Load Calculator UK - Electrical Design & Planning Tools for Electricians

Electrical Installation Design Process

Proper design is crucial for safe, compliant, and cost-effective electrical installations. Follow this systematic approach to ensure your designs meet BS 7671 requirements.

1️⃣

Assess Requirements

Determine the purpose, load requirements, and special conditions of the installation

2️⃣

Calculate Load

Use Load Calculator to determine total connected load and apply diversity factors

3️⃣

Select Main Equipment

Size consumer unit, main switch, and distribution boards based on maximum demand

4️⃣

Design Circuits

Size cables, select protection, and verify voltage drop and Zs compliance

5️⃣

Document & Cost

Create circuit schedules, estimate costs, and prepare installation documentation

Key BS 7671 Design Regulations

Part 3 - Assessment of General Characteristics:

• Regulation 311 - Maximum demand and diversity
• Regulation 313 - Supplies for safety services

Appendix 1 - Diversity Factors:

• Domestic installations diversity table
• Commercial/industrial diversity factors

Load Calculation & Diversity Factors

Maximum demand is rarely equal to the sum of all connected loads. BS 7671 Appendix 1 provides diversity factors to calculate realistic maximum demand for sizing main equipment.

Domestic Diversity (Appendix 1)

Lighting:66% of total load

Heating & Power:100% up to 10A + 50% remainder

Cooking:10A + 30% of remainder + 5A if >13.5kW

Water Heating:100% (instantaneous) or 25% (storage)

Socket Outlets:100% up to 10A + 40% remainder

Commercial/Industrial Diversity

Diversity factors vary significantly based on installation type and usage patterns:

• Office Buildings: 75-90% diversity typical

• Retail: 80-100% (high diversity)

• Industrial: Varies by process (consult with client)

• Hotels: 60-80% depending on size

• Schools: 70-85% diversity

Always verify actual usage patterns with client and consider future expansion

Important: Diversity factors are guidance only. Designer must consider actual usage patterns, simultaneous operation, and safety margins. For critical installations, conservative diversity factors should be used.

Main Equipment Sizing Guide

Consumer Unit / Distribution Board

Select based on maximum demand calculation and number of circuits required

• Main switch rating: ≥ maximum demand current

• Way count: Current circuits + 20-30% spare

• Breaking capacity: Must exceed PSCC at origin

• Amendment 2: AFDD requirements for certain circuits

Supply Cable Sizing

Submain or service cable from meter to consumer unit

• Current capacity: ≥ maximum demand current

• Voltage drop: Typically minimal at short distances

• Installation method: Affects derating

• Common: 25mm² (100A), 35mm² (125A), 50mm² (160A)

Earthing System

TN-S, TN-C-S (PME), or TT system earthing arrangements

• Main earthing conductor: Per BS 7671 Table 54.7

• Main protective bonding: Per Table 54.8

• RCD protection: Required for TT systems

• PME restrictions: Special locations (Reg 722)

Renewable Energy & Future-Proofing

Solar PV Integration

BS 7671 Section 712 covers requirements for solar photovoltaic power supply systems

✓ AC and DC isolators required

✓ Overvoltage protection (SPDs)

✓ Cable sizing for DC side (higher ambient temp)

✓ String monitoring and fault detection

✓ G99 compliance for grid connection

✓ Battery storage integration (Section 551.9)

EV Charging Infrastructure

BS 7671 Section 722 covers electric vehicle charging installations

✓ Dedicated circuit required for each charge point

✓ RCD protection: Type A minimum, Type B preferred

✓ 6mA DC fault detection (IEC 62955)

✓ Load management for multiple chargers

✓ Consider diversity with existing installation

✓ Cable sizing: 6mm² (32A) or 10mm² (40A) typical

Future-Proofing Recommendations

Oversizing Considerations:

• Main switch: 100A minimum for domestic (consider future EV/solar)
• Spare ways: 20-30% for future circuits
• Conduit: 40-50% capacity for future cables

Smart Home Integration:

• Energy monitoring capability
• Network infrastructure (Cat6/fiber)
• USB charging outlets at sockets

Installation Cost Planning

Accurate cost estimation is essential for project planning and client quotations. Consider all material, labor, and testing costs.

Material Costs

• Consumer units / DBs
• Cables and containment
• Protective devices (MCBs, RCDs, AFDDs)
• Accessories and fittings
• Earthing and bonding materials
• Add 10-15% contingency

Labor Costs

• Installation time (per point/circuit)
• First fix and second fix
• Testing and commissioning
• Certification and documentation
• Site supervision
• Factor in skill level required

Additional Costs

• DNO connection fees
• Building control notification
• Certification scheme fees
• Equipment hire
• Site access/parking
• Remedial work allowance

💡 Pro Tip: Use the Installation Cost Estimator to quickly calculate material and labor costs based on industry averages and regional pricing data.