AS/NZS 3008 Table 22: Temperature Derating Factors

Table 22 of AS/NZS 3008.1.1:2017 provides ambient temperature derating factors for cables installed above ground. These factors adjust the base current-carrying capacity (from Tables 13, 14, or 15) when the actual ambient air temperature differs from the 40°C reference temperature.

Unlike BS 7671 and IEC 60364 (which use 30°C as the reference), AS/NZS 3008 uses 40°C because Australian conditions frequently expose cables to elevated ambient temperatures — particularly in roof spaces, outdoor enclosures, plant rooms, and northern Australian locations where summer temperatures routinely exceed 40°C.

Table 22 covers temperatures from 25°C to 80°C and provides factors for three conductor operating temperature ratings:

• 75°C — V-75 (standard PVC/thermoplastic)
• 90°C — V-90 (heat-resistant PVC) and X-90 (XLPE)
• 110°C — Silicone and other high-temperature insulations

For buried cables, use Table 23 instead, which provides factors based on ground (soil) temperature with a 25°C reference.

The following table reproduces the key values from AS/NZS 3008 Table 22:

Because the AS/NZS 3008 reference is 40°C (higher than BS 7671’s 30°C), cables installed in environments below 40°C benefit from an uprating factor greater than 1.0. This is a common situation in:

• Temperate southern Australian cities (Melbourne, Hobart, Adelaide) where typical ambient is 25–35°C
• Air-conditioned plant rooms maintained at 25°C year-round
• Underground car parks and basements where ambient rarely exceeds 30°C
• New Zealand installations where ambient seldom exceeds 30°C

For example, at 25°C ambient with 90°C insulation, the factor is 1.14 — meaning a cable rated at 100 A in Table 13 can actually carry 114 A. This can sometimes allow a smaller cable size, reducing cost and installation effort.

However, uprating should only be applied when the lower ambient temperature can be guaranteed for the cable’s entire service life. A cable installed in an air-conditioned room today may be in a non-conditioned space after a building refurbishment 15 years later.

In hot Australian environments, temperature derating can have a dramatic effect on cable sizing:

Scenario: Cable in outdoor enclosure, inland Queensland
  Maximum ambient temperature: 55°C
  Cable type: V-90 copper multicore
  Design current: 80 A

  Table 22 factor at 55°C for 90°C insulation: 0.84

  Required tabulated rating: 80 / 0.84 = 95.2 A

  Without temperature derating, a 25 mm² cable (87 A, Col 6) suffices.
  With derating, 35 mm² (107 A, Col 6) is needed.

  Impact: One cable size larger = 40% more copper by weight.

The situation is even more severe for V-75 insulation: at 55°C, the factor drops to 0.76, requiring a tabulated rating of 80 / 0.76 = 105.3 A — pushing to the same 35 mm² cable but with less safety margin.

Environments that commonly exceed 40°C in Australian practice include:

• Roof spaces: Can reach 60–70°C in summer
• Outdoor switchboards: 45–55°C with sun exposure
• Engine rooms and plant rooms: 40–50°C from equipment heat
• North-facing walls: Radiant heating can add 10–15°C to ambient

The temperature correction factor is derived from the thermal equation for cable current-carrying capacity. The factor is calculated as:

kt = √((Tmax − Ta) / (Tmax − Tref))

where:

• T_max = maximum conductor operating temperature (75°C for V-75, 90°C for V-90/X-90)
• T_a = actual ambient temperature
• T_ref = reference ambient temperature (40°C)

For example, V-90 cable at 50°C ambient:

kt = √((90 − 50) / (90 − 40))
    = √(40 / 50)
    = √0.80
    = 0.894 ≈ 0.89

This formula also explains why cables with higher insulation ratings are less affected by temperature increases — the larger the (T_max − T_ref) denominator, the less impact each degree of ambient increase has on the square root ratio.

AS/NZS 3008 provides two separate temperature derating tables:

Ground temperatures are more stable than air temperatures and are generally lower in summer. In most Australian locations, the ground temperature at 500 mm depth (standard burial depth) is 20–30°C even when the air temperature exceeds 40°C. This is why Table 23 uses a lower reference (25°C) and covers a narrower temperature range.

Never use Table 22 for buried cables or Table 23 for above-ground installations. The different reference temperatures and the different thermal environments (convective air vs conductive soil) make the factors non-interchangeable.