Correct cable sizing under BS 7671:2018+A2:2022 ensures conductors carry their continuous load without exceeding thermal operating limits. Failing to account for environmental factors, such as bunched containment, thermal insulation, or high ambient temperatures, can cut a conductor's current-carrying capacity by over 50%, transforming compliant designs into severe fire hazards.
Overload Protection Coordination: The Two Limbs of Regulation 433.1.1
Regulation 433.1.1 coordinates three critical parameters: circuit design current (Ib), nominal rating or setting of the protective device (In), and continuous current-carrying capacity of the cable under its installed conditions (Iz). To achieve complete overload protection, the circuit must satisfy both statutory conditions:
The Two Fundamental Overload Conditions (Regulation 433.1.1):
Condition (i): Ib ≤ In ≤ Iz
Condition (ii): I2 ≤ 1.45 × Iz
Where I2 is the current causing effective operation of the protective device within conventional time:
- BS EN 60898 MCBs & BS 88 Fuses: Standard circuit breakers and cartridge fuses are manufactured to operate at I2 ≤ 1.45 × In. Consequently, once Condition (i) is satisfied, Condition (ii) is automatically met.
- BS 3036 Semi-Enclosed Rewireable Fuses: Rewireable fuses have a coarse operating characteristic where I2 = 2.0 × In. Because 2.0 / 1.45 = 1.38, Condition (ii) is not met automatically. To compensate, a correction factor Cc = 1.45 / 2.0 = 0.725 must be applied to the cable calculation under Appendix 4 §4.
Tabulated Current-Carrying Capacity Formula (Appendix 4)
To determine the minimum tabulated current-carrying capacity (It) required from the Appendix 4 current rating tables, divide the nominal protective device rating by the product of all applicable installation correction factors:
Tabulated Current Capacity Formula:
It ≥ In / (Ca × Cg × Ci × Cc × Cs × Cd)
Installation Reference Methods A to G
The base current-carrying capacity (It) of any conductor depends on its ability to shed I2R heat into the surrounding environment. BS 7671 Appendix 4 classifies installation conditions into standardised Reference Methods:
| Reference Method | Physical Installation Configuration | Heat Dissipation Profile |
|---|---|---|
| Method A | Enclosed in conduit in a thermally insulating wall | Poor: trapped in wall insulation |
| Method B | Enclosed in conduit or trunking on a wooden or masonry wall | Moderate: enclosed air layer |
| Method C | Clipped direct to a non-metallic surface or unperforated tray | Good: one face exposed to free ambient air |
| Method D | Underground in ducting or direct in ground (Ref temp: 20 °C) | Dependent on soil thermal resistivity (Cs) and depth (Cd) |
| Method E | Multicore cable in free air on perforated cable tray or brackets | Excellent: continuous natural convection airflow |
| Method F & G | Single-core cables in free air (F: touching; G: spaced ≥ 1 De) | Maximum: Method G achieves highest air ratings |
| Method 100 | Flat twin & earth on plasterboard ceiling with ≤ 100 mm insulation above (Table 4D5) | Standard domestic ceiling void design |
The Derating Correction Factors
1. Ambient Temperature Factor: Ca (Table 4B1 / 4B2)
Standard current ratings in BS 7671 assume a reference ambient temperature of 30 °C for cables in air and 20 °C for cables buried in the ground. In unventilated plant rooms, boiler houses, or hot loft spaces during summer, ambient air temperatures frequently exceed 40 °C to 45 °C.
- For 70 °C thermoplastic (PVC) cables operating at 45 °C ambient: Ca = 0.79 (Table 4B1).
- For 90 °C thermosetting (XLPE / LSOH) cables operating at 45 °C ambient: Ca = 0.87.
2. Grouping / Bunching Factor: Cg (Table 4C1 to 4C5)
When multiple loaded cables run touching or bunched together on containment, thermal dissipation is mutually restricted. Grouping derating accounts for trapped I2R thermal dissipation, not mutual inductance.
- Four circuits bunched together on a surface or enclosed: Cg = 0.65 (Table 4C1 item 1), representing a 35% capacity penalty.
- Spaced cables on perforated trays still experience minor grouping factors (< 1.0) when multiple circuits share the tray, unless dedicated clearances are maintained (Table 4C4).
3. Thermal Insulation Factor: Ci (Regulation 523.9 & Table 52.2)
Thermal insulation encasing a cable creates a severe barrier to conductive heat loss:
- Total Enclosure for ≥ 500 mm: Under Regulation 523.9, where a cable is totally surrounded by thermal insulation for 0.5 m or more, in the absence of more precise data, the current capacity must be taken as 0.5 × It for Reference Method C (clipped direct). Do not apply Ci = 0.50 to Method A, B, or 100 tabulated ratings, as this would double-derate the conductor.
- Shorter Enclosure Lengths (Table 52.2): For conductors ≤ 10 mm² passing through thermal insulation with λ > 0.04 W/(m·K):
- 50 mm in insulation: Ci = 0.88
- 100 mm in insulation: Ci = 0.78
- 200 mm in insulation: Ci = 0.63
- 400 mm in insulation: Ci = 0.51
- ≥ 500 mm in insulation: Ci = 0.50
4. Semi-Enclosed Rewireable Fuse Factor: Cc (BS 3036)
For circuits protected by semi-enclosed fuses to BS 3036, a factor of Cc = 0.725 must be applied to satisfy Condition (ii) of Regulation 433.1.1. For circuits protected by standard MCBs (BS EN 60898), RCBOs (BS EN 61009), or cartridge fuses (BS 88), Cc = 1.0.
5. Buried Cable Factors: Cs & Cd (Method D)
For underground installations, current-carrying capacity is governed by soil thermal resistivity (Cs, Table 4B3, reference 2.5 K·m/W) and burial depth (Cd). Where third-harmonic currents exceed 15% in three-phase supplies, neutral harmonic factor Cf (Appendix 4 §5.5) must also be included.
Worked Examples: Step-by-Step Derating
Example A: High Ambient Temperature & Grouping (Ca & Cg)
A 32 A circuit is protected by a 32 A Type B MCB (In = 32 A) using 70 °C thermoplastic cable. The route runs through an unventilated loft space at 45 °C (Ca = 0.79) bunched with 3 other loaded circuits (4 circuits total, Cg = 0.65). The cable is not encased in insulation (Ci = 1.0, Cc = 1.0).
Tabulated Requirement:
It ≥ 32 / (0.79 × 0.65 × 1.0 × 1.0) = 32 / 0.5135 = 62.3 A
Result: Even though design current is only 32 A, a conductor with a tabulated rating of at least 62.3 A (typically 16 mm² under Reference Method C) must be selected.
Example B: Thermal Insulation Enclosure (Ci)
A 32 A MCB radial circuit in a 30 °C ambient environment (Ca = 1.0, Cg = 1.0, Cc = 1.0) passes through a roof joist void completely encased in thermal quilt for 600 mm.
Tabulated Requirement (Applied to Method C):
It ≥ 32 / 0.50 = 64.0 A
Result: Encasing a cable in thermal insulation for more than 500 mm doubles the required tabulated conductor capacity from 32 A to 64 A.
Voltage Drop Verification: Regulation 525.1 & Appendix 4 Table 4Ab
Selecting a cable that satisfies thermal capacity (Iz ≥ In) is only half the design. The installation must also be verified for voltage drop.
- Functional Requirement (Regulation 525.1): The voltage drop between the origin of the installation and any socket-outlet or equipment terminal must not impair the safe, continuous operation of the load.
- Conventional Design Limits (Appendix 4 Table 4Ab):
- Public LV Supply: 3% for lighting circuits (6.9 V on 230 V single-phase) and 5% for other uses (11.5 V on 230 V).
- Private LV Supply / Generating Sets: 6% for lighting (13.8 V) and 8% for other uses (18.4 V).
- Long Runs (> 100 m): The permitted voltage drop may be increased by 0.005% per metre beyond 100 m, up to a maximum additional allowance of 0.5%.
If calculated voltage drop exceeds Table 4Ab limits, conductor cross-sectional area must be increased until the millivolt-per-amp-per-metre (mV/A/m) drop complies, regardless of thermal adequacy.