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Diversity Calculator

Calculate maximum demand after applying diversity factors per IET On-Site Guide

Reference Info & Formulas
What is Diversity?

Diversity recognizes that not all electrical loads operate simultaneously at full capacity.

Applying diversity factors reduces the calculated maximum demand to a realistic value for sizing the main supply.

Reference

Based on IET On-Site Guide Appendix A and BS 7671 Regulation 311.1

Premises Type
Select the type of installation
Circuits
Add all circuits in the installation

Data: IET On-Site Guide — Appendix A (Diversity)

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Diversity reflects the reality that not all circuits in an installation draw maximum current at the same time.

The calculator applies standard diversity factors from the IET On-Site Guide to each circuit type. This reduces the total from the full connected load down to a realistic maximum demand figure. Each circuit category has its own allowance based on typical usage patterns in domestic premises.

Diversified Load = Connected Load × Diversity Factor
Connected Load
= Full rated current of the circuit (A)
Diversity Factor
= Reduction percentage from IET On-Site Guide

IET On-Site Guide Appendix A

Domestic Diversity Allowances

Standard diversity factors for domestic installations

IET On-Site Guide Appendix A
Circuit TypeDiversity Allowance
Lighting66% of total connected load
Socket outlets (ring)100% of largest + 40% of each remaining
Cooker10A + 30% of remainder + 5A if socket fitted
Immersion heater (thermostat)No diversity (100%)
Underfloor heating (thermostat)No diversity (100%)
Electric showerNo diversity (100%)

These allowances apply to domestic installations only. Commercial and industrial require separate assessment.

Practical Notes

Domestic Installations Only

The diversity factors in the IET On-Site Guide apply specifically to domestic installations. Commercial and industrial premises require a separate assessment based on actual usage patterns, often guided by IET Guidance Note 1 and site-specific load profiles.

Thermostatically Controlled Loads

Items with thermostatic control — immersion heaters, underfloor heating, storage heaters — still take no diversity reduction. During cold-start periods the element draws full current continuously, so the supply must be rated for this worst case even though the thermostat cycles the load during normal operation.

Overnight Full-Load Circuits

Storage heaters and EV chargers often run at full capacity simultaneously during off-peak hours overnight. Diversity does not apply to these loads, and their combined demand can be substantial. Always verify that the overnight load does not exceed supply capacity.

Frequently Asked Questions

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