Cable Derating Factors Explained — Complete Guide

Cable current ratings published in standards are determined under specific reference conditions — a set ambient temperature, a single circuit, no thermal insulation contact, and standard soil conditions for buried cables. Real installations rarely match these reference conditions exactly.

Derating factors (also called rating factors or correction factors) adjust the cable's published current rating to account for the actual installation conditions. When conditions are less favourable than the reference (e.g., higher ambient temperature, multiple circuits grouped together), the derating factor is less than 1.0, reducing the cable's effective current rating. When conditions are more favourable (e.g., ambient temperature below the reference), the factor can exceed 1.0, effectively increasing the rating.

The fundamental formula for applying derating factors is:

I_required = I_n / (k₁ × k₂ × k₃ × ...) — (Eq. 1)

Where I_n is the protective device rating and k₁, k₂, k₃ are the individual derating factors. The cable must be selected from the standard’s current rating tables such that its tabulated rating meets or exceeds I_required.

Multiple derating factors are multiplied together. This means their effects compound — two factors of 0.80 each give a combined factor of 0.64, not 0.60. This compounding is significant: individually modest derating can combine to require a substantially larger cable.

The ambient temperature derating factor adjusts the cable’s current rating when the surrounding air (for above-ground installations) or soil (for buried cables) temperature differs from the standard’s reference temperature.

The underlying physics is straightforward: a cable’s current rating is limited by its maximum operating temperature (e.g., 70°C for PVC, 90°C for XLPE). If the ambient temperature is higher, there is less temperature “headroom” available for the cable’s own heat generation, so it must carry less current.

The general formula for the ambient temperature derating factor is:

k₁ = √((T_max − T_a) / (T_max − T_ref)) — (Eq. 2)

Where T_max is the cable’s maximum operating temperature, T_a is the actual ambient temperature, and T_ref is the reference ambient temperature.

Reference temperatures and key table references:

Example values from AS/NZS 3008 Table 22 (above-ground cables, 90°C rated insulation):

When multiple current-carrying cables are installed in proximity — in the same conduit, on the same tray, or bundled together — they heat each other through mutual thermal coupling. Each cable’s effective current rating is reduced because the surrounding cables add to the thermal environment.

The grouping derating factor depends on:

• Number of circuits: More circuits = more mutual heating = lower factor
• Installation arrangement: Touching cables have more thermal coupling than spaced cables
• Enclosure type: Cables in conduit (fully enclosed) derate more than cables on open tray (some ventilation)

Key grouping factor tables:

Typical grouping factors (cables bunched or in conduit, single layer):

Grouping derating has the most dramatic impact of all derating factors in typical installations. A tray with 12 circuits requires cables rated for more than double the actual load current. This is why separating cable routes and using multiple conduit runs can save significantly on cable cost.

For cables buried directly in the ground or in underground ducts, the thermal resistivity of the surrounding soil determines how effectively heat can be dissipated. Soil thermal resistivity is measured in K·m/W (Kelvin-metres per Watt) — higher values mean the soil is a poorer thermal conductor and the cable must be derated.

The reference soil thermal resistivity values differ between standards:

• AS/NZS 3008: 1.2 K·m/W (typical of moist Australian soil)
• BS 7671: 2.5 K·m/W (conservative, typical of dry UK soil)
• IEC 60364: 2.5 K·m/W (same as BS 7671)

The AS/NZS 3008 reference of 1.2 K·m/W is notably lower (better thermal conductivity) than the BS 7671/IEC value of 2.5 K·m/W. This means that in dry, sandy soil (resistivity 3.0+ K·m/W), AS/NZS 3008 requires more severe derating than BS 7671 would for the same soil.

Typical soil thermal resistivity values:

Soil conditions can change seasonally and with drainage patterns. For critical cables, it is advisable to use the worst-case (driest) soil conditions unless the cable is installed in selected backfill with known thermal properties.

The depth at which a cable is buried affects its current rating because deeper burial means a longer thermal path to the surface and therefore less effective heat dissipation. Standard current ratings assume a reference burial depth (typically 0.5 m for AS/NZS 3008).

From AS/NZS 3008 Table 26, typical depth of burial correction factors:

Depth of burial derating is relatively modest compared to temperature and grouping. However, for cables buried at 1.5 m or deeper (e.g., under roads, in deep trenches), the factor becomes significant enough to affect cable selection, especially when combined with other derating factors.

Cables that pass through or are in contact with thermal insulation material (such as building insulation in walls, ceilings, and floors) require derating because the insulation impedes heat dissipation from the cable surface. This is one of the most severe derating scenarios in domestic installations.

The derating factor depends on how much of the cable is surrounded by insulation:

A factor of 0.50 means the cable can carry only half its published current rating. This is why cables in insulated walls often need to be much larger than expected, and why electricians are advised to route cables clear of insulation wherever possible.

Cables exposed to direct sunlight absorb radiant heat, which raises their surface temperature above the ambient air temperature. This is particularly relevant for:

• Cables on external cable trays and ladders
• Rooftop cables (solar PV arrays, HVAC supply cables)
• Overhead cables and aerial bundled conductors

AS/NZS 3008 Clause 3.3.6 addresses solar radiation by recommending that cables exposed to direct sunlight be treated as operating in an ambient temperature approximately 15°C above the measured air temperature. So if the air temperature is 35°C, the effective ambient for derating purposes is 50°C.

BS 7671 does not provide a specific solar radiation derating factor but notes in the IET Guidance Note 1 that direct solar radiation should be considered. The IET recommends an approach similar to AS/NZS 3008 — adding an appropriate temperature margin.

Black-sheathed cables absorb more solar radiation than light-coloured sheaths. Where solar radiation is a significant concern, specifying cables with a light grey or white outer sheath can reduce the thermal impact by several degrees.

When more than one derating condition applies, the factors are multiplied together:

k_total = k₁ × k₂ × k₃ × ... — (Eq. 3)

For example, consider a cable installed in a conduit in a wall at 45°C ambient with 4 circuits grouped together:

• Ambient temperature derating (45°C, 90°C cable): k₁ = 0.95
• Grouping derating (4 circuits, enclosed): k₂ = 0.65

k_total = 0.95 × 0.65 = 0.618

This means the cable can carry only 61.8% of its published current rating. A circuit requiring a 32 A protective device would need a cable rated for at least 32 / 0.618 = 51.8 A, potentially requiring a cable two sizes larger than the “bare” current rating would suggest.

Good installation design can significantly reduce the impact of derating factors, often allowing smaller and cheaper cables:

1. Separate cable routes: Run high-current circuits in their own conduit or on a separate tray section rather than grouping with many other circuits. Going from 6 circuits (factor 0.57) to 3 circuits (factor 0.70) in a conduit saves a cable size or more.
2. Use open tray instead of conduit: Where permitted, perforated cable tray provides better heat dissipation than enclosed conduit, giving higher base current ratings. Spaced cables on tray have even better ratings.
3. Ventilate enclosed spaces: In switchrooms, risers, and ceiling voids, adequate ventilation keeps ambient temperature closer to the outdoor air temperature, reducing thermal derating.
4. Keep cables clear of insulation: Where cables must pass through insulated walls, use a conduit or cable sleeve that creates an air gap around the cable, or route cables on the room side of the insulation.
5. Use higher-temperature cable insulation: XLPE (90°C) cables have more thermal headroom than PVC (70°C) cables, so they are less affected by high ambient temperatures. The derating factor for the same ambient is closer to 1.0 for XLPE than for PVC.
6. Select appropriate soil backfill: For critical buried cables, use selected thermal backfill (cement-bound sand or thermal grout) with a known low thermal resistivity rather than relying on native soil conditions.

The process for calculating derating factors involves several systematic steps:

• Identify which derating factors apply based on the installation method and site conditions
• Look up the correct factor from the appropriate standard table (AS/NZS 3008, BS 7671, IEC 60364, or NEC)
• Multiply all applicable factors together to determine the combined derating
• Calculate the minimum required cable current rating and select the appropriate cable size
• Document each derating factor individually with its source table reference for verification

Good documentation shows which derating factor had the greatest impact on cable selection, making it clear whether the cable size is driven by temperature, grouping, or other installation conditions.