The complete guide to cable sizing correction factors under BS 7671. Ambient temperature (Ca from Table 4B1), grouping (Cg from Tables 4C1-4C6), thermal insulation (Ci from Regulation 523.9), and semi-enclosed fuse factor (Cf = 0.725). Worked examples, common mistakes, and how to apply the formula It = In / (Ca x Cg x Ci x Cf).
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
The short answer
Correction factors reduce a cable’s tabulated current-carrying capacity to allow for the real installation conditions. The four BS 7671 factors are: Ca for ambient temperature (Table 4B1, 1.0 at 30°C), Cg for grouping with other cables (Tables 4C1–4C6), Ci for thermal insulation (Reg 523.9 — 0.5 if totally surrounded over 0.5m), and Cf for a BS 3036 semi-enclosed fuse (0.725). Apply them so the required tabulated rating It ≥ In ÷ (Ca × Cg × Ci × Cf) — the more onerous the conditions, the larger the cable you need.
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01 · BS 7671 Cable Sizing
Correction factors are multipliers used in the cable sizing process to account for real-world installation conditions that reduce a cable's ability to carry current safely. The current-carrying capacity values in BS 7671 Appendix 4 are based on a set of reference conditions — a single circuit, installed in an ambient temperature of 30 degrees Celsius, with no thermal insulation, and protected by an MCB or HRC fuse. When the actual conditions differ from these references, correction factors must be applied to derate the cable accordingly.
There are four correction factors in the BS 7671 cable sizing methodology: Ca for ambient temperature, Cg for grouping (multiple circuits installed together), Ci for thermal insulation, and Cf for semi-enclosed fuses. Each factor is a decimal value less than or equal to 1.0. Multiplying them together gives the overall derating, and dividing the protective device rating by this product gives the minimum tabulated current rating the cable must have.
The fundamental formula is:
It = In ÷ (Ca × Cg × Ci × Cf)
It = minimum tabulated current rating | In = rated current of protective device | Ca, Cg, Ci, Cf = correction factors
You then select a cable from the appropriate Appendix 4 table where the tabulated current-carrying capacity Iz is equal to or greater than It. If any factor is missed or applied incorrectly, the cable will be undersized for the actual conditions, leading to overheating, insulation degradation, and a potential fire risk.
Work your own factors out below: pick the installation method, ambient temperature, grouping, insulation and protective device, and it applies Tables 4B1, 4C1 and 4C2 and the Cf and buried-cable factors, then shows the derated rating. Free, no sign-up.
Calculate cable current carrying capacity with BS 7671 derating factors
From BS 7671 current capacity tables
Standard is 2.5 K·m/W
Empty: Design Current (Ib), Base Current Rating.
Iz = It × Ca × Cg × Ci × Cs × Cf × Cc — the tabulated current from Appendix 4, reduced by every rating factor that applies. Size the cable so Iz is not less than the rating of the protective device (In).
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02 · BS 7671 Cable Sizing
The ambient temperature correction factor Ca accounts for the fact that cable current-carrying capacity tables in Appendix 4 are based on an ambient temperature of 30 degrees Celsius. When cables are installed in environments hotter than 30 degrees Celsius, their ability to dissipate heat is reduced, and the cable must be derated. Ca values are found in Table 4B1 of BS 7671 Appendix 4.
25°C
1.03
30°C
1.00
35°C
0.94
40°C
0.87
45°C
0.79
50°C
0.71
55°C
0.61
60°C
0.50
Thermosetting cables (XLPE, LSF) have different Ca values because they have a higher maximum conductor temperature of 90 degrees Celsius compared to 70 degrees Celsius for PVC. This means thermosetting cables are less affected by elevated ambient temperatures — the same Table 4B1 gives a separate column for 90 °C thermosetting insulation.
25°C
1.02
30°C
1.00
35°C
0.96
40°C
0.91
45°C
0.87
50°C
0.82
55°C
0.76
60°C
0.71
At 40 °C, 90 °C thermosetting cable derates to 0.91 versus 0.87 for 70 °C PVC — one reason thermosetting is favoured for hot routes.
Common situations where Ca must be applied include loft spaces (35 to 50 degrees Celsius in summer), plant rooms, airing cupboards, near hot water cylinders, commercial kitchens, server rooms, and South-facing roof voids. If the ambient temperature is 30 degrees Celsius or below, Ca = 1.0 and has no effect on the calculation.
Cables routed directly beneath photovoltaic modules are subjected to heating from the underside of the module. Reg 712.523.101 requires that for the design and sizing of such cables, the ambient temperature shall be considered to be at least 70 °C. At 70 °C the Ca factor for a 70 °C PVC cable drops to 0.50 — the same magnitude as total thermal insulation enclosure. XLPE or LSOH cables with a 90 °C conductor operating temperature are standard for under-module strings precisely because their Ca value at 70 °C is significantly less severe.
03 · BS 7671 Cable Sizing
The grouping correction factor Cg accounts for the mutual heating effect when multiple circuits are installed together. Each current-carrying cable generates heat, and when cables are bunched or touching, they share that heat, raising the temperature of every cable in the group. This reduces the safe current each individual cable can carry.
Cg values are found in Tables 4C1 to 4C6 of BS 7671 Appendix 4. The correct table depends on the installation arrangement. Table 4C1 is the one you reach for most often — it covers a single circuit or a group bunched in air, on a surface, embedded or enclosed (including cables in the same conduit or trunking), and also single-layer arrangements on a wall, floor or cable tray. Tables 4C2 to 4C6 cover specialised cases such as cables buried directly in the ground, single cables in buried ducts, groups of multicore cables, groups of single-core cables, and cables in in-floor concrete troughs.
1 circuit
1.00
2 circuits
0.80
3 circuits
0.70
4 circuits
0.65
5 circuits
0.60
6 circuits
0.57
7 circuits
0.54
9+ circuits
0.50
An important exception comes from Regulation 523.5: where a cable in a group is expected to carry not more than 30% of its grouped current-carrying capacity, it may be ignored when obtaining the rating factor for the rest of the group. This allows diversity to be taken into account at the cable sizing stage, reducing the impact of grouping in installations where not all circuits are fully loaded at the same time. The 30% allowance must not then be applied a second time to any adjacent cable grouping calculation.
Grouping is the correction factor most frequently encountered in practice because cables commonly share routes — leaving a consumer unit through a common hole, running through shared voids, or installed together in conduit or trunking. Every electrician must count the number of circuits sharing a route and apply the correct Cg factor for the worst-case section of the cable run.
04 · BS 7671 Cable Sizing
The thermal insulation correction factor Ci is the most punishing of all correction factors. Thermal insulation prevents a cable from dissipating the heat generated by current flow, causing the conductor temperature to rise above its safe operating limit. BS 7671 Regulation 523.9 sets out the requirements. Note that Reg 523.7 covers parallel conductors and equal load sharing — some older guides incorrectly cite it for thermal insulation. The correct reference is Reg 523.9 alone.
Use the reference method, not Ci — Cables clipped to joists or studwork with insulation laid between the joists or packed into the stud wall are covered by the reference methods for that situation (Regulation 523.9): Method A, or Methods 100 to 103 in Table 4D5 for twin and earth. Pick the method that matches the installation and read the rating directly.
Ci = 0.50 — If the cable is completely enclosed in thermal insulation for a continuous length of more than 0.5 metres, the current-carrying capacity shall be taken as 0.5 times the capacity for that cable clipped direct to a surface and open (Reference Method C). This is an extremely severe derating that typically forces a significant increase in cable size. It applies when cables are run through insulation in lofts, walls, or floors where the insulation completely surrounds the cable.
Important: If the cable is installed in conduit or trunking (Reference Method A or B), the base Iz is already lower than Method C. You must still work from the Method C figure when applying the 0.5 factor per Reg 523.9 — using the conduit rating as the base would result in systematic under-sizing.
Appendix 4 Section 2.6 of BS 7671 provides Ci derating factors for cables totally surrounded by thermal insulation for short lengths (up to 10 mm² conductors, insulation thermal conductivity above 0.04 W m⁻¹ K⁻¹). Typical values: 50 mm in insulation Ci = 0.88; 100 mm Ci = 0.78; 200 mm Ci = 0.63; 400 mm Ci = 0.51; 500 mm or more Ci = 0.50.
The derating for cables in insulation for less than 0.5 m depends on three factors, not length alone: the size of the conductor, the length of cable within the insulation, and the thermal conductivity of the insulation material (OSG Reg 2.6). Different insulation products — mineral wool, rigid foam board, and blown fibre — have different thermal conductivities, so the published derating factors assume insulation with a thermal conductivity above 0.04 W m⁻¹ K⁻¹. For conductors above 10 mm² or insulation with lower conductivity, more precise information is required.
50 mm
0.88
100 mm
0.78
200 mm
0.63
400 mm
0.51
≥500 mm
0.50
These factors apply to conductors up to 10 mm² in insulation with a thermal conductivity (λ) greater than 0.04 W m⁻¹ K⁻¹. Above 0.5 m of full enclosure, Reg 523.9 fixes the worst case at 0.5 × the Reference Method C rating, in the absence of more precise information.
The Ci factor is the one most commonly forgotten by electricians, particularly in domestic loft installations where insulation depths have increased from 100mm to 270mm or more in recent years. A cable that was adequately sized when the loft had 100mm of insulation may now be undersized if additional insulation has been laid over it, totally enclosing the cable.
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05 · BS 7671 Cable Sizing
The semi-enclosed fuse correction factor Cf = 0.725 applies only when the circuit is protected by a BS 3036 semi-enclosed (rewirable) fuse. This factor compensates for the poor fusing characteristics of BS 3036 fuses, which have a fusing factor of approximately 2.0 — meaning they may not blow until the current reaches twice their rated value.
By comparison, an MCB to BS EN 60898 has a much tighter operating characteristic — BS 7671 treats the conventional operating current (I2) of an MCB, RCBO or cartridge fuse as not exceeding 1.45 times the device rating (Reg 433.1.1). HRC fuses to the BS 88 series are similarly precise. Because a BS 3036 fuse can allow significantly higher currents to flow for longer before it blows, the cable must be rated to handle those currents without overheating — hence the 0.725 derating factor (which is 1.45 / 2, restoring the same degree of protection afforded by other devices).
BS 3036 semi-enclosed (rewirable) fuse
0.725
MCB — BS EN 60898
1.00
RCBO — BS EN 61009-1
1.00
HRC fuse — BS 88 series
1.00
Cartridge fuse — BS 88-3 / BS 1362
1.00
MCCB — BS EN 60947-2
1.00
Only the BS 3036 rewirable fuse attracts a Cf below 1.0. For every other device on this list Cf = 1.0 and drops out of the calculation.
Cf = 0.725 applies only when the protective device is a BS 3036 semi-enclosed fuse. For MCBs (BS EN 60898), RCBOs (BS EN 61009-1), HRC fuses (BS 88 series), and cartridge fuses (BS 88-3), Cf = 1.0 and has no effect on the calculation. BS 3036 fuses are rarely installed in new work but are commonly encountered during periodic inspection of older installations, particularly those with rewirable fuse boards.
If you are assessing an existing installation with BS 3036 fuses during an EICR, you must apply Cf = 0.725 when verifying the existing cable sizes. If the cable was originally sized without this factor (as was common in older installations designed before this requirement was introduced), the cable may be undersized for the fuse protecting it. This is a common finding on older installations and the appropriate EICR observation code depends on the magnitude of the shortfall and the actual loading of the circuit.
The BS 7671 Schedule of Inspections requires the inspector to confirm the “adequacy of cables for current-carrying capacity with regard for the type and nature of installation” — verified against Section 523 (item 8.10 on the model schedule of inspections). That assessment implicitly covers the applicable correction factors: ambient temperature, grouping, and thermal insulation. An existing circuit whose conductor cross-sectional area does not stand up once the relevant correction factors are applied is a verifiable shortfall against Section 523, not merely a legacy sizing decision. Record it with an appropriate EICR observation code and cross-reference the correction factor calculation.
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06 · BS 7671 Cable Sizing
In practice, multiple correction factors often apply simultaneously. A cable leaving a consumer unit may be grouped with other circuits (Cg), pass through a loft space at elevated temperature (Ca), and be in contact with thermal insulation (Ci). All applicable factors must be multiplied together, and the protective device rating divided by this product to determine the minimum tabulated current rating.
It = In ÷ (Ca × Cg × Ci × Cf)
The combined effect of multiple correction factors can be dramatic. Consider a circuit protected by a 32A MCB with three circuits grouped (Cg = 0.70), ambient temperature of 35 degrees Celsius (Ca = 0.94), and the cable passing through 50mm of insulation (Ci = 0.88):
It = 32 ÷ (0.94 × 0.70 × 0.88 × 1.0)
It = 32 ÷ 0.579 = 55.3A
Without any correction factors, a 32A circuit would need a cable rated for just 32A. With all three factors applied, the cable must be rated for 55.3A — a massive increase that could mean the difference between 4mm² and 10mm² cable, with significant cost and installation implications.
This is precisely why cable sizing must be done correctly at the design stage. An electrician who skips the correction factor calculation and simply selects a cable based on the MCB rating alone is likely to install an undersized cable that will overheat under full load conditions. Over time, this overheating degrades the cable insulation and increases the risk of fire.
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07 · BS 7671 Cable Sizing
A 9.5kW electric shower is to be installed on a dedicated radial circuit. The cable route runs through a loft space at 35 degrees Celsius, grouped with 2 other circuits, touching insulation on one side. Protected by an MCB.
Design current: Ib = 9,500 ÷ 230 = 41.3A
Protective device: 45A Type B MCB (next standard rating above 41.3A)
Correction factors: Ca = 0.94 (35°C, 70°C PVC) | Cg = 0.70 (3 circuits bunched and touching, Table 4C1 row 1) | Ci = 0.88 (passing through 50mm of insulation) | Cf = 1.0 (MCB)
Combined factor: 0.94 × 0.70 × 0.88 × 1.0 = 0.579
Required It: 45 ÷ 0.579 = 77.7A
Cable selection: the cable must be chosen from the correct Appendix 4 table so that its tabulated Iz meets or exceeds 77.7A under the chosen reference method. The grouping and insulation conditions push this circuit well beyond the cable a 45A MCB alone would suggest — the difference between an installed cable that survives and one that overheats.
A ring final circuit protected by a 32A Type B MCB. Cable runs through an insulated timber-frame wall where it is totally enclosed in insulation for 2 metres. Ambient temperature 30 degrees Celsius, no other circuits grouped at the insulated section.
Correction factors: Ca = 1.00 (30°C) | Cg = 1.00 (single circuit at worst point) | Ci = 0.50 (totally enclosed >0.5m) | Cf = 1.0 (MCB)
Combined factor: 1.00 × 1.00 × 0.50 × 1.0 = 0.50
Required It: 32 ÷ 0.50 = 64.0A
Result: the ring would need each leg to carry an effective It of 64A through the enclosed section — far beyond what a standard 2.5mm² twin and earth ring leg can deliver once halved by Ci = 0.50. The practical fix is to reroute the cable so it is not totally surrounded by insulation, or to redesign the circuit. This is exactly why Reg 523.9 advises that cables should preferably not be installed where they are liable to be covered by thermal insulation.
08 · BS 7671 Cable Sizing
The most common and most dangerous mistake. Many electricians select cables based solely on the MCB rating without considering the installation conditions. A 32A MCB does not mean a 32A-rated cable is sufficient — it means you need a cable that can carry at least 32A after all derating factors have been applied. Under adverse conditions, this could require a cable rated for 50A or more.
Grouping must be assessed at the worst point along the cable route — typically where the most cables are bundled together, such as at the consumer unit or where cables pass through a common hole in a joist. Counting only the cables at the load end rather than at the point of maximum grouping leads to an incorrect (too high) Cg value.
With modern loft insulation depths of 270mm or more, cables laid on ceiling joists can be completely buried in insulation. This triggers the Ci = 0.50 factor, halving the cable capacity. Many electricians fail to check whether insulation has been added since the original installation, particularly during periodic inspections.
The formula divides the protective device rating (In) by the correction factors, not the design current (Ib). Dividing Ib by the factors would give a lower value than required, potentially resulting in an undersized cable. The correct approach is: It = In / (Ca x Cg x Ci x Cf), then select a cable with Iz equal to or greater than It.
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