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All-in-One Circuit Designer & Cable Calculator | TMUK Group Ltd

All-in-One Circuit Design Engine

Cable Sizing, Protection Selection & Diversity Calculator

1. Circuit Specifications
kW
Metres
2. Installation Environment
Cg
Required Cable Size -- Base: Copper PVC Multi-Core
Protective Device (In) -- Type B/C MCB or RCBO Suggested
Calculated Design Current (Ib) 0.00 A Volt Drop: --
⚠ VD WARNING: Calculated drop exceeds statutory limits. The engine has automatically increased the cable size to maintain compliance.
⚠ BS 7671 RING WARNING: Ring Final Circuits must not serve a floor area exceeding 100 m² and must be wired in min 2.5mm² cable. Ensure 3-Phase is NOT selected.
⚠ OVERLOAD: The required capacity exceeds standard domestic/commercial radial sizes (35mm²). Please use the Heavy Industrial Sizing tool for this load.

Circuit Design Methodology & Instructions

Designing a safe, compliant electrical installation requires more than just picking a standard cable size. To meet the strict requirements of BS 7671:2018+A3:2024, every circuit must be assessed through the lens of the fundamental equation: Ib ≤ In ≤ Iz. This tool automates the selection process while accounting for length, load, and installation environment.

How to Use This Tool

1
Define the Load: Select your circuit type. For Radial Power, use the toggle to input your load in Amps, Watts, or Kilowatts. If you are wiring a commercial motor or large heater, ensure the Supply Phase is set to 3-Phase (400V) and adjust the Power Factor accordingly.
2
Assess Length & Diversity: Enter the total run length in metres. Diversity (Regulation 311.1) is applied automatically to reduce the calculated load if you are confident not all equipment will run at 100% capacity simultaneously.
3
Set Environmental Factors: Select how the cable is installed. Cables in thermal insulation (Method 103) or grouped with other circuits (Cg) have significantly reduced current-carrying capacities.

Technical Constraints & Regulations

1. Protective Device Selection (In)

This engine does not simply match the cable to your load (Ib). It mathematically selects the nearest standard circuit breaker (In) that is greater than your load, and then ensures the cable capacity (Iz) is greater than the breaker. This guarantees the breaker will trip before the cable melts.

2. Correction Factors (It = In / [Ca × Cg × Ci])

The "Effective Capacity" of a cable is dictated by its environment. This tool applies factors for:

  • Ca (Ambient Temperature): Standard rating is at 30°C. If your run passes through a hot loft space at 40°C, the cable capacity is reduced by approx 13%.
  • Cg (Grouping): When multiple circuits run together in the same trunking, they generate mutual heat. 2 circuits together require a 0.80 multiplier.
  • Ci (Thermal Insulation): Cables totally surrounded by insulation (Method 103) require a massive 0.50 reduction factor.

3. Voltage Drop (Regulation 525)

BS 7671 mandates that voltage drop from the origin to any point in the installation should not exceed 3% for lighting (6.9V on 230V) and 5% for other uses (11.5V on 230V). On longer runs, the tool will automatically check if the voltage drop is exceeded and will seamlessly bump up the cable size to maintain compliance.

Professional Disclaimer: This TMUK Group Ltd engineering tool is a design aid. It does not account for Earth Fault Loop Impedance (Zs) limits for automatic disconnection of supply. A final site inspection and full verification must be carried out by a competent person before installation.