📖 UK Cable Size Calculator - Professional Cable Sizing Tool
Welcome to the UK Cable Size Calculator, a professional electrical engineering tool designed to help electricians, engineers, and DIY enthusiasts determine the correct cable size for electrical installations in compliance with BS 7671 (IET Wiring Regulations).
This cable size calculator considers current-carrying capacity, voltage drop, installation method, ambient temperature, and grouping factors to recommend the minimum safe cable size. Whether you're working on a domestic ring main, an industrial sub-main, or an EV charger installation, this tool provides accurate results based on the 18th Edition IET Wiring Regulations.
📐 How to Calculate Cable Size
Cable size calculation is a critical step in electrical design. The wrong cable size can lead to overheating, voltage drop issues, and fire hazards. The cable size calculation formula follows a systematic approach defined by BS 7671:
Step 1: Determine the Design Current
The design current (Ib) is the maximum current the circuit is expected to carry under normal operation. It is calculated using the formula:
Ib = Load (Watts) ÷ Voltage (Volts)
For example, a 7kW electric shower at 230V draws 7000 ÷ 230 = 30.4A.
Step 2: Apply Correction Factors
BS 7671 requires derating the cable's current-carrying capacity based on installation conditions:
- Ambient Temperature (Ca): Cables in higher temperatures need derating. Standard ambient temperature is 30°C for air and 20°C for buried cables.
- Grouping Factor (Cg): When multiple cables are installed together, their current capacity is reduced due to mutual heating.
- Thermal Insulation (Ci): Cables surrounded by thermal insulation require significant derating.
Effective Current = Ib ÷ (Ca × Cg × Ci)
Step 3: Select the Minimum Cable Size
Using the BS 7671 cable ampacity tables, select the smallest cable size whose current-carrying capacity (after correction) is greater than the effective current.
Step 4: Check Voltage Drop
Voltage drop calculation is essential to ensure appliances receive adequate voltage. BS 7671 limits voltage drop to:
- 3% for lighting circuits (6.9V at 230V)
- 5% for other circuits (11.5V at 230V)
Voltage Drop = (mV/A/m × Ib × L) ÷ 1000
Where mV/A/m is the cable's voltage drop per metre per ampere, and L is the cable length. If the voltage drop exceeds the limit, a larger cable is required.
📊 Cable Size Calculation Formula
The complete cable size calculation formula can be expressed as:
Minimum Cable Size = max(
Sizecurrent: Itz × Ca × Cg ≥ Ib
Sizevoltage: (mV/A/m × Ib × L) ≤ Voltage Drop Limit
)
🔌 AC Cable Size Calculator
This AC cable size calculator works for both single-phase and three-phase AC circuits. The calculator automatically adjusts for the system voltage selected. For three-phase circuits, the current is lower for the same power:
Three-Phase Current: I = P ÷ (√3 × V × PF)
Single-Phase Current: I = P ÷ V
🛡️ Armoured Cable Size Calculator
The armoured cable (SWA) size calculator is essential for outdoor and buried installations. SWA (Steel Wire Armoured) cable provides mechanical protection and is commonly used for:
- Sub-mains to outbuildings (garages, sheds)
- Electric vehicle (EV) charger installations
- Garden lighting and power circuits
- Industrial and commercial installations
- Underground cable runs
SWA cables have higher current ratings than PVC insulated cables due to the XLPE insulation operating at 90°C.
📋 BS 7671 Cable Ampacity Reference Tables
The following table shows cable ampacity values for common cable sizes based on BS 7671 Table 4D2A (Twin & Earth) and Table 4D4A (SWA):
| Cable Size (mm²) | Twin & Earth (Method C) | SWA (Method C) | SWA (Method D - Buried) |
|---|
| 1.0 | 15A | — | — |
| 1.5 | 19.5A | 27A | 25A |
| 2.5 | 27A | 36A | 33A |
| 4 | 36A | 49A | 43A |
| 6 | 46A | 62A | 53A |
| 10 | 63A | 85A | 71A |
| 16 | 85A | 110A | 91A |
| 25 | — | 146A | 116A |
📏 Voltage Drop Calculation
Voltage drop calculation is critical for long cable runs. The voltage drop per metre per ampere (mV/A/m) depends on cable size:
| Cable Size (mm²) | mV/A/m (2-core) | mV/A/m (3-core) |
|---|
| 1.0 | 44 | 44 |
| 1.5 | 29 | 29 |
| 2.5 | 18 | 18 |
| 4 | 11 | 11 |
| 6 | 7.3 | 7.3 |
| 10 | 4.4 | 4.4 |
| 16 | 2.8 | 2.8 |
| 25 | 1.75 | 1.75 |
Example: A 6mm² cable carrying 30A over 40 metres has a voltage drop of (7.3 × 30 × 40) ÷ 1000 = 8.76V (3.8% at 230V). This is acceptable for a power circuit (5% limit).
💡 Common Cable Sizing Applications
- Lighting Circuits: 1.0mm² or 1.5mm² depending on load and length
- Ring Mains: 2.5mm² Twin & Earth (32A protection)
- Radial Circuits: 2.5mm² (20A) or 4mm² (32A)
- Cooker Circuits: 4mm² to 10mm² depending on rating (6mm² for up to 10kW, 10mm² for 15kW)
- Electric Showers: 6mm² (up to 9.5kW), 10mm² (9.5-12kW)
- EV Chargers (7kW): 6mm² or 10mm² SWA depending on length
- Sub-Mains: 10mm² to 25mm² depending on load and distance
📌 Important Notes for Cable Sizing
- Always consider BS 7671 (IET Wiring Regulations) for professional installations
- This calculator provides estimates only - consult a qualified electrician for final design
- Consider short-circuit current and protective device coordination in final design
- Different installation methods significantly affect cable capacity
- Ambient temperature correction is essential for lofts, attics, and hot environments
- Grouping corrections apply when cables are bunched together
❓ Frequently Asked Questions
How to calculate cable size for a given load? ▼
Calculate the design current (I = P ÷ V), apply correction factors (temperature, grouping), and select the smallest cable whose corrected ampacity exceeds the design current. Then check voltage drop.
What is the cable size calculation formula? ▼
The formula is: Minimum Cable Size = max(Size based on current capacity, Size based on voltage drop). Current capacity requires Itz × Ca × Cg ≥ Ib. Voltage drop requires (mV/A/m × Ib × L) ≤ Vd_limit.
What is the difference between CPM and RPM in cable sizing? ▼
In cable sizing, we use current-carrying capacity (ampacity) not CPM/RPM. The cable's current rating must be greater than the circuit's design current after applying correction factors.
What cable size for a 7kW EV charger? ▼
A 7kW EV charger draws approximately 30.4A at 230V. Depending on cable length and installation method, 6mm² SWA (up to 35-40m) or 10mm² SWA (longer runs) is typically recommended.
How to calculate voltage drop in cable? ▼
Voltage Drop = (mV/A/m × Ib × L) ÷ 1000, where mV/A/m is from cable tables, Ib is design current, and L is cable length in metres. For 230V circuits, the drop should not exceed 5% (11.5V).
What is BS 7671? ▼
BS 7671 is the UK standard for electrical installations, also known as the IET Wiring Regulations. It sets the requirements for safe electrical design, installation, and inspection.
What is the maximum voltage drop allowed in the UK? ▼
BS 7671 limits voltage drop to 3% for lighting circuits and 5% for other circuits (e.g., sockets, appliances). This ensures equipment operates within acceptable voltage limits.
How do I choose cable size for a 3-phase motor? ▼
Calculate full load current (FLC) using I = P ÷ (√3 × V × PF × η), apply correction factors, and select a cable with sufficient ampacity. Voltage drop must also be checked.
What is the difference between SWA and twin & earth cable? ▼
SWA (Steel Wire Armoured) has a steel armour layer for mechanical protection, suitable for outdoor and buried use. Twin & Earth is unarmoured, typically used indoors for domestic wiring.
What are the common cable sizes for domestic wiring? ▼
Common domestic cable sizes: 1.0mm² (lighting), 1.5mm² (lighting/radials), 2.5mm² (ring mains), 4mm² (cooker/small showers), 6mm² (showers/EV chargers), 10mm² (large showers/sub-mains).