Diversity & Maximum Demand Calculator

Calculate the maximum demand for electrical installations by applying diversity factors according to BS 7671:2018 On-Site Guide Appendix A

Maximum Demand with Diversity

Calculate the maximum demand for an installation by applying diversity factors according to BS 7671 On-Site Guide Appendix A. Add all circuits and their loads below.

Select the type of installation - diversity factors vary by premises type

Circuits & Loads

A

Design current for this circuit

Number of identical circuits

Typical Load Values (for reference)
Lighting
  • • LED bulb: ~0.04A (10W)
  • • LED downlight: ~0.03A (7W)
  • • Circuit (10 lights): ~0.5A
Heating
  • • Storage heater: ~13A (3kW)
  • • Panel heater: ~8.7A (2kW)
  • • Underfloor (per m²): ~0.65A
Appliances
  • • Electric shower: ~41A (9.5kW)
  • • Electric cooker: ~48A (11kW)
  • • Washing machine: ~10A
  • • EV charger (7kW): ~30A

To convert Watts to Amps (single-phase): Amps = Watts ÷ 230V

Understanding Diversity and Maximum Demand

When designing electrical installations, it's important to recognize that not all circuits will operate at full load simultaneously. Diversity (also called demand factor or coincidence factor) is the principle of applying reduction factors to the total connected load to determine the actual maximum demand of an installation. This is required by BS 7671 Regulation 311.1.

What is Diversity?

Diversity is the reduction factor applied to the total connected load to account for the fact that not all equipment will be used at maximum capacity at the same time. This allows for more economical sizing of main switches, cables, and supply equipment.

BS 7671 Requirement

Regulation 311.1 states: "In determining the maximum demand of an installation, diversity may be taken into account." Guidance on diversity factors is provided in the IET On-Site Guide Appendix A.

Application

Diversity should only be applied by competent persons with relevant knowledge and experience of the installation type and usage patterns. Conservative estimates should be used where uncertainty exists.

Diversity Factors by Premises Type (BS 7671 On-Site Guide Appendix A)

Domestic (Individual Households/Flats)

  • • Lighting: 66% of total connected load
  • • Heating: 100% up to 10A + 50% above 10A
  • • Cooking: 10A + 30% of remainder (+ 5A if socket on cooker control)
  • • Water Heaters (instant): 100% largest + 100% second + 25% remainder
  • • Water Heaters (storage): 100% (no diversity)
  • • Floor Warming: 100% (no diversity)
  • • Storage Heaters: 100% (no diversity)
  • • Socket Outlets: 100% of largest appliance + 40% at every other point

Small Shops/Stores

  • • Lighting: 90% of total connected load
  • • Heating: 100% up to 10A + 75% above 10A
  • • Socket Outlets: 75% of total connected load

Small Offices

  • • Lighting: 90% of total connected load
  • • Heating: 100% up to 10A + 80% above 10A
  • • Socket Outlets: 75% of total connected load

Small Hotels/Guest Houses

  • • Lighting: 75% of total connected load
  • • Heating: 100% up to 10A + 80% above 10A
  • • Socket Outlets: 60% of total connected load
  • • Water Heaters (instant): 100% largest + 80% remainder

When to Apply Diversity

Diversity factors can be applied to:

  • Main Switch Sizing: Determine the required rating of the main switch or incomer based on actual maximum demand rather than total connected load
  • Supply Cable Sizing: Size main supply cables based on maximum demand including appropriate derating factors
  • Distribution Board Selection: Select distribution board ratings appropriate to the maximum demand of circuits connected
  • Generator Sizing: Size backup generators or UPS systems based on realistic maximum demand

Important Considerations

  • ⚠️ Special Knowledge Required: Diversity application requires knowledge and experience - consult IET Guidance if unsure
  • ⚠️ No Single Formula: No single diversity formula applies to all installations - consider actual usage patterns
  • ⚠️ Future Expansion: Consider potential future load increases when applying diversity
  • ⚠️ Critical Circuits: Do not apply diversity to critical circuits that must remain operational (fire alarms, emergency lighting, etc.)
  • ⚠️ Distribution Boards: Distribution boards must be rated for the total connected load without diversity
  • ⚠️ Circuit Protection: Individual circuit protection must be based on circuit design current without diversity

Example: 3-Bedroom House Calculation

Connected Load:

  • • Lighting (5 circuits × 2A): 10A
  • • Sockets (4 circuits × 32A): 128A
  • • Electric cooker: 48A (11kW)
  • • Electric shower: 41A (9.5kW)
  • • Immersion heater: 13A (3kW)

Total Connected Load: 240A

After Diversity:

  • • Lighting: 10A × 66% = 6.6A
  • • Sockets: 32A + (96A × 40%) = 70.4A
  • • Cooker: 10A + (38A × 30%) = 21.4A
  • • Shower: 41A × 100% = 41A
  • • Immersion: 13A × 100% = 13A

Maximum Demand: 152.4A → Use 160A or 200A main switch

Standards & References

  • • BS 7671:2018+A2:2022 Regulation 311.1 - Assessment of general characteristics (Maximum demand)
  • • IET On-Site Guide Appendix A - Maximum demand and diversity
  • • IET Guidance Note 1: Selection & Erection - Chapter 3 (Assessment of general characteristics)
  • • Table A1 - Current demand to be assumed for points of utilization and current-using equipment
  • • Table A2 - Allowance for diversity