Cable Sizing Calculator UK
BS 7671:2018+A2:2022 compliant electrical cable sizing with voltage drop analysis and derating factors
Cable Size Calculator
BS 7671:2018+A2:2022 compliant cable sizing with voltage drop analysis
Method C = highest capacity. Methods B/E derated for reduced air flow.
Calculate Cable Size for Your Specific Project
Select your project type for tailored calculations, tips, and guidance:
EV Charger
7kW, 22kW home & commercial
Electric Shower
8.5kW to 10.8kW installations
Cooker Circuit
Ovens, hobs & range cookers
Garden Office
Sheds, workshops, outbuildings
Hot Tub & Spa
Spas, swim spas, jacuzzis
Immersion Heater
Hot water cylinder circuits
Solar PV & Battery
Inverter & storage systems
Air Source Heat Pump
£7,500 BUS grant eligible
Underfloor Heating
Electric UFH supply circuits
Garage & Workshop
Power tools, welders, compressors
Sauna
Electric sauna heater installations
Air Conditioning
Split systems & multi-split AC
Swimming Pool
Pumps, heaters, lighting circuits
Electric Gates
Gate motors & access control
CCTV & Security
Camera systems & alarm power
Annex & Granny Flat
Submains for self-contained annexes
Shed & Summer House
Garden building power supplies
Outdoor Lighting
Garden, security & festoon lights
Storage Heaters
Economy 7 & panel heater circuits
Ring Main & Sockets
Ring final & radial socket circuits
Commercial Kitchen
Restaurants, cafés & catering
Server Room
Data centres & IT infrastructure
Caravan & Marina
Holiday parks & boat hookups
Farm & Agricultural
Barns, milking parlours, workshops
Shop & Retail
Commercial fit-outs & small business
Ground Source Heat Pump
GSHP & borehole systems
Battery Storage
Home battery & retrofit systems
Commercial EV Charging
Workplace, car parks & fleet
⚡ Important Compliance Notes
- • This calculator follows BS 7671:2018+A2:2022 (18th Edition) requirements
- • Results assume standard conditions (30°C ambient, no grouping factors, PVC/XLPE copper conductors)
- • Additional derating may apply for: grouped circuits, high ambient temperatures, sub-surface installations, or special conditions
- • Always consult a qualified electrician and verify calculations with current BS 7671 tables and manufacturer data
- • This calculator is a guide only and does not replace professional design
How to Use the Cable Sizing Calculator
Choosing the correct cable size is critical for electrical safety and compliance with BS 7671:2018+A2:2022 (the 18th Edition wiring regulations). Our free cable sizing calculator helps UK electricians determine the appropriate cable size for any electrical installation in seconds.
Step-by-step: Enter load current (amps or kW), specify cable run length, select installation method, indicate if lighting circuit, and click calculate. Results show recommended cable size based on BS 7671 current-carrying capacity tables.
What This Calculator Does
This professional-grade tool calculates the minimum cable size required for electrical circuits in UK installations. It accounts for load current, installation methods, and derating factors to ensure compliance with BS 7671:2018+A2:2022.
✓ What It Calculates
- • Minimum cable size for current
- • Derating factors by method
- • Voltage drop limits (5% or 3%)
- • Installation method capacity
✓ Why It Matters
- • Prevents dangerous overheating
- • Ensures BS 7671 compliance
- • Reduces project delays
- • Protects professional reputation
Quick Cable Sizing Reference (Method C)
Standard PVC/XLPE copper conductors, clipped direct to surface (Method C), 30°C ambient, single-phase 230V. Always verify with current BS 7671 tables for your specific installation conditions.
| Cable Size (mm²) | Max Current (Amps) | Common Uses | MCB Rating |
|---|---|---|---|
| 1.5 | 13.5A | Low-power circuits | 10A |
| 2.5 | 18A | Lighting circuits | 16A |
| 4 | 24A | Appliances, cookers | 20A |
| 6 | 32A | Heavy loads, showers | 32A |
| 10 | 41A | Submain cables | 40A |
| 16 | 57A | Main feeds, large loads | 63A |
| 25 | 76A | Main board supplies | 100A |
Note: These are approximate values for Method C. Method B and Method E installations require derating. Always consult BS 7671 Table 433 for exact values.
Installation Methods & Derating Factors
Cable current-carrying capacity varies significantly by installation method due to heat dissipation differences. BS 7671 defines derating factors for each method.
Method C - Clipped Direct to Surface (Derating: 1.0x)
Cable clipped or fixed to a wall, ceiling, or surface. Excellent air circulation. Highest current capacity. Use this when possible. Examples: surface conduit, external wall runs.
Method B - Enclosed in Conduit/Trunking (Derating: 0.7x)
Cable enclosed in rigid conduit, flexible conduit, or trunking. Restricted air flow = more heat. Derated to 70% of Method C. Examples: buried conduit, metal trunking, PVC trunking.
Method E - In Cable Tray or Basket (Derating: 0.85x)
Cable laid in cable tray, basket, or cleats. Moderate air circulation. Derated to 85% of Method C. Examples: cable tray installations, clipped to tray.
Example: A 6mm² cable rated for 32A in Method C would be derated to: Method B = 32 × 0.7 = 22.4A; Method E = 32 × 0.85 = 27.2A
Why Correct Cable Sizing Matters
Incorrect cable sizing is one of the most common electrical installation defects. The consequences are serious:
❌ Undersized Cables
- • Dangerous overheating and fire risk
- • Excessive voltage drop
- • Equipment malfunction
- • BS 7671 non-compliance
- • EICR failures
⚠️ Oversized Cables
- • Unnecessary material costs
- • Wasted installation space
- • Reduced project profitability
- • Inefficient circuit protection
- • Harder to bend and route
✓ Correctly Sized Cables
Optimal safety, efficiency, and cost. BS 7671 compliant. Professional standard. Protects lives and property. Ensures customer satisfaction and protects your reputation.
Common Cable Sizing Mistakes
Professional electricians know that mistakes in cable sizing are costly and dangerous. Here are the most common errors:
Ignoring derating factors
Using Method C capacity for Method B or E installations. This is the most common mistake and creates fire risk.
Forgetting voltage drop calculations
Long cable runs require voltage drop checking. 5% limit for power, 3% for lighting. Violations cause equipment malfunction.
Not accounting for grouping
Multiple cables in conduit share heat and require additional derating. BS 7671 Table 4C provides factors.
Wrong installation method
Misidentifying whether cable is clipped to surface, in conduit, or in tray. Dramatically affects capacity.
Not checking ambient temperature
High ambient (lofts, sunlit areas) or high external surface temperatures require additional derating.
Ignoring future load growth
Size cables for potential future loads. It's cheaper to install slightly larger cable now than upgrade later.
⏱️ Save Time On Every Job
Professional electricians save 10-15 minutes per circuit using our calculator. That's hours saved every installation, allowing you to complete more jobs without compromising on safety or BS 7671 compliance. Plus, documented calculations prove you followed regulations during EICR inspections.
Frequently Asked Questions
Q: Is this calculator compliant with BS 7671:2018+A2:2022?
Yes, all calculations follow the 18th Edition requirements including current-carrying capacity tables (Table 433), voltage drop limits (Regulation 525.31), and derating factors (Table 4C). Results are designed to ensure compliance with modern wiring regulations.
Q: Can I use this for three-phase installations?
Yes, the calculator supports both single-phase (230V) and three-phase (400V) installations with appropriate voltage drop calculations. Enter current for your system voltage.
Q: What if I have multiple derating factors?
Apply all relevant factors cumulatively. For example: grouping factor × temperature factor × installation method factor. The calculator guides you through standard conditions (30°C, no grouping). For complex installations, always verify with BS 7671 Table 4C.
Q: How is voltage drop calculated?
Voltage drop = (Current × Resistance × Length × 2) ÷ 1000. BS 7671 limits to 5% for power circuits (11.5V at 230V) or 3% for lighting (6.9V). Longer runs or higher currents require larger cables.
Q: What's the difference between cable size and current rating?
Cable size (1.5mm², 2.5mm², etc.) determines its current rating. The same cable size has different ratings by installation method. Method C = full rating. Methods B/E = reduced ratings due to derated heat dissipation.
Q: Is this free to use?
Yes, completely free with no hidden costs, registration required, or usage limits. Built by electricians for electricians to save time and ensure BS 7671 compliance.
Q: Can I rely on this calculator for professional work?
This calculator provides accurate guidance for standard installations and saves time. However, always verify your calculations against current BS 7671 tables, check for special conditions (high ambient, grouping, sub-surface), and document your design process. Professional responsibility requires due diligence.
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