Cable Sizing and Derating Factors: BS 7671 Guide

Proper cable sizing is the foundation of electrical safety and thermal longevity. Failing to account for installation conditions - such as cables bundled together on a tray or buried under loft insulation - can derate a conductor's current-carrying capacity by more than 50%, turning compliant cables into severe fire hazards.

A derating factor (or correction factor) is a multiplier applied to a conductor's base current rating to compensate for installation conditions that restrict heat dissipation. When circuits run through high ambient temperatures, group together with other loaded cables, or become encased in thermal insulation, conductors cannot cool efficiently. Derating ensures the selected cable has sufficient current-carrying capacity to carry its load continuously without exceeding safe thermal limits or causing insulation breakdown.

How Do You Size a Cable for Overload Protection? (Regulation 433.1.1)

Under BS 7671, the fundamental requirement for cable overload protection coordinates three critical design values: design current (Ib), nominal protective device rating (In), and effective cable current-carrying capacity (Iz).

In practice, the overcurrent protective device rating must be large enough to carry the full circuit design load without nuisance tripping, but small enough to protect the cable's effective current-carrying capacity under its actual installed conditions.

To determine the minimum tabulated current-carrying capacity (It) required from the BS 7671 Appendix 4 tables, divide the nominal protective device rating by the product of all applicable correction factors:

Where each variable represents:

• It: Minimum required tabulated current-carrying capacity from BS 7671 Appendix 4 tables.
• In: Nominal rating or current setting of the protective device.
• Ca: Correction factor for ambient temperature (Table 4B1).
• Cg: Correction factor for grouping with other loaded circuits (Table 4C1).
• Ci: Correction factor for conductors in thermal insulation (Regulation 523.9 & Table 52.2).
• Cc: Correction factor for semi-enclosed BS 3036 rewireable fuses.

The Four Correction Factors: Ca, Cg, Ci and Cc

Every installation route presents thermal constraints that reduce how effectively a conductor sheds heat. When multiple factors apply simultaneously, their derating multipliers compound, rapidly escalating the required tabulated cable rating.

How Does Ambient Heat Derate a Cable? (Ca, Table 4B1)

• Standard current-carrying capacity tables assume a reference ambient temperature of 30°C for cables in air (or 20°C for cables in the ground).
• In unventilated lofts or boiler rooms reaching 45°C, 70°C thermoplastic (PVC) cable capacity is derated by a factor of 0.79 (from Table 4B1).
• 90°C thermosetting (XLPE/LSOH) cables offer higher thermal margins and higher temperature thresholds in elevated ambient temperatures.
• When ambient temperature matches standard reference conditions (30°C in air), Ca is 1.0.

How Much Capacity Do Bunched Cables Lose? (Cg, Table 4C1)

• When multiple loaded multicore cables run bunched together or touching on cable trays, mutual inductive and resistive heating restricts heat dissipation to ambient air.
• Bunching 4 loaded cables together reduces capacity by a factor of 0.65 (from Table 4C1), which represents a 35% reduction in allowable current.
• As the number of grouped loaded circuits increases, mutual heating worsens, demanding substantially larger conductors to prevent insulation overheating.
• Best Practice: Maintain at least one cable diameter spacing between high-current sub-mains on containment to avoid grouping derating penalties (Cg = 1.0).

What Does Loft Insulation Do to a Cable? (Ci, Regulation 523.9 and Table 52.2)

• Thermal insulation in ceilings, lofts, and stud walls creates an extreme thermal barrier around conductors.
• A cable completely surrounded by thermal insulation for a length of > 500 mm requires a severe correction factor of 0.50 under Regulation 523.9 & Table 52.2 (cutting allowable capacity in half).
• A cable touching one side of a plasterboard ceiling with thermal insulation above uses Reference Method 100 (Table 4D5).
• Where a cable is clipped directly in free air without thermal barrier contact, Ci is 1.0.

Why Does a Rewireable Fuse Need a Bigger Cable? (Cc, BS 3036)

• If a circuit is protected by a BS 3036 rewireable fuse, a correction factor of 0.725 must be applied due to the coarse operating characteristic and high fusing factor (1.45).
• Because a BS 3036 fuse requires up to 1.45 times its rated current to operate under sustained overload, conductor capacity must be significantly oversized to prevent fire risk during minor overload conditions.
• For circuits protected by standard MCBs or cartridge fuses, Cc is 1.0.

Which Derating Factor Applies to Your Installation?

Use this quick reference summary to identify which BS 7671 reference, condition, and factor symbol apply during cable sizing calculations:

Note: For cables touching one side of a plasterboard ceiling with thermal insulation above, Reference Method 100 (Table 4D5) is used.

Worked Examples: Derating in Practice

The following three worked examples demonstrate how environmental and installation factors compound to alter the required tabulated current-carrying capacity (It).

Example A: Loft Heat and Bunching Together (Ca and Cg)

Consider a 32 A circuit protected by a 32 A Type B MCB, so In = 32 A.

• The cable passes through an unventilated loft at 45 C using 70 C thermoplastic cable, so Ca = 0.79.
• The route is bunched with three other cables, four in total, so Cg = 0.65.
• The cable is not in thermal insulation, so Ci = 1.0.
• The circuit is protected by an MCB (not a rewireable fuse), so Cc = 1.0.

Applying the tabulated capacity formula:

Key takeaway: A 32 A circuit needs a cable tabulated at over 62 A, nearly double, and nothing about the circuit itself changed. Only where the cable runs changed.

Example B: The Same Run on a Rewireable Fuse (Cc)

Consider the exact same installation route as Example A, but installed on an older distribution board protected by a BS 3036 rewireable fuse, so Cc = 0.725.

• In = 32 A (BS 3036 rewireable fuse).
• Loft ambient temperature at 45 C for 70 C thermoplastic cable: Ca = 0.79.
• Bunched with three other cables (four in total): Cg = 0.65.
• BS 3036 rewireable fuse derating factor: Cc = 0.725.
• Not surrounded by thermal insulation: Ci = 1.0.

Applying the tabulated capacity formula:

Key takeaway: The same cable route now needs a tabulated capacity of 86 A. The coarse tripping characteristics of a BS 3036 fuse impose a severe thermal requirement on conductor selection.

Example C: Thermal Insulation on Its Own (Ci)

Consider a 32 A MCB circuit installed where ambient temperature is standard (30 C so Ca = 1.0) and the cable is not grouped with other circuits (Cg = 1.0), but the cable is surrounded by thermal insulation for more than 500 mm, so Ci = 0.50.

• In = 32 A (32 A MCB, so Cc = 1.0).
• Ambient temperature 30 C: Ca = 1.0.
• Single circuit (not grouped): Cg = 1.0.
• Surrounded by thermal insulation for more than 500 mm: Ci = 0.50.

Applying the tabulated capacity formula:

Key takeaway: One factor on its own can be as punishing as two together. Encasing a cable in thermal insulation for more than 500 mm doubles the required tabulated conductor rating from 32 A to 64 A.

How Do You Check Voltage Drop? (Regulation 525)

After satisfying current-carrying capacity (Iz) and selecting a conductor that meets the required tabulated capacity (It), you must confirm that voltage drop between the origin of the installation and the load terminals complies with Regulation 525:

• Lighting circuits: Maximum 3% (6.9 V on a 230 V supply).
• Other uses (power, heating, motors): Maximum 5% (11.5 V on a 230 V supply).

If the calculated voltage drop exceeds these limits due to long circuit lengths, the cable conductor size must be increased to a larger cross-section, even if current-carrying capacity (Iz) already satisfies the overcurrent protection requirements of Regulation 433.1.1