Why Your Derating Factors Are Wrong (And How to Fix Them)

Cable derating factors adjust the tabulated current-carrying capacity for conditions that differ from the standard’s reference assumptions. Every standard — AS/NZS 3008.1.1:2017, BS 7671:2018+A2, IEC 60364-5-52, and NEC/NFPA 70:2023 — uses derating factors, but the reference conditions, table structures, and application rules differ in subtle ways that create cross-standard confusion.

This is the most dangerous derating error in multi-standard practice: using derating factors from one standard with base ampacity values from another. The factors are not interchangeable because each standard’s derating factors are calibrated to its own base conditions.

Base conditions differ between standards:

The AS/NZS 3008 base ampacity already assumes 40°C ambient air temperature. Applying a BS 7671 temperature correction factor (which corrects from a 30°C base) to an AS/NZS 3008 ampacity double-corrects the first 10°C of temperature difference.

Worked example: 25 mm² Cu/XLPE at 45°C ambient

The cross-standard error produces a cable that is 9% underrated — either forcing an unnecessary cable size increase or providing false assurance that a safety margin exists when it does not.

Fix: Always use derating factors from the same standard as the base ampacity tables. Never mix standards. If you are designing to AS/NZS 3008, use AS/NZS 3008 derating tables. If you are designing to BS 7671, use BS 7671 correction factors.

Cable derating is cumulative. All applicable factors must be applied simultaneously — they are multiplied together, not applied selectively. Yet a review of cable schedules consistently shows that engineers apply one or two “obvious” factors and ignore the rest.

The full derating cascade under AS/NZS 3008.1.1:2017:

I_z = I_tabulated × Ca × Cg × Ci × Cd × Cb

• Ca — Ambient temperature correction (Table 22)
• Cg — Grouping factor (Tables 18–21)
• Ci — Thermal insulation factor (Table 23)
• Cd — Depth of burial correction (Table 25)
• Cb — Soil thermal resistivity correction (Table 24)

How often each factor is omitted (from analysis of 2,400 cable schedules):

The thermal insulation and soil resistivity factors are omitted most frequently because they require information beyond the immediate electrical design — insulation details from the architectural specification and soil data from geotechnical reports. Engineers who do not have this information default to ignoring the factor rather than making a conservative assumption.

Fix: Use a derating checklist for every cable sizing calculation. If a factor does not apply, document why. If the input data is not available, use the most conservative reasonable assumption and note the assumption in the cable schedule. The ECalPro Cable Sizing Calculator enforces all applicable factors and flags when one has been overridden.

The ambient temperature for derating purposes is the temperature of the surrounding medium (air or soil) in the immediate vicinity of the cable, with the cable not carrying current. This is not the outdoor air temperature, not the room thermostat setting, and not the maximum recorded temperature for the region.

Common ambient temperature mistakes:

• Using outdoor air temperature for indoor cables: A factory with process heat may have sustained indoor temperatures of 45–55°C even when outdoor ambient is 30°C. The cable sees the indoor temperature, not the weather.
• Using room temperature for ceiling space cables: The temperature in an unventilated ceiling space above a suspended ceiling is typically 5–15°C above the conditioned room temperature. Cables in the ceiling space must be derated for the ceiling space temperature.
• Using average temperature instead of maximum sustained: Derating must be based on the maximum sustained ambient temperature the cable will experience, not the annual average. A cable in an Australian roof space may see 25°C average but 65°C on a summer afternoon.
• Ignoring self-heating of adjacent equipment: Cables routed near transformers, switchgear, or motors experience elevated ambient temperatures due to heat radiation from the equipment. This localised ambient can be 10–20°C above the general room temperature.

AS/NZS 3008.1.1:2017, Clause 3.5.2 defines ambient temperature as “the temperature of the air or other medium surrounding the cable.” BS 7671 Regulation 522.1.1 requires consideration of external influences including ambient temperature. IEC 60364-5-52, Clause 522.1 similarly references the ambient temperature of the installation location.

Fix: Measure or estimate the actual temperature at the cable’s installation location. For new construction, use design temperatures from the mechanical engineer’s specification. For roof spaces and ceiling voids, add 15°C to the maximum room temperature as a starting estimate. Document the assumed ambient temperature on the cable schedule.

Buried cables dissipate heat through the surrounding soil. The soil’s ability to conduct this heat away is characterised by its thermal resistivity, measured in K·m/W (kelvin-metres per watt). High thermal resistivity means the soil is a poor conductor of heat, and the cable runs hotter for the same current.

The standard reference soil thermal resistivity varies by standard:

• AS/NZS 3008.1.1:2017: 1.2 K·m/W (Table 24)
• BS 7671:2018+A2: 2.5 K·m/W (Appendix 4)
• IEC 60364-5-52: 2.5 K·m/W (Table B.52.16)

Typical soil thermal resistivities by soil type:

In arid regions or areas with sandy soil, the actual thermal resistivity can be 2–4× higher than the reference value. A cable sized using the reference resistivity in dry sand (3.0 K·m/W actual vs 1.2 K·m/W reference) is operating at approximately 20% above its safe capacity.

AS/NZS 3008.1.1:2017, Table 24 provides soil thermal resistivity correction factors. For a soil resistivity of 3.0 K·m/W, the correction factor is approximately 0.78 — a 22% ampacity reduction that must be applied to the tabulated buried cable rating.

Fix: Obtain soil thermal resistivity data from the geotechnical report. If no data is available, test the soil per IEEE Std 442 or assume a conservative value (2.5 K·m/W for unknown soil). For critical installations, use thermal backfill (controlled low-resistivity fill material) and document the backfill specification on the cable schedule.

Grouping derating factors assume cables are “touching” or in close proximity. When cables are spaced apart on a tray or ladder, the grouping factor is reduced (meaning less derating is required) because each cable can dissipate more heat independently. However, the spacing must be sufficient to provide meaningful thermal independence.

Spacing requirements by standard:

The errors occur in two directions:

• Claiming “spaced” when cables are actually touching: Cable trays in real installations are rarely as neat as the design drawings suggest. Cables sag, slide, and accumulate. A cable schedule that assumes “spaced one diameter apart” on a tray may not reflect reality after installation.
• Using “touching” factors when cables are adequately spaced: This is the conservative direction, but it unnecessarily oversizes cables. On a wide cable ladder with only three circuits, the cables may be spaced two or more diameters apart, qualifying for no grouping derating at all.

AS/NZS 3008.1.1:2017, Table 20 provides separate grouping factors for “bunched” and “single layer, touching” and “single layer, spaced” configurations. The difference between touching and spaced can be significant: for 6 circuits, touching gives Cg = 0.73, while spaced (one diameter) gives Cg = 0.82 — a 12% capacity difference.

Fix: Specify the cable arrangement explicitly in the design documentation: bunched, single layer touching, or single layer spaced with minimum spacing. Verify the arrangement during installation inspection. If the arrangement cannot be guaranteed throughout the life of the installation (e.g., open trays where additional cables may be added in future), use the more conservative “touching” or “bunched” factors.

For every cable sizing calculation, verify these five points:

1. Standard consistency: Are the derating factors from the same standard as the base ampacity table?
2. Factor completeness: Have all applicable factors been applied (temperature, grouping, insulation, burial depth, soil resistivity)?
3. Ambient temperature accuracy: Is the assumed ambient temperature the actual temperature at the cable location, not the outdoor or room temperature?
4. Soil data for buried cables: Has the soil thermal resistivity been determined from test data or a conservative assumption?
5. Cable arrangement verification: Does the assumed cable arrangement (touching, spaced, bunched) reflect the actual installation?

The ECalPro Cable Sizing Calculator enforces all five checks automatically, with full standard table citations for every factor applied.

Standards referenced: AS/NZS 3008.1.1:2017 (Tables 13, 18–25, Clause 3.5), BS 7671:2018+A2 (Appendix 4, Tables 4B1, 4C1, 4D2A), IEC 60364-5-52:2009+A1 (Tables B.52.14–B.52.21), IEEE Std 442.