Electrical Load Calculator - Maximum Demand

Calculate maximum demand with diversity factors for UK electrical installations according to BS 7671

Calculate Electrical Load

Calculate maximum demand with UK diversity factors

Basic Loads

Watts

Total connected lighting load (diversity applied automatically)

outlets

13A socket outlets (diversity applied automatically)

Cooking Appliance

How to Use the Load Calculator

This calculator helps you determine the maximum demand of an electrical installation by applying diversity factors according to BS 7671 Appendix 1.

Step-by-Step Guide

  1. Enter Lighting Load - Total wattage of all lighting circuits. The calculator automatically applies 66% diversity after the first 10A.
  2. Specify Socket Outlets - Count all 13A socket outlets. Diversity is applied based on quantity (100% for first 10, reducing for additional outlets).
  3. Add Cooking Appliance - If you have an electric cooker, enter its rated power. Tick the box if the cooker control unit includes a socket outlet.
  4. Include Water/Space Heating (optional) - Add any electric water heating (immersion heaters) or space heating (storage heaters). These loads have 100% diversity (no reduction).
  5. Review Results - See your total connected load, maximum demand after diversity, and recommended supply configuration.

Understanding the Results

  • Total Connected Load: Sum of all loads if operating simultaneously (unlikely in practice)
  • Maximum Demand: Realistic peak demand after applying diversity factors (what you actually need to design for)
  • Estimated Current: The current draw at maximum demand (at 230V)
  • Recommended Supply: Suggested main fuse/MCB rating and meter tail cable size

Important Note

This calculator provides initial design estimates. Final installation must be verified by a qualified electrician considering all site-specific factors including future expansion, special loads, and DNO requirements.

UK Diversity Factors Explained

Diversity factors recognize that not all electrical loads operate simultaneously. BS 7671 Appendix 1 provides guidance on appropriate diversity for different load types.

Lighting

Diversity: 66% after first 10A

It's unlikely all lights will be on simultaneously, especially in larger installations. The first 2.3kW (10A at 230V) is counted at 100%, remainder at 66%.

Socket Outlets

Diversity: Varies by quantity

  • 1-10 outlets: 100%
  • 11-20 outlets: First 10 at 100%, next at 50%
  • 21-30 outlets: Further reduction to 40%
  • 30+ outlets: Additional outlets at 30%

Based on the principle that more sockets means lower probability of simultaneous use.

Cooking Appliances

Diversity: First 10A at 100%, remainder at 30%, plus 5A for socket

Not all rings/oven/grill operate at full power simultaneously. Example: 11kW cooker = 10A (100%) + 38A (30%) = 23.4A demand

Water & Space Heating

Diversity: 100% (No diversity)

These loads are typically thermostatically controlled and may run for extended periods. For safety and performance, full capacity must be available.

Why Use Diversity?

Applying appropriate diversity factors:

  • Prevents oversizing of supply equipment and cables
  • Reduces installation costs
  • Reflects realistic usage patterns in domestic and commercial installations
  • Is accepted practice and required by BS 7671

Reference: BS 7671:2018+A2:2022 Appendix 1 - British Standard for electrical installations (IET Wiring Regulations)