--- title: "Cable Sizing Design Guide: How to Size a Cable to AS/NZS 3008.1.1:2025, Step by Step" url: "https://scaleset.com.au/design-guide/cable-sizing" description: "Cable sizing step by step to AS/NZS 3008.1.1:2025: the seven checks, correction factors and an 80 A submain worked with the calculator’s own figures." updated: "2026-10-21" standards: - "AS/NZS 3008.1.1:2025 (Selection of cables: current carrying capacity, voltage drop and short-circuit temperature)" - "AS/NZS 3000:2018 (Wiring Rules: Clauses 2.5.3.1, 2.5.4.2, 3.6.2 and 5.7.4, Table 5.1)" author: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))" reviewed_by: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))" publisher: "ScaleSet Pty Ltd (ACN 703 144 736, ABN 38 703 144 736)" site: "ScaleSet" license: "https://scaleset.com.au/terms" usage: "Cite and link. Do not reproduce or train on this content." --- # Cable Sizing Design Guide: How to Size a Cable to AS/NZS 3008.1.1:2025, Step by Step Source: https://scaleset.com.au/design-guide/cable-sizing A step-by-step guide to sizing a low voltage cable for Australian and New Zealand installations, following the method the ScaleSet Cable Size Calculator runs. It sets out what decides a cable size under AS/NZS 3008.1.1:2025 Clause 2.2 (current carrying capacity, voltage drop and short-circuit temperature), the inputs a design needs before it starts, and the seven checks a size must pass: current carrying capacity against the device (AS/NZS 3000 Clause 2.5.3.1, Ib ≤ In ≤ Iz), voltage drop (AS/NZS 3000 Clause 3.6.2), device breaking capacity (AS/NZS 3000 Clause 2.5.4.2), the adiabatic short-circuit check on the active and the earth (AS/NZS 3008.1.1 Equation 5.3), earth fault loop impedance (AS/NZS 3000 Clause 5.7.4, Zs × Ia ≤ Uo) and the earth conductor size (AS/NZS 3000 Table 5.1). It explains the correction factors for ambient temperature, grouping, depth and soil, then works an 80 A three-phase submain on a shared perforated tray through all seven checks with the calculator’s own figures, and shows what to change when a check fails. ## Key facts - AS/NZS 3008.1.1:2025 Clause 2.2: the minimum cable size is the smallest that meets current carrying capacity, voltage drop and the short-circuit temperature limit. - The cable is coordinated with its device, not its load: Ib ≤ In ≤ Iz under AS/NZS 3000 Clause 2.5.3.1. - Correction factors for ambient, grouping, depth and soil multiply, so the rating that counts is Iz = Iz(tab) × CF. - Voltage drop is read at the conductor operating temperature, and the whole installation shares the 5 % of AS/NZS 3000 Clause 3.6.2. - A size passes only when the fault checks pass too: breaking capacity, the adiabatic check on active and earth, and Zs × Ia ≤ Uo. ## Who this page is for Electrical engineers, electrical designers, licensed electricians and apprentices sizing or checking low voltage cables to AS/NZS 3008.1.1 and AS/NZS 3000 in Australia and New Zealand. ## Relevant standards - AS/NZS 3008.1.1:2025 (Selection of cables: current carrying capacity, voltage drop and short-circuit temperature) - AS/NZS 3000:2018 (Wiring Rules: Clauses 2.5.3.1, 2.5.4.2, 3.6.2 and 5.7.4, Table 5.1) ## What this tool does - What decides a cable size under AS/NZS 3008.1.1 Clause 2.2, and which requirement wins on short and long runs. - The seven checks the ScaleSet cable calculator runs, in its order, each with its clause and formula. - Correction factors for ambient temperature, grouping, depth of laying and soil, and how they multiply. - An 80 A submain worked through every check with the calculator’s own figures, pinned to the engine by a test. - What to change when a check fails, and the ScaleSet calculator that works each check on its own. ## How to size a cable to AS/NZS 3008.1.1:2025 1. **Find the design current and the device** — Take Ib from the maximum demand of the circuit and choose a protective device rating In at or above it, with its type, breaking capacity and clearing time. 2. **Size for current carrying capacity** — Pick the AS/NZS 3008.1.1 current table and column for the cable and installation method, multiply the base rating by every correction factor, and take the smallest size where Iz ≥ In ≥ Ib. 3. **Check the voltage drop** — Read the a.c. resistance and reactance at the operating temperature, work Vd = K × Ib × L × Zc ÷ 1000, and keep it within this cable’s share of the 5 % total. Step the size up if it is over. 4. **Check the short circuit** — Confirm the device breaking capacity is at least the prospective fault current, and that each active and earth conductor passes Smin = I × √t ÷ k. 5. **Check the earth fault loop and the earth** — Confirm Zs × Ia ≤ Uo so the device disconnects in time, and that the earth is at least the AS/NZS 3000 Table 5.1 size for the active. ## How to size a cable to AS/NZS 3008.1.1:2025, step by step ### What decides a cable size AS/NZS 3008.1.1:2025 Clause 2.2 sets three requirements for a cable: it carries the design current without overheating (current carrying capacity), it delivers the supply voltage within the allowed drop (voltage drop), and it survives a short circuit without passing its temperature limit. The minimum size is the smallest conductor that meets all three, and the selection procedures for each are in AS/NZS 3008.1.1 Clauses 2.3.1, 2.3.2 and 2.3.3. Which requirement wins depends on the run. Current carrying capacity decides most short runs inside a building, where grouping and thermal insulation take the rating down. Voltage drop decides most long runs. The short-circuit limit rarely decides a low voltage cable on its own, but it can for a small cable close to a large transformer. ### What you need before you start The design current Ib, which for a submain or consumer mains is the maximum demand from an AS/NZS 3000 Appendix C assessment. The protective device: its rating In, type, breaking capacity and clearing time. The cable (conductor, insulation, construction) and its installation method, which together pick the AS/NZS 3008.1.1 current table and column. The conditions along the route: ambient temperature, the circuits it is grouped with, depth and soil. The route length and the share of the 5 % voltage drop this cable may use. And the supply at the board the cable leaves: its prospective short-circuit current and its earth fault loop impedance Ze. ### The seven checks Current carrying capacity: Iz = Iz(tab) × CF × Nset, and Ib ≤ In ≤ Iz (AS/NZS 3000 Clause 2.5.3.1, Equation 2.1); a fuse is held to Iz ≥ In ÷ 0.9 (Equation 2.3). Voltage drop: Vd = K × Ib × L × Zc ÷ 1000, with K = √3 for three-phase and 2 for single-phase and Zc from the AS/NZS 3008.1.1 Section 4 resistance and reactance at operating temperature. The drop from the point of supply stays within 5 % (AS/NZS 3000 Clause 3.6.2). Device breaking capacity: at least the prospective short-circuit current where the device is installed (AS/NZS 3000 Clause 2.5.4.2). Short circuit, active and earth: the adiabatic check of AS/NZS 3008.1.1 Equation 5.3, Smin = I × √t ÷ k with k from AS/NZS 3008.1.1 Table 5.1, applied to one conductor at a time and to the earth at both ends of the run. Earth fault loop impedance: Zs × Ia ≤ Uo (AS/NZS 3000 Clause 5.7.4), where Ia is the current that operates the device in the required time. Earth conductor size: at least the AS/NZS 3000 Table 5.1 size for the active it runs with. ### A worked example: an 80 A submain on a shared tray An 80 A three-phase submain runs 55 m from the main switchboard to a distribution board on a perforated tray, touching three other circuits in one row, at 40 °C. It is protected by a 100 A MCCB with its magnetic trip at 5 × In and a 25 kA breaking capacity; the fault level at the main switchboard is 10 kA and Ze is 0.10 Ω. The cable is four-core and earth X-90 copper, and this run may use 3 % of the drop. AS/NZS 3008.1.1 Table 3.35 gives 0.78 for four circuits on the tray. At 25 mm² the base rating of 119 A falls to 92.8 A, below the 100 A breaker, so 25 mm² fails. At 35 mm² the 147 A base rating corrects to 114.7 A, and 114.7 A ≥ 100 A ≥ 80 A passes. The drop at 35 mm² is 5.13 V, 1.28 % of 400 V. The adiabatic minimum is 14.0 mm² for the active and 3.22 mm² for the earth. Ia is 600 A (5 × 100 A with the IEC 60947-2 tolerance of 1.2), so Zs may be at most 0.383 Ω; the loop comes to 0.264 Ω. With a 10 mm² earth, the AS/NZS 3000 Table 5.1 size for 35 mm², all seven checks pass. These are the ScaleSet cable calculator's own figures for these inputs. ### When a check fails Fix the check that decided the size first. For current carrying capacity, space the circuits, move the cable to its own tray or ladder, or take it out of thermal insulation before choosing a bigger size. For voltage drop, step the size up, shorten the route or move the board closer. For a short-circuit or breaking capacity failure, a faster or energy-limiting device, or one with a higher breaking capacity. For the earth fault loop, a bigger earth, a device that trips at a lower current, or a shorter run. ## Frequently asked questions ### How do you size a cable in Australia? Under AS/NZS 3008.1.1:2025 the cable is the smallest size that meets current carrying capacity, voltage drop and the short-circuit temperature limit, and AS/NZS 3000 adds the fault checks. Start from the design current and the protective device, apply the correction factors for the installation so that Ib ≤ In ≤ Iz, then check the voltage drop, the device breaking capacity, the adiabatic short-circuit check on the active and earth, the earth fault loop impedance and the earth size. The ScaleSet Cable Size Calculator runs all seven. ### Is a cable sized to the load or to the circuit breaker? To the circuit breaker. AS/NZS 3000 Clause 2.5.3.1 requires Ib ≤ In ≤ Iz, so the cable’s corrected rating Iz is compared with the device rating In, not with the load. An 80 A load on a 100 A breaker needs a cable rated at least 100 A after correction factors. ### What correction factors apply to cable current carrying capacity? AS/NZS 3008.1.1:2025 gives factors for grouping (Tables 3.33 to 3.43), ambient air and soil temperature (Tables 3.44 and 3.45), depth of laying (Tables 3.46 and 3.47) and soil thermal resistivity (Table 3.48). They multiply. Thermal insulation is handled by reading the cable from its own installation column rather than by a factor. ### Why does a cable pass current carrying capacity but still fail? Because current is one of seven checks. A long run can fail voltage drop, a cable close to a large transformer can fail the short-circuit check, and a long run on a large breaker can fail the earth fault loop impedance check, Zs × Ia ≤ Uo. The cable calculator names the check that decided the size so the right one is fixed first. ## Related ScaleSet pages - [Cable Size Calculator](https://scaleset.com.au/calculator/cable-selection) - [Correction Factor Calculator](https://scaleset.com.au/calculator/correction-factor) - [Voltage Drop Calculator](https://scaleset.com.au/calculator/voltage-drop) - [Maximum Demand Calculator](https://scaleset.com.au/calculator/max-demand) - [Earthing Cable Size Calculator](https://scaleset.com.au/calculator/earth-cable-size) - [Selectivity Calculator (Breaker Discrimination)](https://scaleset.com.au/calculator/selectivity) - [Cable Tray Sizing Calculator](https://scaleset.com.au/calculator/cable-tray) - [Conduit Sizing Calculator](https://scaleset.com.au/calculator/conduit-sizing) - [Voltage Rise Design Guide](https://scaleset.com.au/design-guide/voltage-rise) - [All Design Guides](https://scaleset.com.au/design-guide) - [Calculation Verification](https://scaleset.com.au/verification) ## About this page Published by ScaleSet Pty Ltd (ACN 703 144 736, ABN 38 703 144 736), Sydney, Australia. Methods built and reviewed by Wisam Tozah (CPEng, NER, MIEAust) for ScaleSet Pty Ltd. How the methods are tested and reviewed: https://scaleset.com.au/verification. Profile: https://scaleset.com.au/creator. ScaleSet is a design aid: your inputs and results are not reviewed or certified, and responsibility for the design stays with the engineer who issues it. Usage: this page may be read, indexed, retrieved and cited, including as grounding for an AI answer, with attribution to ScaleSet and a link to https://scaleset.com.au/design-guide/cable-sizing. Reproducing it, training a model on it, or using it to build a derivative or competing site is not licensed. Terms: https://scaleset.com.au/terms. Machine-readable policy: https://scaleset.com.au/ai-usage-policy.json Site index for assistants: https://scaleset.com.au/llms.txt — full content: https://scaleset.com.au/llms-full.txt