What is electrical load calculation and why is it important?

Electrical load calculation determines the maximum power demand a property will draw from the supply. It's essential for sizing the main supply (60A, 80A, 100A), ensuring the consumer unit can handle all circuits, and preventing overloading which causes tripped breakers or fire risk. Every new installation or major upgrade should start with a load calculation.

What are diversity factors in BS 7671?

Diversity factors recognise that not all electrical loads operate simultaneously at full capacity. BS 7671 Appendix 1 Table 1A provides standard factors: lighting at 66%, socket outlets with first 10A at 100% then 30%, cookers at 10A plus 30% of remainder. These factors convert total connected load into realistic maximum demand.

How do I know if I need a supply upgrade?

If your calculated maximum demand exceeds your current supply rating (typically 60A, 80A, or 100A), you need an upgrade. Signs include frequently tripping main switch, wanting to add high-demand items (EV charger, heat pump, electric shower), or the DNO advising your supply is at capacity. Contact your DNO (Distribution Network Operator) for upgrades.

What is the typical supply rating for a UK house?

Older properties often have 60A or 80A supplies. Modern properties typically have 100A single-phase supplies. High-demand properties with EV chargers, heat pumps, and electric heating may need 100A+ or even three-phase supply. Check your main fuse or service head for your current rating.

How do I calculate load for an EV charger installation?

A 7.4kW home EV charger draws 32A continuously - this is added at 100% diversity as it's a sustained load. Add this to your existing maximum demand. If total exceeds supply capacity, you may need a supply upgrade or smart load management system that limits charging when other high loads are active.

What diversity applies to electric showers?

Electric showers are calculated at 100% diversity (no reduction) because when in use, they draw full rated power. A 10.8kW shower draws 47A - nearly half a 100A supply. Multiple electric showers in a property significantly impact maximum demand calculations.

How do I calculate socket outlet diversity?

BS 7671 uses a stepped approach: first 10A (2.3kW) at 100%, remainder at 30%. So a 32A ring main (7.36kW connected) calculates as: 2.3kW + (5.06kW × 0.3) = 3.82kW maximum demand. This recognises you won't use every socket at full capacity simultaneously.

What about cooker diversity calculations?

Cookers use: first 10A at 100%, plus 30% of remainder, plus 5A if there's a socket outlet. A 45A cooker circuit (10.35kW) calculates as: 2.3kW + (8.05kW × 0.3) + 1.15kW = 5.87kW. This accounts for not using all hobs and oven at maximum simultaneously.

Should I include future loads in my calculation?

Yes - always plan ahead. If you're likely to add an EV charger, heat pump, or additional circuits within 5-10 years, include them now. It's much cheaper to install adequate supply capacity upfront than to upgrade later. Aim for at least 20% headroom on your supply.

What's the difference between connected load and maximum demand?

Connected load is the sum of all circuit ratings - the theoretical maximum if everything ran at once. Maximum demand applies diversity factors to calculate realistic peak draw. A house might have 50kW connected load but only 15kW maximum demand. Supplies are sized to maximum demand, not connected load.

How do storage heaters affect load calculations?

Storage heaters charge overnight on Economy 7/10 tariffs, typically 1.7-3.4kW each. They're calculated at 100% diversity since they all charge simultaneously. Multiple storage heaters can create significant overnight demand - ensure your supply and main switch can handle the total.

Do I need a three-phase supply?

Three-phase is typically needed when single-phase demand exceeds 100A (23kW), or for large motors/equipment requiring three-phase power. Common in larger houses with heat pumps + EV chargers + electric heating. Three-phase provides 3× the capacity but requires different equipment and higher standing charges.

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Electrical Load Calculator UK

Calculate maximum demand with BS 7671 diversity factors

Domestic & commercial • Supply sizing • All circuit types

Load Calculation Quick Facts

Diversity factors reduce connected load to realistic maximum demand
Standard domestic supply: 60A (older), 80A (standard), 100A (high demand)
EV chargers and heat pumps typically require 100A supply minimum
Always allow 20%+ headroom for future additions
Contact your DNO if calculated demand exceeds current supply

Calculate Maximum Demand

1. Property Type

Quick Start Templates

2. Add Circuits

Complete Your Electrical Design

Load calculation is step one. Once you know your maximum demand, continue with cable sizing and voltage drop verification:

Load Calculation

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Cable Sizing →

Size cables for each circuit

Voltage Drop →

Verify compliance

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Understanding Maximum Demand

Maximum demand is the highest rate of electrical power a property is likely to draw at any given moment. It's always lower than the connected load because diversity factors account for the reality that not everything runs simultaneously at full power.

For example, a house might have 50kW of connected load (all circuits at maximum), but the maximum demand might only be 15kW because you're never using every socket, every light, the cooker, shower, and immersion heater all at once.

Example: Typical 3-Bed House

2× Lighting circuits (6A each): 2.76kW connected → 1.82kW after diversity

2× Ring mains (32A each): 14.72kW connected → 6.02kW after diversity

1× Cooker (45A): 10.35kW connected → 5.87kW after diversity

1× Shower (10.8kW): 10.8kW connected → 10.8kW (no diversity)

Total: 38.6kW connected → 24.5kW maximum demand (63A)

This property would need a minimum 80A supply, with 100A recommended for headroom. Adding an EV charger (7.4kW) would push demand to ~32kW (nearly 80A), making 100A essential. Learn more about sizing cables for these circuits.

UK Domestic Supply Ratings

Common supply ratings and their suitability for different property types

Supply	Max kW	Typical Property	Suitable For
60A	13.8	Older properties, basic loads	No EV, no electric heating, gas heating
80A	18.4	Standard modern property	Electric shower + cooker, possible small EV charger
100A	23.0	Higher demand property	EV charger + heat pump OR multiple high-demand circuits
3-Phase	69.0	Large/all-electric properties	Multiple EVs, heat pump, all-electric, large property

kW calculated at 230V single phase (400V for three-phase). Actual capacity may vary - check with your DNO.

Common Circuit Loads & Diversity

Circuit Type	Typical Rating	Typical Load	Diversity Factor
Lighting (per circuit)	6A	1.4kW	66%
Ring Main 32A	32A	7.4kW	10A + 30%
Electric Shower	40-50A	8-11kW	100%
Electric Cooker	32-45A	8-12kW	10A + 30%
EV Charger 7.4kW	32A	7.4kW	100%
Heat Pump (ASHP)	20-32A	4-8kW	80%
Immersion Heater	16A	3kW	100%
Storage Heater	13A	2-3kW	100%

Diversity factors per BS 7671 Appendix 1. "10A + 30%" means first 10A at 100%, remainder at 30%.

EV Chargers & Heat Pumps

The transition to electric vehicles and heat pumps is significantly increasing domestic electrical demand. Both are high-load, long-duration circuits that require careful consideration.

EV Charger (7.4kW)

• 32A dedicated circuit required
• 100% diversity (continuous load)
• Often triggers 100A supply upgrade
• Consider smart charging for load management

EV charger cable sizing →

Air Source Heat Pump

• Typically 4-12kW depending on property
• 80% diversity (thermostat controlled)
• May run simultaneously with immersion backup
• Combined with EV often needs 100A+

Calculate heating load →

Practical Installation Tips

• Always allow 20% headroom on supply for future additions
• Check existing supply capacity before designing - look at main fuse rating
• Consider load management systems for properties near supply limits
• DNO supply upgrades can take 6-12 weeks - factor into project timelines
• Document your load calculation for Building Control and Part P certification

Related Tools for Electricians

After calculating load, you'll need to size your cables correctly. Use our cable sizing calculator with full derating factors, then verify voltage drop for longer runs. For containment, check our conduit fill calculator. See all our electrical calculators.

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