the Cable Sizing Calculator handles the maths for you.

The three checks every cable must pass

AS/NZS 3008.1.1 and AS/NZS 3000 break cable selection into three parallel constraints. The smallest standard size that satisfies all three is the answer.

1. Current carrying capacity (Iz). The cable must carry the design current Ib continuously without exceeding its insulation temperature rating. Iz comes from the AS/NZS 3008.1.1 base rating tables, derated for ambient temperature, grouping, and depth of burial.
2. Voltage drop (Vd). The drop from the point of supply to the most remote point of the installation must not exceed 5 percent of the nominal supply voltage (AS/NZS 3000 Clause 3.6). For 230 V single-phase that is 11.5 V; for 400 V three-phase that is 20 V line-to-line.
3. Earth fault loop impedance (Zs). The total fault loop impedance must be low enough that the upstream protective device disconnects within the AS/NZS 3000 Table 5.1 time. The Table 5.1 limit is 0.4 seconds for socket outlets up to 32 A and 5 seconds for fixed appliances and distribution circuits.

Short runs on small loads are usually limited by Iz. Long runs on heavy loads are usually limited by Vd. Circuits with weak upstream protection (long cable feeds from a small substation) can be limited by Zs. Always run all three checks; the binding constraint is not always obvious from the inputs.

What changed in AS/NZS 3008.1.1:2025 versus the 2017 edition

The 4th edition was published in December 2025 and is now the binding reference for Australian cable sizing. Five practical changes affect day-to-day design.

• Updated base current rating tables. New ratings for modern conductor and insulation combinations, including improved low-smoke zero-halogen sheaths.
• Revised grouping derating. The Kg factors for closely-packed cables now align more closely with IEC 60364-5-52, which generally tightens the derating compared to 2017.
• Expanded installation method coverage. Solar PV DC string circuits and EV charging infrastructure now have their own table sets rather than being approximated from general-purpose installation methods.
• Adjusted ambient temperature references. The standard Australian reference of 40 degrees Celsius is now applied more consistently across in-air and buried tables, so some derating factors shift slightly.
• New harmonisation with AS/NZS 5033. Solar DC cable sizing references the latest 5033 method directly instead of carrying a parallel calculation in 3008.1.1.

Some 2017 cable sizes will calculate slightly differently in 2025; always size new work to the 2025 edition rather than mixing references. The Cable Sizing Calculator uses the 2025 tables throughout.

Step 1: Determine the design current Ib

The design current is the steady-state load the cable must carry. How you calculate it depends on the load type.

• Single-phase load. Ib equals power in watts divided by (line voltage times power factor). A 4 kW kettle on 230 V at near-unity power factor draws about 17 A.
• Three-phase balanced load. Ib equals power divided by (1.732 times line voltage times power factor). A 22 kW EV charger on 400 V at unity power factor draws about 32 A per phase.
• Motor. Use the nameplate full load current multiplied by the service factor (typically 1.0 to 1.15 for general purpose induction motors). For continuous duty add another 25 percent margin per AS/NZS 3000 motor rules.
• Diversified residential load. Use AS/NZS 3000 Appendix C Table C1 to calculate maximum demand from the individual circuits, then convert kVA to amps for the main.

Apply any future-load allowance at this step (it is much cheaper to add 25 percent now than to retrofit a thicker cable in five years). The output of step 1 is the design current Ib that every other check sizes around.

Step 2: Pick cable construction

Cable construction sets the base rating table you will use in step 3 and the per-metre R and X values you will use in step 5. Three choices matter most.

• Conductor material. Copper is the default for small and medium cables. Aluminium is cost-effective for larger feeders (35 mm squared and up) and longer runs, but it needs special terminations to avoid creep failure.
• Insulation. V-90 PVC has a 90 degree Celsius conductor rating and a 75 degree termination rating; suitable for most general work. X-90 XLPE has the same conductor rating but tolerates higher short-time fault temperatures, which matters for fault clearing margin.
• Sheath. TPS twin and earth for residential and light commercial. Multi-core in conduit for switchboards and motor circuits. Mineral-insulated metal-sheathed (MIMS) for fire-rated circuits.

Step 3: Look up the base current rating

AS/NZS 3008.1.1:2025 organises base ratings by installation method. The five common methods are.

• Method A. Single core or multi-core cable in conduit on a thermally insulated wall. The most thermally restrictive common method.
• Method B. Cable on the surface of a wall.
• Method C. Cable clipped direct to a non- insulated wall or supported on a non-perforated tray.
• Method D. Cable buried in the ground, in conduit or directly buried.
• Method E. Cable on a perforated cable tray or in free air with at least one cable diameter of clearance. The most thermally generous common method.

Find the matching table for your conductor and insulation combination, look up the cable size, and read off the base rating. For example, a 6 mm squared copper PVC twin and earth on Method A has a base rating around 36 A (refer to AS/NZS 3008.1.1:2025 Table 4 for exact figures). The Conduit Fill Calculator checks the parallel constraint that the cable group fits the chosen conduit.

Step 4: Apply derating factors

The table base rating assumes a single isolated cable at the reference ambient temperature. Real installations rarely match those assumptions. Three multiplicative derating factors correct for the deviation.

• Ka, ambient temperature factor. Reference is 40 degrees Celsius for in-air installations and 25 degrees Celsius for buried. Hotter conditions reduce capacity. A 50 degree ambient on an in-air cable typically takes Ka to about 0.85, meaning 15 percent capacity loss.
• Kg, grouping factor. When multiple loaded circuits share a tray or conduit they heat each other, so the base rating must drop. Kg comes from AS/NZS 3008.1.1 Table 22 and depends on the number of cables and how they are spaced. Six bunched cables typically gives Kg around 0.55.
• Kd, depth of burial factor. Buried cables deeper than the reference depth lose convective cooling. Cables buried at 1 metre take Kd around 0.95; deeper installations get smaller factors.

Multiply the base rating by all three factors to get Iz. Confirm Iz is greater than Ib. If not, step up to the next standard size and recheck.

Step 5: Voltage drop calculation with R and X tables

Voltage drop in volts is calculated per phase as:

Vd = Ib * L * (R * cos(phi) + X * sin(phi))

where Ib is the design current in amps, L is the route length in metres divided by 1000 (because R and X are in milliohms per metre), R and X are the cable per-metre resistance and reactance from AS/NZS 3008.1.1 Tables 30 to 32, and cos phi is the load power factor. For three-phase, the line-to-line drop is 1.732 times the calculated phase drop.

For cables under 25 mm squared, reactance is small and the equation simplifies to Vd equals Ib times L times R times cos phi. For cables 35 mm squared and larger, X becomes a meaningful share of the impedance and must be included. The Voltage Drop Calculator handles both regimes automatically and uses the AS/NZS 3008.1.1 R and X tables directly.

Confirm Vd stays under 5 percent of nominal (or your project- specific tighter limit). If not, step up to the next cable size and recheck. For long runs voltage drop usually wins, forcing a cable size much larger than current carrying capacity alone would require.

Step 6: Earth fault loop impedance and disconnection times

The earth fault loop impedance Zs is the sum of every impedance in the fault path:

1. Supply system impedance Ze (transformer plus upstream cable)
2. Phase conductor impedance from the supply to the fault point
3. Earth conductor impedance from the fault point back to the MEN link

The prospective earth fault current is the supply phase voltage (230 V) divided by Zs. AS/NZS 3000 Table 5.1 then sets the maximum time the protective device may take to clear that current: 0.4 seconds for socket outlet circuits up to 32 A, 5 seconds for fixed appliances and distribution circuits.

Find the protective device operating curve, look up the time at the calculated fault current, and confirm it falls under the Table 5.1 limit. The Earth Conductor Sizing Calculator handles the earth conductor side; the Cable Sizing Calculator combines all three impedances and runs the disconnection time check automatically.

Worked example: residential 32 A circuit, 50 metre run

A residential cooktop and oven combination is fed from the switchboard 50 metres away on a single-phase 230 V supply. Design current Ib is 32 A, power factor 0.95. The cable will run in conduit on a wall (Method A) and the ambient is 35 degrees Celsius. No grouping; no depth derating.

Try 6 mm squared V-90 copper twin and earth.

• Iz check. Method A base rating around 36 A. Ka at 35 degrees is 1.04 (slightly above reference). Kg and Kd are 1.0. Iz equals 36 times 1.04, about 37 A. Iz of 37 A exceeds Ib of 32 A. Pass.
• Vd check. R for 6 mm squared copper is about 3.7 milliohms per metre at 75 degrees. X is about 0.1 milliohms per metre. Vd equals 32 times 50 divided by 1000 times (3.7 times 0.95 plus 0.1 times 0.31), giving roughly 5.6 V or 2.4 percent of 230 V. Pass.
• Zs check. Assume Ze of 0.35 ohms (typical urban supply). Phase impedance for 50 metres of 6 mm squared copper is about 0.19 ohms; earth conductor (4 mm squared) adds about 0.27 ohms. Zs is roughly 0.81 ohms. Fault current is 230 divided by 0.81, about 284 A. A 32 A curve C MCB clears in well under 0.1 seconds at 284 A, easily inside the 0.4 second limit. Pass.

Result: 6 mm squared copper V-90 twin and earth is the smallest standard size that satisfies all three checks.

Worked example: commercial 100 A three-phase feeder

A commercial subboard is fed from the main switchboard 80 metres away on a 415 V three-phase supply. Design current Ib is 100 A balanced; power factor 0.85. The cable runs on a perforated cable tray (Method E) shared with five other loaded power feeders. Ambient is 40 degrees Celsius (the reference). No depth derating.

Try 35 mm squared X-90 copper four-core.

• Iz check. Method E base rating around 144 A. Ka is 1.0 at the reference. Kg for six bunched cables on a tray is about 0.80. Iz equals 144 times 0.80, about 115 A. Iz of 115 A exceeds Ib of 100 A. Pass.
• Vd check. R for 35 mm squared copper at 90 degrees is around 0.65 milliohms per metre. X is around 0.09 milliohms per metre. Phase Vd equals 100 times 80 divided by 1000 times (0.65 times 0.85 plus 0.09 times 0.53), giving about 4.8 V per phase, or 8.3 V line-to-line. As a percentage of 415 V, that is 2.0 percent. Pass.
• Zs check. Combined phase plus earth impedance for 80 metres of 35 mm squared copper plus a 16 mm squared earth conductor is about 0.07 ohms. With a generous Ze of 0.05 ohms, Zs is around 0.12 ohms. Phase voltage divided by Zs gives about 1900 A fault current. A 125 A curve B MCB clears in under 0.1 seconds at 1900 A. Pass.

Result: 35 mm squared X-90 copper four-core on a 125 A breaker. If the breaker is set higher, recheck the disconnection time margin. If the cable count on the tray grows from six to ten, the grouping factor tightens and 50 mm squared may be required.

Common pitfalls

• Ignoring the binding constraint. Designers who size only on Iz miss long-run voltage drop failures. Always run all three checks.
• Using one-way length when the run is loop length. For single-phase circuits the relevant length for voltage drop is the one-way route length (the formula already includes the return path implicitly through the impedance values), but for three-phase line-to-line it depends on the convention used in the table. Stay consistent with whichever R and X table set you choose.
• Hot ceiling spaces ignored. An in-roof PV string cable in a 60 degree Celsius summer roof void needs Ka around 0.65, not the reference 1.0. Use site-specific ambient for any cable run inside an unventilated cavity.
• Parallel cables not derated. Two parallel cables count as a group of two in step 4, even when they are on the same circuit. Without grouping derating the design is non-compliant.
• Forgetting the earth conductor. AS/NZS 3000 Table 5.2 sets minimum earth conductor sizes. A 35 mm squared phase paired with a 4 mm squared earth conductor will fail the disconnection time check on a high-impedance fault.

Where the calculator fits in

The ElecCalc Cable Sizing Calculator runs steps 1 to 6 in parallel. Enter design current, voltage, power factor, route length, installation method, and ambient conditions, and the calculator returns the smallest standard cable size that satisfies all three checks. It also shows the binding constraint (Iz, Vd, or Zs) so you know which lever to pull if the result is bigger than expected.

For voltage drop on a fixed cable size, the Voltage Drop Calculator back-solves percentage drop. For the earth conductor side of the fault loop, use the Earth Conductor Sizing Calculator. And before you finalise the conduit selection, run the Conduit Fill Calculator to confirm the cable group fits with margin.

The companion Earthing System Design Guide covers the substation and electrode side of the fault loop in detail, which is the other half of getting Zs right.