Cable Sizing Calculator AS3008

What is AS/NZS 3008.1.1?

AS/NZS 3008.1.1 is the Australian and New Zealand standard that governs the selection of cables for alternating current installations up to and including 0.6/1 kV. It covers:

• Current-carrying capacities for different conductor sizes, insulation types, and installation methods.
• Correction factors for ambient temperature above the standard reference.
• Derating for cables grouped together in the same conduit, tray, or enclosure.
• Additional derating when cables pass through thermal insulation.

If you are wiring anything in Australia — from a bedroom power point to a factory sub-main — this standard determines which cable you must use. The standard works alongside AS/NZS 3000 (the Wiring Rules), which sets out broader installation requirements including maximum voltage drop, protection coordination, and earthing.

How does cable sizing actually work?

There are three things that determine the minimum cable size for a circuit:

1. The load current (Ib): This is the maximum continuous current the circuit will carry during normal operation. For a 2400W single-phase heater on 230V, the load current is 2400 ÷ 230 = 10.4A.
2. The installation method: How the cable is physically installed affects how well it can shed heat. A cable clipped directly to a masonry wall in open air can dissipate heat much better than a cable crammed inside a conduit with five other circuits. AS/NZS 3008 breaks installation methods into columns — each column in the current-carrying capacity tables corresponds to a different installation scenario.
3. Derating factors: Even after selecting the right column, you must apply correction factors that reduce the cable's effective rating:
   • Grouping: When multiple circuits share a conduit or tray, they heat each other up. Two circuits grouped together in a conduit typically require a derating factor of 0.80 (the cable loses 20% of its rated capacity). By the time you have 7-9 circuits in one conduit, the factor drops to around 0.50.
   • Ambient temperature: The standard tables assume a reference ambient of 40°C for PVC and 40°C for XLPE in Australian conditions. If your site regularly hits 50°C (common in roof spaces, engine rooms, or North Queensland), you apply a correction that further reduces capacity.
   • Thermal insulation: If a cable passes through wall or ceiling insulation, the insulation traps heat. AS/NZS 3008 applies a flat 0.5 derating factor for cables completely surrounded by thermal insulation.

The formula is straightforward:

You then look up the AS/NZS 3008 table for your cable type and installation method, and pick the smallest cable whose tabulated rating meets or exceeds that value.

Worked example — sizing a cable for a hot water system

Say you are wiring a 4.8 kW single-phase electric hot water system in a residential garage. The cable runs through the ceiling space in PVC conduit alongside one other circuit (a power point run).

This is one size up from what you might pick if you just looked at the raw 20.9A load and thought 2.5 mm² or 4 mm² was enough. The derating factors pushed it up. After sizing, you must still verify voltage drop for the actual route length using the Voltage Drop Calculator.

Common mistakes sparkies make with cable sizing

• Ignoring grouping: Pulling four circuits through the same conduit without applying derating. The cables look fine on paper but overheat in practice because nobody accounted for mutual heating.
• Wrong ambient assumption: Using 40°C as the default when the cable runs through a tin roof space in summer. Parts of Western Australia, Queensland, and inland NSW hit 50°C+ in roof cavities.
• Skipping insulation derating: Running TPS through a section of wall batts without applying the 0.5 factor. This halves the cable's capacity and is one of the most dangerous oversights.
• Confusing cable size with protection size: The circuit breaker rating protects the cable — the cable does not protect the load. Sizing the cable correctly comes first, then you select a breaker whose rating sits between the load current and the cable's derated capacity.

When to use XLPE instead of PVC

XLPE (cross-linked polyethylene) insulated cables have a higher continuous operating temperature (90°C vs 75°C for PVC). This means they can carry more current for the same conductor size. On long runs where voltage drop is marginal, switching from PVC to XLPE can sometimes let you use a smaller cable, saving material cost.

However, XLPE cable is more expensive per metre. The cost saving on conductor size needs to outweigh the price difference on insulation type. For most standard domestic work, V-90 PVC/PVC (TPS) is the default. XLPE becomes more relevant on sub-mains runs, commercial feeders, and solar DC strings where the extra capacity matters.