What is the maximum voltage drop allowed in Australian electrical installations?

AS/NZS 3000:2018 Clause 3.6 limits voltage drop to 5% of nominal voltage from the point of supply to any point in the installation. That is 11.5 V on a 230 V circuit and 20 V on a 400 V circuit. When supply comes from a dedicated on-site substation, the limit increases to 7%.

What three checks must every cable pass under AS/NZS 3008.1?

Every cable must satisfy current-carrying capacity (Tables 4-21), voltage drop (Tables 40-51), and short-circuit withstand (Tables 52-53) under AS/NZS 3008.1.1:2017. The final cable size is the largest result from all three checks.

How do you calculate voltage drop using AS/NZS 3008 tables?

Voltage drop = current (A) x cable length (m) x mV/A.m value from Tables 40-51, divided by 1000. For single-phase circuits, multiply the three-phase table value by 1.155. The result must stay within the allocated voltage drop budget for that circuit segment.

What derating factors affect cable current-carrying capacity in Australia?

AS/NZS 3008.1.1:2017 Tables 22-29 cover derating for cable grouping, ambient air temperature above 40 degrees C, soil temperature, burial depth, and soil thermal resistivity. Multiply all applicable factors together and apply to the base current rating from Tables 4-21.

What is the difference between PVC and XLPE cable insulation for sizing?

PVC (V-90) operates at 75 degrees C with a short-circuit limit of 160 degrees C. XLPE (X-90) operates at 90 degrees C with a short-circuit limit of 250 degrees C. The higher temperature rating gives XLPE cables greater current-carrying capacity for the same conductor size.

Cable Sizing & Voltage Drop | CCC Engineering Design Memo

Section 1

What You Need to Know

Every cable in a building must carry its load current without overheating and deliver voltage to the equipment without dropping too far. AS/NZS 3008.1 sets the rules for picking the right cable size. AS/NZS 3000 caps the total voltage drop at 5% from the point of supply to any load.

Get it wrong, and you get flickering lights, motors that overheat, and an installation that fails compliance. This memo covers the three checks every cable must pass: current-carrying capacity, voltage drop, and short-circuit rating.

Section 2

The Rules

Three checks, one cable size. Every cable must satisfy current-carrying capacity, voltage drop, and short-circuit withstand. The final size is the largest result from all three (AS/NZS 3008.1.1:2017, Cl 1.1)
Total voltage drop: max 5%. From the point of supply to any point in the installation. That is 11.5 V on a 230 V circuit and 20 V on a 400 V circuit (AS/NZS 3000:2018, Cl 3.6)
Dedicated substation: max 7%. When supply comes from an on-site substation dedicated to the installation (AS/NZS 3000:2018, Cl 3.6)
Current ratings depend on installation conditions. Cable type, insulation, installation method, and ambient temperature all affect the base rating. Australian tables assume 40°C air and 25°C ground (AS/NZS 3008.1.1:2017, Tables 4–21)
Apply derating factors. Grouped cables, high ambient temperatures, burial depth, and soil thermal resistivity all reduce the current rating. Multiply all applicable factors together (AS/NZS 3008.1.1:2017, Tables 22–29)
Voltage drop in mV/A.m. Look up the value from the standard tables. For single-phase, multiply the three-phase value by 1.155 (AS/NZS 3008.1.1:2017, Tables 40–51)
Short-circuit withstand. The insulation must not exceed its fault temperature limit: 160°C for PVC, 250°C for XLPE (AS/NZS 3008.1.1:2017, Cl 5.3, Tables 52–53)

Section 3

What This Means in Practice

Take a 50 kW three-phase load at 400 V with a power factor of 0.85. The design current is about 85 A. A 16 mm² three-core XLPE cable rates at roughly 87 A under standard conditions, so it looks like it fits on current alone. But add a 50 m cable run, and the voltage drop tells a different story. At 2.55 mV/A.m, the drop is 85 A × 50 m × 2.55 / 1000 = 10.8 V, which is 2.7% of 400 V. That leaves room within the 5% limit, but only if the upstream consumer mains and sub-mains have not already used up the budget.

A common voltage drop split is 0.5% for consumer mains, 1.5–2% for sub-mains, and 2.5% for final subcircuits. If the sub-mains run is long, it eats into the budget and forces larger cables downstream. Lighting circuits are even tighter: most designers cap them at 3% total to avoid visible dimming.

Derating catches people out. A cable rated at 87 A in free air at 40°C might only carry 65 A when bundled with three other circuits in a shared conduit in a 50°C ceiling void. You must multiply all applicable derating factors together before comparing against the design current. Skipping this step is the most common cause of non-compliance on site.

Section 4

Key Design Decisions

1

Copper vs Aluminium Conductors

Copper has higher conductivity, smaller cable diameter, and better connections. Aluminium costs less per metre but needs larger cross-sections for the same current rating and requires special termination techniques to prevent oxidation and loosening.

Trade-off: Aluminium saves material cost on large sub-mains and feeders (typically 95 mm² and above), but needs bigger cable trays, larger glands, and compatible lugs. Copper is standard for final subcircuits and most commercial runs under 95 mm².

2

PVC vs XLPE Insulation

XLPE-insulated cables operate at 90°C versus 75°C for PVC. That higher temperature rating gives XLPE a larger current-carrying capacity for the same conductor size, and a higher short-circuit withstand (250°C vs 160°C).

Trade-off: XLPE cables typically cost more per metre than PVC but often allow a smaller conductor size, which saves on cable tray space and installation labour. For heavily loaded circuits or hot environments, XLPE usually wins.

3

Voltage Drop Budget Allocation

You have 5% total to work with (or 7% from a dedicated substation). Decide upfront how to split it across consumer mains, sub-mains, and final subcircuits. Front-loading the budget on final subcircuits means shorter permissible sub-mains runs.

Trade-off: Allocating more voltage drop to sub-mains allows smaller feeder cables but forces larger (more expensive) cables on the final subcircuits, and vice versa. Run the numbers both ways and pick the lowest total cable cost.

4

Economic Cable Sizing for Continuous Loads

For equipment running 24/7 (chillers, pumps, data centre feeds), the energy lost as heat in the cable resistance adds up. Upsizing by one or two cable sizes reduces I²R losses and can pay back within 2–5 years through lower electricity bills.

Trade-off: Higher upfront cable cost versus lower running cost. For short cable runs or intermittent loads, the payback is too slow to justify. For long runs on 24/7 plant, it is almost always worth it.

Section 5

Who Needs to Act

Section 6

References

AS/NZS 3008.1.1:2017, Electrical installations — Selection of cables — Part 1.1: Cables for alternating voltages up to and including 0.6/1 kV — Typical Australian installation conditions
AS/NZS 3000:2018, Electrical installations (known as the Australian/New Zealand Wiring Rules) (including Amendment 2:2021)
National Construction Code 2022, Volume One
IEC 60364-5-52, Low-voltage electrical installations - Part 5-52: Selection and erection of electrical equipment - Wiring systems (international reference)

Electrical Cable Sizing and Voltage Drop

What You Need to Know

The Rules

What This Means in Practice

Key Design Decisions

Copper vs Aluminium Conductors

PVC vs XLPE Insulation

Voltage Drop Budget Allocation

Economic Cable Sizing for Continuous Loads

Who Needs to Act

Need this engineered for your project?

References

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