How is domestic maximum demand calculated under BS 7671?

Under BS 7671 Regulation 311.1 and IET On-Site Guide Table A1, maximum demand applies standard diversity factors to connected loads: 66% for domestic lighting, 10A + 30% remainder + 5A for cooking appliances, 100% of the largest shower + 100% of the second, and 100% of the largest socket circuit + 40% of all other socket circuits.

Can diversity be applied to electric vehicle (EV) charging points?

No. Under BS 7671 Regulation 722.311, EV charging equipment is sized for 100% continuous unmanaged duty unless an Active Load Management (ALM) or Dynamic Load Balancing (DLB) system complying with Regulation 722.311.201 is installed to dynamically curtail charging current during peak household demand.

What is the 60 A continuous thermal stress threshold under ENA EREC G12/4?

ENA EREC G12/4 guidelines state that single-phase service equipment subjected to continuous simultaneous loads (such as EV charging and heat pump compressors) exceeding 60 A experiences accelerated thermal stress. Distribution Network Operators (DNOs) require formal assessment and notification before connecting continuous loads above 60 A.

← Back to Tools Directory EV Charging & Earthing Sizer →

Maximum Demand & Diversity Sizer

Assess maximum demand and circuit diversity for single-phase electrical installations under BS 7671:2018+A4:2026 Regulation 311.1 and IET On-Site Guide Appendix A (Table A1). Benchmarks calculated maximum demand against UK Distribution Network Operator (DNO) service head cut-out fuses (60 A, 80 A, 100 A), checks the 60 A continuous thermal stress limit (ENA EREC G12/4), and generates formatted text for electrical certification schedules.

1-Click Mobile Presets:Tap for instant on-site sizing

Select a preset above to auto-fill common circuit values

Supply & Premises ParametersDNO Cut-Out Fuse RatingStandard single-phase incoming supply fuse rating from the DNO.Premises ClassificationGoverns lighting diversity factor (66% domestic vs 90% commercial under Table A1).

Domestic & General Circuit LoadsConnected Lighting Load (Watts)Sum of all fixed lighting points. Table A1 applies 66% (domestic) or 90% (commercial).Cooking Appliances (Cooker / Hob / Oven Total kW)Total connected rating. Table A1 applies 10 A + 30% remainder + 5 A socket allowance.

Cooker control unit includes 13 A socket-outlet (+5 A)

Largest Space Heater (kW)Table A1 item 3: 100% of largest heating unit is assessed.Remaining Space Heaters (Total kW)Assessed at 75% of remaining load (or 100% if thermal storage / underfloor).

Thermal storage / underfloor heating (100% duty on all heaters)

Electric Shower 1 (kW)Instantaneous water heater / electric shower (largest unit).Electric Shower 2 (kW: if present)Second instantaneous shower or instantaneous water heater.

Priority changeover switch fitted between showers (interlocked)

Standard 30 A / 32 A Ring Final Socket CircuitsTable A1 item 9: 100% largest circuit (32 A) + 40% of all other socket circuits (12.8 A each).Standard 20 A Radial Socket Circuits20 A radial circuits evaluated at 40% (8 A each) after the largest circuit.

Modern Electrified Loads (Continuous Duty)Electric Vehicle Charger (EVSE kW)BS 7671 Reg 722.311: 100% continuous duty factor (no diversity without ALM).

Active Load Management (ALM) / Dynamic Load Balancing fitted (Reg 722.311.201)

Heat Pump Compressor Rating (kW)Rated compressor full-load running current at nominal 230 V.Heat Pump Auxiliary / Backup Immersion Heater (kW)Auxiliary direct electric element for legionella cycles or emergency backup.

Immersion interlocked against simultaneous compressor heating

Maximum Demand Assessment

Diversified Maximum Demand0.0 A0.0 kW @ 230 V single-phase

Undiversified Connected Load0.0 A (0.0 kW)

Diversity Load Reduction0.0 A (0.0%)

DNO Cut-Out Capacity (80 A)ADEQUATE

Utilization: 0.0%Headroom: 0.0 A

Calculating headroom...

Circuit Breakdown (OSG Table A1)

Load Group	Connected	Rule Applied	Demand

Show Full Mathematical Derivation (AM2 Working Standard)

BS 7671:2018+A4:2026

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

BS 7671 & IET On-Site Guide Diversity Principles

BS 7671:2018+A4:2026 Regulation 311.1 requires that for every design, the maximum demand of the installation shall be assessed. In domestic installations, diversity allows circuits to be sized for realistic simultaneous consumption rather than the arithmetic sum of connected loads. However, the rapid adoption of high-power continuous loads requires strict engineering vigilance:

1. Why Modern Electrified Loads Defeat Traditional Diversity

Traditional Table A1 diversity factors were developed when household loads were intermittent (kettles, toasters, cooking). Modern electrified loads break these assumptions:

Electric Vehicle Supply Equipment (EVSE): Under Regulation 722.311, no diversity factor may be applied to an EV charging point unless Active Load Management (ALM) or Dynamic Load Balancing (DLB) to Regulation 722.311.201 is installed. A 7.4 kW charger draws a constant 32.17 A for 4 to 8 consecutive hours.
Heat Pumps: Heat pump compressors run continuously for extended cycles during winter peak periods, coinciding with evening domestic lighting and cooking peaks. Auxiliary immersion heaters must be locked out by interlocks during compressor operation.
ENA EREC G12/4 Continuous 60 A Threshold: Energy Networks Association (ENA) standard G12/4 notes that single-phase service heads subjected to continuous currents >60 A experience elevated thermal fatigue. Installations exceeding 60 A continuous draw require formal DNO notification.

2. Cooking Appliance Clamping Rule

Under IET OSG Table A1 item 2, household cooking appliances are calculated as:

Demand (A) = 10 A + 30% × (I_flc - 10 A) + 5 A (if control unit has socket)

This tool incorporates a safe mathematical clamp: if the appliance full load current is ≤ 10 A (e.g. a small plug-in induction hob), the demand is capped at I_flc to avoid artificially inflating a small load.

3. Instantaneous Water Heating & Shower Interlocks

Table A1 item 4 assesses multiple instantaneous water heaters at 100% of the largest unit, 100% of the second unit, and 25% of any remaining units. Two 9.5 kW showers running simultaneously draw 41.30 A + 41.30 A = 82.6 A, instantaneously overloading an 80 A cut-out fuse on their own. Installing a mechanical or contactor-based shower priority changeover switch interlocks the circuits so only one shower can run at once, cutting water heating demand by half.