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

Enter Lighting Load - Total wattage of all lighting circuits. The calculator applies the On-Site Guide allowance — 66% of the total connected lighting load.
Specify Socket Outlets - Count all 13A socket outlets. The calculator applies the On-Site Guide rule — 100% of the largest point + 40% of every other.
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.
Include Water/Space Heating (optional) - Add any electric water heating (immersion heaters) or space heating (storage heaters). These loads have 100% diversity (no reduction).
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% of the total connected lighting load

It's unlikely all lights will be on simultaneously, especially in larger installations. The On-Site Guide (Appendix A, domestic) takes 66% of the whole lighting load.

Socket Outlets

Diversity: 100% of the largest + 40% of every other

On-Site Guide (Appendix A) domestic socket-outlet rule. The calculator models each outlet as a 13A point — an aggregate approximation of the per-circuit rule (the dedicated diversity calculator takes an explicit circuit list).

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)

Electrical Load Calculator - Maximum Demand

Calculate Electrical Load

Basic Loads

Cooking Appliance