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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

Circuit Specifications
W
Metres
Installation Environment
Cg
Required Cable Size -- Base: Copper Twin & Earth
Protective Device (In) -- Type B MCB/RCBO Suggested
Design Current (Ib) 0.00 A Volt Drop: --
VD WARNING: Calculated drop exceeds limits. Cable size has been auto-increased to maintain compliance.
BS 7671 WARNING: Ring Final Circuits must not serve a floor area exceeding 100 m². Ensure continuity of all conductors.

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 Lighting, enter the number of points and the wattage per lamp (e.g., 40W for standard LED arrays). For Ring Finals, the tool automatically locks to a 32A design base as per Appendix 15.
2
Assess Length & Diversity: Enter the total run length in metres. Diversity (Regulation 311.1) is applied automatically to lighting loads (90% of total connected load) but can be adjusted for fixed-load radial circuits.
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. Diversity (Regulation 311.1)

Diversity allows us to account for the fact that not all points on a circuit will be used at full capacity simultaneously. For a lighting circuit with 15 points, calculating at 100% capacity would result in oversized cables and protection. Applying a standard 90% factor provides a more realistic Design Current (Ib).

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 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 (like your 25m lighting example), the tool will automatically check if the voltage drop is exceeded and will prompt an increase in cable size to maintain compliance.

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