Calculate the correct cable size for electrical installations according to British Standards BS 7671:2018+A2:2022
Enter your installation parameters to calculate the required cable size
Calculating the correct cable size for UK electrical installations requires consideration of several critical factors according to BS 7671 wiring regulations. This guide explains the complete process step-by-step.
The design current is the maximum current your circuit will carry under normal operating conditions.
BS 7671 requires cable sizing to account for installation conditions that affect heat dissipation:
Standard is 30°C. For loft spaces (45°C) or other hot environments, apply temperature correction from BS 7671 Table 4B1.
Multiple cables grouped together generate more heat. Correction factors from BS 7671 Table 4C1.
Divide your design current by the overall correction factor:
Example: 32A circuit, in conduit (0.95), at 30°C (1.0), with 2 cables (0.8):
It = 32A ÷ (0.95 × 1.0 × 0.8) = 42.1A required capacity
Choose the cable size from the appropriate BS 7671 table based on your cable type and installation method:
Select the smallest cable size that has a current rating ≥ It (your calculated capacity)
BS 7671 Regulation 525 limits voltage drop to prevent poor performance:
Voltage Drop Formula (Single Phase):
mV/A/m values are found in BS 7671 voltage drop tables (e.g., Table 4D5)
If voltage drop exceeds limits, increase cable size and recalculate.
Given: 9.5kW shower, 230V, 20m cable run, clipped direct, 30°C ambient
Step 1: Design current (Ib) = 9500W ÷ 230V = 41.3A
Step 2: Correction factors: Ca=1.0, Cg=1.0, Ci=1.0
Step 3: Required capacity: It = 41.3A ÷ 1.0 = 41.3A
Step 4: From Table 4D5, 6mm² twin & earth = 47A (clipped direct) ✓
Step 5: Voltage drop: (18 × 20 × 41.3) ÷ 1000 = 14.87V = 6.5%
⚠️ Voltage drop exceeds 5% limit!
Solution: Upgrade to 10mm² cable (voltage drop = 3.9%) ✓
Final answer: 10mm² twin & earth cable with 45A MCB
💡 Pro Tip: Always use the calculator above for accurate results. Manual calculations are prone to errors, especially when multiple correction factors apply. The calculator automatically applies all BS 7671 requirements and checks voltage drop compliance.
This cable size chart shows current ratings for common UK cable types. Values are for copper conductors at 30°C ambient with PVC insulation (70°C). Always verify with BS 7671 tables for your specific installation.
| Cable Size | Clipped Direct | In Conduit | Max MCB | Typical Use |
|---|---|---|---|---|
| 1.0mm² | 15.5A | 13.5A | 10A | Lighting circuits |
| 1.5mm² | 20A | 17.5A | 16A | Lighting, immersion heaters |
| 2.5mm² | 27A | 24A | 20A | Ring mains, radial sockets |
| 4.0mm² | 37A | 32A | 32A | Cooker circuits, high-power radials |
| 6.0mm² | 47A | 41A | 40A | Electric showers, small cookers |
| 10.0mm² | 64A | 57A | 50A | Large cookers, EV chargers |
| 16.0mm² | 85A | 76A | 63A | Sub-mains, large loads |
Reference: BS 7671 Table 4D5 (Flat twin and earth cables, copper conductors, 70°C PVC)
| Cable Size | Clipped/Tray | Buried Direct | Max MCB | Typical Use |
|---|---|---|---|---|
| 2.5mm² | 28A | 35A | 25A | Garden lighting, small outbuildings |
| 4.0mm² | 37A | 45A | 32A | Garages, workshops |
| 6.0mm² | 47A | 57A | 40A | EV chargers, hot tubs |
| 10.0mm² | 64A | 78A | 50A | Larger outbuildings, sub-mains |
| 16.0mm² | 84A | 102A | 63A | Commercial sub-mains |
| 25.0mm² | 110A | 133A | 100A | Main supplies, large installations |
Reference: BS 7671 Table 4D4A (Armoured 70°C PVC cables, 2-core for single phase)
Here are typical cable sizes for common domestic and commercial installations. Always calculate for your specific cable length and installation method.
7.5kW shower: 6mm² cable, 32A MCB
8.5kW shower: 6mm² cable, 40A MCB
9.5kW shower: 10mm² cable, 40A MCB
10.5kW+ shower: 10mm² cable, 45A MCB
Current = Power ÷ 230V (e.g., 9500W ÷ 230V = 41.3A)
Single oven (2-3kW): 2.5mm², 20A MCB
Double oven (5-6kW): 4mm², 32A MCB
Range cooker (8-10kW): 6mm², 40A MCB
Large range (12kW+): 10mm², 45A MCB
Note: Apply diversity for cookers with multiple rings
3.6kW (16A): 2.5mm² cable
7.4kW (32A): 6mm² cable (up to 20m)
7.4kW (32A): 10mm² cable (20-40m)
22kW (32A 3-phase): 6mm² 4-core SWA
Most home EV chargers are 7.4kW single phase
Standard (3kW): 2.5mm², 16A MCB
Long run (3kW, 20m+): 4mm², 16A MCB
Heat-resistant flex required at the heater connection
Standard ring: 2.5mm², 32A MCB
Floor area: Max 100m² per ring
Spurs: Max 1 per socket, same cable size
Radial circuits: 2.5mm²/20A (20m²) or 4mm²/32A (50m²)
Domestic: 1.0mm² or 1.5mm², 6A MCB
Max per circuit: 12 lighting points
Voltage drop: Max 3% (6.9V)
Use 1.5mm² for longer runs or LED driver loads
13A plug-in: Existing socket circuit
32A dedicated: 6mm² SWA
40A large spa: 10mm² SWA
RCD protection required - outdoor special location
Light use: 4mm² SWA, 32A MCB
With heating: 6mm² SWA, 40A MCB
Workshop: 10mm² SWA, 63A MCB
SWA buried 450mm deep or in ducting
Small split (1-2kW): 2.5mm², 16A MCB
Medium (3-5kW): 4mm², 20A MCB
Large (7kW+): 6mm², 32A MCB
Check manufacturer specs - motor starting current
Important: These are typical values only. Always calculate cable size for your specific installation considering cable length, installation method, ambient temperature, and grouping with other cables. Use the calculator above for accurate sizing.
Follow these detailed calculations for the most common UK electrical installations. All examples use BS 7671:2018+A2:2022 (18th Edition) standards.
Scenario:
Step 1 - Calculate Design Current:
Ib = 7400W ÷ 230V = 32.2A
Step 2 - Apply Correction Factors:
Step 3 - Required Capacity:
It = 32.2A ÷ 1.0 = 32.2A
Step 4 - Select Cable from BS 7671 Table 4D4A (SWA):
6mm² SWA buried direct = 57A rating ✓ (exceeds 32.2A)
Step 5 - Check Voltage Drop:
From Table 4D4A: 6mm² SWA = 7.3 mV/A/m
Vd = (7.3 × 25 × 32.2) ÷ 1000 = 5.88V
Percentage = (5.88 ÷ 230) × 100 = 2.56% ✓ (under 5% limit)
✓ Final Answer:
Use 6mm² 3-core SWA cable with 32A Type B MCB
Note: Ensure RCD protection (30mA) as per BS 7671 Regulation 722.531.2
Scenario:
Step 1 - Calculate Design Current:
Ib = 8500W ÷ 230V = 37A
Step 2 - Apply Correction Factors:
Step 3 - Required Capacity:
It = 37A ÷ 1.0 = 37A
Step 4 - Select Cable from BS 7671 Table 4D5:
6mm² twin & earth clipped direct = 47A ✓ (exceeds 37A)
Note: 4mm² = 37A would be borderline, use 6mm² for safety margin
Step 5 - Check Voltage Drop:
From Table 4D5: 6mm² = 7.3 mV/A/m
Vd = (7.3 × 12 × 37) ÷ 1000 = 3.24V
Percentage = (3.24 ÷ 230) × 100 = 1.41% ✓ (well under 5%)
✓ Final Answer:
Use 6mm² twin & earth cable with 40A Type B MCB
Scenario:
Step 1 - Calculate Design Current with Diversity:
Total load: 10kW = 43.5A at 230V
Apply cooker diversity:
Design current (Ib) = 10 + 10 + 5 = 25A
Step 2 - Required Capacity:
It = 25A ÷ 1.0 = 25A (no derating needed)
Step 3 - Select Cable:
4mm² twin & earth = 37A ✓ OR 6mm² = 47A for longer runs
Step 4 - Voltage Drop Check (using 6mm²):
Vd = (7.3 × 8 × 25) ÷ 1000 = 1.46V = 0.63% ✓
✓ Final Answer:
Use 6mm² twin & earth cable with 32A Type B MCB
Note: Diversity applied per BS 7671 Appendix 15 for cooking appliances
The current rating of 6mm² cable depends on the installation method. Here are the ratings from BS 7671:
| Installation Method | Current Rating | Max MCB | Reference |
|---|---|---|---|
| Twin & Earth - Clipped Direct | 47A | 40A | Table 4D5 Method C |
| Twin & Earth - In Conduit/Trunking | 41A | 32A | Table 4D5 Method B |
| SWA - Clipped to Surface | 47A | 40A | Table 4D4A Method C |
| SWA - Buried Direct | 57A | 50A | Table 4D4A Method D |
Important: These ratings assume 30°C ambient temperature and single circuit. Apply correction factors for grouped cables or high temperatures.
10mm² cable is commonly used for larger loads like big cookers, sub-mains, and high-power equipment:
| Cable Type & Method | Rating (Amps) | Max MCB | Typical Use |
|---|---|---|---|
| Twin & Earth - Clipped Direct | 64A | 50A | Large cookers, shower pumps |
| Twin & Earth - In Conduit | 57A | 50A | Protected high-power circuits |
| SWA - Buried Direct | 78A | 63A | Outbuilding sub-mains |
Real-world example: A 40A shower (9.2kW) with 15m cable run would need 10mm² cable to keep voltage drop under 5%. Using 6mm² would result in 6.8% voltage drop (too high).
These common errors can result in failed inspections, dangerous installations, or expensive rewiring. Understanding these traps is essential for BS 7671 compliance.
Modern homes with 270mm+ loft insulation create a critical derating scenario that many electricians miss. When cables are buried in thermal insulation, heat cannot dissipate.
| Reference Method | Scenario | Derating Factor (Ci) |
|---|---|---|
| Method 100 | Clipped to joist, <100mm insulation | 0.89 |
| Method 101 | Clipped to joist, >100mm insulation | 0.5 |
| Method 102 | In stud wall, not touching inner wall | 0.89 |
| Method 103 | In stud wall, touching inner wall | 0.5 |
Real Impact: A 2.5mm² cable rated at 27A clipped direct becomes just 13.5Aunder Method 101. This is insufficient for a 32A ring circuit. The circuit requires upsizing to 4mm² or 6mm².
Reference: BS 7671 Regulation 523.9 and Table 4A2 (Amendment 3)
BS 7671 Regulation 433.1.1 requires that the current causing effective operation of the protective device (I₂) must not exceed 1.45 times the cable's current-carrying capacity (Iz).
For modern MCBs (BS EN 60898), this is automatically satisfied because I₂ = 1.45 × In. However, for BS 3036 rewirable fuses, the fusing factor is approximately 2.0.
Critical: When using rewirable fuses, you must apply an additional derating factor of 0.725 to the cable capacity. A 27A cable becomes effectively 19.6A.
Many older domestic consumer units still use rewirable fuses. Always check before sizing cables.
SWA cables with XLPE insulation are rated for 90°C operation and have higher current ratings than 70°C PVC cables. However, this creates a hidden problem.
The Problem: Most domestic consumer units, MCBs, and terminal blocks are rated for70°C maximum. Running an XLPE cable at 90°C will conduct excessive heat into the terminals, causing nuisance tripping or terminal damage.
Solution: When using XLPE/SWA cables with standard 70°C terminals, use the 70°C current ratings from BS 7671 tables, not the 90°C ratings. The benefit of XLPE is only realised when both the cable AND terminals are rated for 90°C.
Voltage drop is cumulative through an installation. Many electricians calculate the final circuit drop but forget the sub-main contribution.
Total Voltage Drop = Sub-main + Final Circuit
Example: 1.5% (sub-main) + 4% (final circuit) = 5.5% (FAIL)
Best Practice: Budget your 5% allowance carefully:
Calculations may suggest a cable size that is electrically correct but physically impossible.
Example: High derating factors on a 32A ring circuit might suggest 10mm² cable. However, 10mm² cable will not fit into the terminals of a standard 13A double socket. Maximum terminal capacity is typically 2.5mm² or 4mm².
Solutions:
Cable sizing is not just about current capacity. It requires simultaneous verification of thermal conditions, voltage drop, protective device coordination, physical terminations, and earth fault loop impedance. Use this calculator and always verify calculations against your specific installation conditions.
For commercial, industrial, and complex residential installations, these advanced factors must be considered beyond basic current and voltage drop calculations.
Modern LED lighting, IT equipment, and Variable Speed Drives (VSDs) are non-linear loads that create harmonic currents. The third harmonic (150Hz) is particularly problematic in three-phase systems.
The Physics: Third harmonic currents from all three phases are in-phase (0° apart instead of 120°). They don't cancel at the neutral point—they add together.
| 3rd Harmonic Content | Phase Reduction Factor | Neutral Sizing |
|---|---|---|
| 0-15% | 1.0 | Standard (same as phase) |
| 15-33% | 0.86 | Standard (same as phase) |
| 33-45% | 0.86 | Neutral = Phase size |
| >45% | By calculation | Neutral may exceed phase |
Reference: BS 7671 Regulation 523.6.3 and Appendix 4, Section 11
BS 7671 contains an often-overlooked provision that can significantly reduce cable costs on commercial installations.
The Rule: Cables carrying no more than 30% of their grouped ratingmay be excluded from grouping factor calculations.
Practical Application:
Reference: BS 7671 Table 4C1, Note 3
Induction motors draw 6-8 times their Full Load Current (FLC) during Direct-On-Line (DOL) starting. While cable current capacity is sized for running current, voltage drop during starting can cause problems.
Starting Voltage Drop: A cable sized for 5% voltage drop at running current will experience 30-40% drop during starting (6-8× current). If this exceeds 15%, the motor may stall or the contactor may chatter.
Sizing Approach for Motors:
The Circuit Protective Conductor (CPC/Earth) must withstand the thermal stress of a fault without damage. BS 7671 Regulation 543.1.3 provides the adiabatic equation:
Variables:
k values:
SWA Cable Note: The steel armour of SWA cables often serves as the CPC. Due to the lower k factor (51 vs 115), the armour may be insufficient for high fault currents. Always verify with the adiabatic equation and add external earth cable if needed.
Cable selection must ensure the earth fault loop impedance (Zs) is low enough to guarantee protective device operation within required disconnection times.
Ze = External earth fault loop impedance
TN-C-S (PME): 0.35Ω typical
TN-S: 0.8Ω typical
R1+R2 = Resistance per metre (mΩ/m)
L = Cable length (m)
Cr = Temperature correction (1.2 for 70°C)
Practical Impact: Type C MCBs require much lower Zs than Type B for the same rating. This often dictates using larger cables or shorter runs with Type C devices.
| MCB Rating | Type B Max Zs | Type C Max Zs |
|---|---|---|
| 16A | 2.73Ω | 1.37Ω |
| 32A | 1.37Ω | 0.68Ω |
| 63A | 0.70Ω | 0.35Ω |
For complex installations, always reference:
This calculator complies with BS 7671:2018+A2:2022 (18th Edition) - Requirements for Electrical Installations (IET Wiring Regulations).
Under normal service conditions, the voltage drop between the origin of the installation and the socket outlet or appliance shall not exceed:
The calculator applies derating factors for different installation methods as per Appendix 4 of BS 7671. Factors considered include:
Important: This calculator is a tool for initial sizing estimates. Final cable selection should be verified by a qualified electrician considering all installation-specific factors including grouping, thermal insulation, and harmonic currents.
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