Cable Derating Factor Methodology — Ambient Temperature, Grouping, and Thermal Resistivity

Cable current-carrying capacity tables in all major standards are based on specific reference conditions: a defined ambient temperature, a single circuit in isolation, and (for buried cables) a defined soil thermal resistivity. When actual installation conditions differ from these reference conditions, the tabulated current rating must be adjusted by applying derating factors (also called correction factors or ampacity adjustment factors).

The general derating equation is:

I_z = I_tab x Ca x Cg x Ci x Cs x Cd     — (Eq. 1)

Where:
  I_z   = effective current-carrying capacity under actual conditions (A)
  I_tab = tabulated current rating at reference conditions (A)
  Ca    = ambient temperature correction factor
  Cg    = grouping (mutual heating) factor
  Ci    = thermal insulation factor
  Cs    = soil thermal resistivity factor (buried cables only)
  Cd    = depth of burial factor (buried cables only)

The fundamental requirement is that the effective current-carrying capacity must equal or exceed the protective device rating:

I_z ≥ I_n     — (Eq. 2)

Or equivalently, the minimum tabulated current rating of the cable must be:

I_tab(min) = I_n / (Ca x Cg x Ci x Cs x Cd)     — (Eq. 3)

This document details each derating factor, its physical basis, and the standard-specific tables used by ECalPro.

Cable current ratings are tabulated at a reference ambient temperature. When the actual ambient temperature differs, the current capacity must be corrected because the allowable temperature rise (and hence the allowable I2R heating) changes.

Reference ambient temperatures by standard:

The correction factor is derived from the maximum conductor temperature and the ambient temperature:

Ca = sqrt[(T_max - T_actual) / (T_max - T_ref)]     — (Eq. 4)

Where:
  T_max    = maximum operating temperature of cable insulation (deg C)
  T_actual = actual ambient temperature (deg C)
  T_ref    = reference ambient temperature (deg C)

For a PVC-insulated cable (T_max = 75 deg C) in air per AS/NZS 3008 Table 22:

Note that AS/NZS 3008 uses a 40 deg C reference for cables in air (reflecting Australian climate conditions), while BS 7671 and IEC use 30 deg C. This means the same cable has a higher tabulated current in BS 7671 than in AS/NZS 3008, but when corrected to the same actual ambient temperature, the effective ratings converge. ECalPro applies the correct standard-specific table automatically based on the user's standard selection.

When multiple current-carrying cables are installed in proximity, the heat generated by each cable contributes to the temperature rise of its neighbours. The grouping factor reduces the current rating to account for this mutual heating effect.

Key standard references:

• AS/NZS 3008.1.1:2017, Table 22 — Correction factors for grouping
• BS 7671:2018+A3, Appendix 4, Table 4C1-4C5 — Rating factors for groups of cables
• IEC 60364-5-52, Table B.52.17-B.52.21 — Reduction factors for groups
• NEC 310.15(C)(1), Table 310.15(C)(1) — Adjustment factors for more than 3 current-carrying conductors

BS 7671 Table 4C1 — Cables bunched in air, on a surface, or enclosed (single layer):

Circuits vs cables: A critical distinction is between the number of circuits and the number of cables. A single-phase circuit comprises 2 current-carrying conductors (line + neutral), and a three-phase circuit comprises 3. The grouping factor is applied based on the number of circuits, not individual conductors, except in the NEC which counts individual current-carrying conductors (NEC 310.15(C)(1)).

Loaded circuits: Per BS 7671 Note 2 to Table 4C1, if some cables in a group are known to carry a current not exceeding 30% of their current-carrying capacity, they may be excluded from the group count. ECalPro allows the user to specify the number of loaded circuits separately from the total number of installed cables, applying this relaxation where the standard permits it.

The multiplicative application of derating factors (Ca x Cg x Ci x Cs) is based on the assumption that each derating condition affects the cable's thermal environment independently. This assumption is rooted in the thermal circuit analogy used in IEC 60287 for cable rating calculations.

In the thermal circuit model, the cable conductor temperature is:

T_conductor = T_ambient + I^2 x R_ac x (T_1 + T_2 + T_3 + T_4)     — (Eq. 5)

Where:
  T_1 = thermal resistance of insulation
  T_2 = thermal resistance of bedding/armour
  T_3 = thermal resistance of outer covering
  T_4 = thermal resistance of surrounding medium (air, soil)

Each derating factor modifies a specific term in this equation:

• Ca (ambient temperature) modifies T_ambient — a higher ambient reduces the allowable temperature rise (T_max - T_ambient), which reduces the permissible I2R losses, hence a lower current rating.
• Cg (grouping) modifies T_4 — adjacent cables add heat sources, effectively increasing the thermal resistance of the surrounding medium for any individual cable.
• Cs (soil resistivity) also modifies T_4 — higher soil resistivity increases the thermal resistance between the cable surface and the ground surface.
• Ci (thermal insulation) adds an additional thermal barrier around the cable, increasing T_4.

Because each factor independently modifies the thermal equilibrium equation, they are multiplied rather than added. However, there are cases where the independence assumption is not perfectly valid:

• Grouping in high ambient temperature: the grouping factors in standards are derived at reference ambient temperature. When both Ca and Cg are significantly less than 1.0, the combined derating may be slightly conservative.
• Deeply buried grouped cables: the soil thermal resistivity and grouping interact because the heat from adjacent cables raises the soil temperature locally, which is not the same as the bulk ambient soil temperature.

In practice, the conservative nature of multiplicative derating is accepted as appropriate for safety-critical cable sizing. Per IEC 60287-1-1 Clause 2.2.1.2, more precise calculations using the full thermal circuit model can be performed for critical installations where the combined derating seems excessively conservative.

For cables buried directly in the ground or in ducts, the soil thermal resistivity significantly affects the current rating. Soil with higher thermal resistivity impedes heat dissipation, requiring a reduction in current to prevent overheating.

Reference soil thermal resistivity by standard:

Note the significant difference: AS/NZS 3008 uses 1.2 K.m/W as the reference (typical for moist Australian soils), while BS 7671 uses 2.5 K.m/W (conservative for UK conditions). This means AS/NZS 3008 tabulated ratings are higher for buried cables, but require more aggressive derating if the actual soil is drier.

Typical soil thermal resistivity values:

Per IEC 60287-3-1 Clause 4.2, if no site-specific data is available, a value of 1.0 K.m/W is acceptable for most temperate climates with normal moisture levels. For arid or semi-arid regions (much of Australia, Middle East, parts of the US Southwest), values of 2.0-3.0 K.m/W should be assumed unless geotechnical testing confirms otherwise.

ECalPro accepts user-specified soil thermal resistivity or defaults to the standard-specific reference value. The correction factor is applied per the relevant standard's table (e.g., BS 7671 Table 4A4, AS/NZS 3008 Table 22 Column 9).

When cables pass through or are in contact with thermal insulation, their ability to dissipate heat is severely restricted. Per BS 7671 Regulation 523.7 and AS/NZS 3008 Clause 3.5.4, a derating factor must be applied based on the extent of contact with the insulation.

BS 7671 Table 52.2 — Derating for cables in thermal insulation:

These factors represent a substantial reduction in current capacity. A cable passing through 400 mm of building insulation loses nearly half its current rating. This has significant implications for cables penetrating insulated walls and ceilings in modern energy-efficient buildings.

Per AS/NZS 3008.1.1:2017 Clause 3.5.4.1, if a cable is totally surrounded by thermal insulation for a length of more than the values in Table 22, the cable must be treated as if it is enclosed in an insulated wall for its entire length. ECalPro applies the appropriate standard-specific factor based on the user's selection of insulation contact condition.

Because each standard uses different reference conditions, the same physical installation can yield different derating factors depending on which standard is applied. ECalPro enables side-by-side comparison by computing the combined derating factor under each standard for identical installation conditions.

Example: 4 circuits grouped in conduit, 45 deg C ambient, copper/XLPE:

Note: NEC grouping factors are generally less aggressive than BS 7671/IEC because NEC counts current-carrying conductors rather than circuits, and the adjustment factors in NEC Table 310.15(C)(1) are based on 4-6 conductors = 0.80, versus BS 7671 which gives 0.65 for 4 circuits.

This cross-standard comparison highlights why a multi-standard tool is essential for international engineering firms. A cable sized to AS/NZS 3008 for an Australian project may not be adequate if the same installation were assessed under BS 7671 due to the different reference ambient temperatures and grouping methodologies.

The ECalPro cable sizing calculator applies derating factors through the following workflow:

1. Standard Selection: User selects AS/NZS 3008, BS 7671, IEC 60364, or NEC. This determines the reference conditions and applicable derating tables.
2. Installation Method: User selects from standard-specific installation methods (e.g., BS 7671 Reference Methods A1, A2, B, C, D, E, F, G; AS/NZS 3008 Table 3 installation methods). This determines the base current-carrying capacity table.
3. Ambient Temperature: User enters the actual ambient temperature. ECalPro looks up Ca from the appropriate table (AS/NZS 3008 Table 22, BS 7671 Table 4B1, etc.) or calculates it using Eq. 4.
4. Grouping: User specifies the number of grouped circuits, arrangement (bunched, single layer, trefoil), and optionally the number of loaded circuits. ECalPro applies Cg from the appropriate table.
5. Soil Conditions: For buried cables, user specifies soil thermal resistivity and burial depth. ECalPro applies Cs and Cd factors.
6. Thermal Insulation: If applicable, user specifies the insulation contact condition and ECalPro applies Ci.
7. Combined Factor: ECalPro computes the combined derating factor (Ca x Cg x Ci x Cs x Cd) and the minimum required tabulated current rating per Eq. 3.
8. Cable Selection: ECalPro selects the smallest standard cable cross-section whose tabulated current rating meets or exceeds the minimum required value.

Each factor is reported individually with its source citation (e.g., "Ca = 0.87, BS 7671 Table 4B1, Column XLPE 90 deg C, Row 45 deg C"), enabling full traceability and peer review of the design.