BS 7671 Appendix 4: Correction Factors Explained

BS 7671 current-carrying capacity tables (4D1A through 4D5A) are calculated under specific reference conditions: 30°C ambient air temperature, single circuit, no thermal insulation contact. When actual installation conditions differ from these references, correction factors (also called derating factors or rating factors) must be applied to adjust the effective cable capacity.

The correction factors are multiplicative — they are combined into a single overall factor that reduces (or occasionally increases) the cable’s usable current rating. The four main correction factors in BS 7671 are:

• C_a — Ambient temperature correction (Tables 4B1, 4B2)
• C_g — Grouping correction (Tables 4C1–4C5)
• C_i — Thermal insulation correction (Regulation 523.7)
• C_c — Protective device correction (Table 4B3, BS 3036 fuses only)

The required tabulated current rating is calculated as:

It = In / (Ca × Cg × Ci × Cc)

where I_n is the nominal rating of the protective device and I_t is the minimum tabulated current rating the cable must have.

Table 4B1 provides correction factors for ambient air temperatures different from the 30°C reference. Table 4B2 provides equivalent factors for ground temperatures different from the 20°C reference (used for buried cables, Methods D1/D2).

Table 4B1 — Air temperature correction factors (selected values):

Key insight: At 50°C ambient, a PVC cable loses 29% of its capacity (C_a = 0.71), while an XLPE cable loses only 18% (C_a = 0.82). This is because XLPE has a larger temperature headroom — the difference between its maximum operating temperature (90°C) and the ambient is greater.

When multiple circuits or cables share the same enclosure, tray, or route, mutual heating reduces each cable’s effective current capacity. Tables 4C1 through 4C5 provide grouping factors for different arrangements:

Table 4C1 — Cables bunched in conduit, trunking, or on a surface (selected values):

Grouping has the largest impact of all correction factors in typical installations. With 6 circuits bunched in a conduit, the cable capacity is reduced by 43% (C_g = 0.57). This often drives cable sizes up significantly.

When a cable is enclosed in or in contact with thermal insulation (such as building insulation in walls, ceilings, or floors), heat dissipation is severely restricted. Regulation 523.7 specifies:

A cable totally surrounded by thermal insulation for a length exceeding 1 metre has its capacity halved (C_i = 0.50). This is a severe derating and is a common issue in modern energy-efficient buildings with extensive insulation.

Where the length in insulation is less than 0.5 m (such as a cable passing through an insulated stud wall), the factor is less severe at 0.89. Engineers should always try to minimise the length of cable enclosed in thermal insulation.

The correction factor C_c applies only when the circuit is protected by a BS 3036 semi-enclosed (rewirable) fuse. These fuses have a higher fusing factor (1.9 × rating) compared to modern BS 88 HRC fuses or MCBs (typically 1.45 × rating), meaning the cable must be able to carry a higher current before the fuse operates.

For BS 3036 fuses: C_c = 0.725.

For all other protective devices (MCBs, RCBOs, BS 88 fuses): C_c = 1.0 (no correction needed).

In modern installations, BS 3036 fuses are rarely used in new work, so C_c is typically 1.0. However, when working on older installations that still have rewirable fuses, this factor must be applied.

When multiple correction factors apply, they are multiplied together to give the overall correction factor. The combined factor can result in a very significant increase in the required cable size:

Example: Cable in conduit on wall, 40°C ambient, 4 grouped circuits, BS 3036 fuse

  Ca = 0.87  (Table 4B1, 40°C, PVC)
  Cg = 0.65  (Table 4C1, 4 circuits bunched)
  Ci = 1.00  (no thermal insulation)
  Cc = 0.725 (BS 3036 fuse)

  Overall factor = 0.87 × 0.65 × 1.00 × 0.725 = 0.410

  For a 32 A circuit: It = 32 / 0.410 = 78.1 A

  Without derating, a 10 mm² cable (57 A in Method B1) would suffice.
  With derating, a 25 mm² cable (101 A in Method B1) is needed.

  The combined derating increased the required cable by 2.5 sizes.

This demonstrates why it is critical to account for all applicable derating factors. Omitting even one factor can result in an undersized cable that may overheat under normal operating conditions.