How to Read a Cable Sizing Table (AS/NZS 3008 Table 13 Explained)

I have watched experienced engineers stare at AS/NZS 3008.1.1:2017 Table 13 for minutes, scrolling back and forth between columns, unsure which one applies. The table is dense. It has dozens of columns spanning multiple insulation types, conductor configurations, and installation methods. But once you understand its structure, you can read any value in seconds.

This tutorial breaks down exactly how Table 13 is organised and walks through a specific lookup so you never second-guess your column selection again.

What Table 13 Actually Contains

Table 13 provides the current carrying capacity in amperes for cables installed in air. These are the base ratings before any derating factors are applied.

AS/NZS 3008.1.1, Table 13 — Sustained current rating for cables in air

The table is organised along two axes:

• Rows: Conductor cross-sectional area in mm2 (1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630)
• Columns: A combination of insulation type, number of cores, and installation method

The values in the cells are amperes at the reference ambient temperature of 40 degrees C for cables in air. This reference temperature is specific to AS/NZS 3008 — BS 7671 uses 30 degrees C, which is why you cannot simply borrow values between standards.

Understanding the Column Groups

Table 13 is divided into major column groups by insulation type:

V-75 (PVC, 75 degrees C maximum conductor temperature)

This is standard PVC insulation. Lower maximum temperature means lower current ratings compared to XLPE. Common in building wiring, light commercial, and residential.

V-90 (PVC, 90 degrees C maximum conductor temperature)

Heat-resistant PVC. Higher rating than V-75 but still lower than XLPE. Used where higher temperature tolerance is needed but XLPE cost is not justified.

X-90 (XLPE, 90 degrees C maximum conductor temperature)

Cross-linked polyethylene. The workhorse insulation for industrial and commercial installations. Higher current ratings than PVC for the same conductor size. Standard in mining, heavy industry, and infrastructure.

Within each insulation group, columns are further divided by:

1. Number of loaded cores: 2-core (single-phase) or 3/4-core (three-phase). Single-core cables have their own sub-columns for trefoil and flat formation.
2. Installation method: Referenced by number from Table 3.

Understanding Installation Methods (Table 3)

Before you can select the correct column in Table 13, you must identify your installation method from Table 3. Here are the most common ones:

AS/NZS 3008.1.1, Table 3 — Description of installation methods

The method number tells Table 13 which column to use. Higher-numbered methods generally allow better heat dissipation and therefore higher current ratings. A cable on a perforated tray (Method 13) can carry more current than the same cable in conduit (Method 3), because the tray allows air to circulate around the cable.

Worked Example: Reading the Table

Let us look up a specific value. Our cable is:

• 16 mm2 conductor size
• 4-core (three-phase plus neutral)
• X-90 (XLPE) insulation
• Installed on perforated cable tray (Method 13)

Step 1: Find the correct insulation group

Navigate to the X-90 section of Table 13. Skip past V-75 and V-90 columns.

Step 2: Find the core count sub-group

Within X-90, find the columns for multicore cables (not single-core). Then find the 3 or 4 loaded core columns. For a 4-core cable supplying a three-phase load with a neutral that carries negligible current (balanced load), you use the 3-loaded-conductor columns. If the neutral carries significant current (harmonic-laden loads), you must treat it as a 4-loaded-conductor cable, which has lower ratings.

Step 3: Find the installation method column

Within the 3-loaded-core sub-group, find the column for Method 13 (perforated cable tray).

Step 4: Read the value at the correct row

Go down to the 16 mm2 row. The value you read is approximately 86 A.

This means a 16 mm2 4-core X-90 copper cable on a perforated tray can carry 86 A continuously at 40 degrees C ambient, in isolation (no grouping), before any derating is applied.

Where Engineers Go Wrong

Confusion 1: 2-Core vs 4-Core Columns

The 2-core column gives higher values than the 4-core column for the same cable size. This is because two loaded conductors generate less heat per unit length than four. A junior engineer looking up a three-phase cable who accidentally reads the 2-core column will overestimate the cable's capacity. Always verify you are in the correct core count sub-group.

Confusion 2: V-75 vs V-90 vs X-90

A 10 mm2 cable rated under V-75 might carry 54 A, while the same physical conductor size under X-90 might carry 65 A. The difference is purely the maximum operating temperature of the insulation. Using the wrong insulation column can easily lead to a one-size error in cable selection.

Confusion 3: Trefoil vs Flat for Single-Core Cables

Single-core cables have separate columns for trefoil formation (three cables bundled in a triangle) and flat formation (three cables in a row, either touching or spaced). Flat spaced gives the highest current rating because each cable has maximum exposure to ambient air. Trefoil gives lower ratings but offers better electromagnetic balance and lower induced voltages in cable sheaths.

AS/NZS 3008.1.1, Table 13, Note 3 — Single-core cable formations

Confusion 4: Forgetting the Reference Temperature

Table 13 values are at 40 degrees C ambient. If your installation is at 50 degrees C (common in engine rooms, roof spaces, and tropical outdoor installations), you must apply the temperature derating factor from Table 22 before the table value is usable.

AS/NZS 3008.1.1, Table 22 — Rating factors for ambient air temperature

Comparing Methods: How Much Does It Matter?

To illustrate the impact of installation method, here are the approximate current ratings for the same cable (16 mm2 4C X-90 copper, 3 loaded cores) across different methods:

The difference between conduit and free air is roughly 27%. This is not negligible. An engineer who selects a cable based on the free air rating and then installs it in conduit may have an undersized cable.

The Tables You Need Beyond Table 13

Table 13 gives you the starting point. A complete cable sizing calculation also requires:

AS/NZS 3008.1.1, Table 14 — Sustained current rating for cables buried direct in ground

Practice Exercise

Try this yourself before checking the answer:

Question: What is the current rating for a 25 mm2 2-core V-75 copper cable installed in conduit in air (Method 3)?

Steps:

1. Find the V-75 insulation group in Table 13
2. Find the 2-core (or 2 loaded conductor) sub-group
3. Find the Method 3 column
4. Read the value at the 25 mm2 row

Answer: Approximately 77 A at 40 degrees C ambient, single circuit, no grouping.

If you got that right, you can read any value from Table 13. The structure is identical across all insulation types and installation methods. Find your insulation, find your core count, find your installation method, read the value. Then apply your derating factors from Tables 21 and 22 to get the actual usable current capacity for your specific installation.