Diversity Factors Explained: A Practical Guide for Electricians
How to apply diversity factors to electrical circuit loads — tables, worked examples, and when diversity can and cannot be applied.
What Are Diversity Factors?
In any electrical installation with multiple circuits, it is unlikely that every circuit will draw its maximum rated load at the same time. A domestic kitchen circuit rated at 32A does not run at 32A continuously — the kettle boils for a few minutes, the toaster runs briefly, and the microwave cycles on and off. Diversity factors account for this reality by allowing the designer to reduce the assessed total maximum demand of the installation.
Without diversity, you would need to size the main cable, cut-out fuse, and supply capacity to handle the sum of every circuit at full load simultaneously. This would be unnecessarily expensive and impractical. Diversity is a recognised design tool that produces a more realistic assessment of the actual maximum demand the installation will place on the supply.
Key Distinction
Diversity applies to the total installation demand when sizing the main incoming cable and supply. It does not change the rating of individual circuit protective devices or the cable sizes on individual circuits — those are determined by the full design current of each circuit.
Where Do Diversity Factors Come From?
A common misconception is that diversity factors are specified in BS 7671 (the IET Wiring Regulations). They are not. BS 7671 Regulation 311.1 requires the designer to assess the maximum demand of every circuit, but it does not prescribe how diversity should be calculated.
The practical guidance comes from the IET On-Site Guide, Table 1B, which provides diversity allowances specifically for small domestic and similar installations. These values are based on decades of experience and statistical analysis of how domestic loads behave in practice.
It is important to understand that Table 1B is guidance, not a regulation. The designer has the discretion to apply more or less diversity depending on the specific circumstances of the installation. However, deviating from the published guidance requires sound engineering justification.
Related Course
18th Edition (2382)
Understanding BS 7671 requirements for maximum demand assessment is covered in the 18th Edition course.
How to Apply Diversity by Circuit Type
The IET On-Site Guide groups circuits by type and provides a diversity allowance for each. The general principles for common domestic circuit types are as follows.
Lighting Circuits
Lighting diversity is typically assessed at 66% of the total connected lighting load. The reasoning is straightforward: not every light in a dwelling will be switched on at the same time. In a house with 2,400W of total connected lighting, the assessed demand would be approximately 1,600W.
Cooking Appliances
For electric cookers (including hobs and ovens on a single circuit), the IET On-Site Guide uses a well-established formula: 10A plus 30% of the remainder of the total connected load plus 5A if a socket outlet is incorporated in the cooker control unit. For example, a cooker rated at 12kW (approximately 52A at 230V) would be assessed as: 10A + 30% of (52A - 10A) = 10A + 12.6A = 22.6A. This reflects the fact that all rings, oven elements, and grill are rarely at maximum output simultaneously.
Socket Outlet Circuits (Ring Finals and Radials)
For socket outlets on ring and radial circuits, the typical diversity approach is 100% of the largest circuit plus 40% of each remaining circuit. In a dwelling with three ring final circuits rated at 32A each, the assessed demand would be: 32A + (0.4 x 32A) + (0.4 x 32A) = 32A + 12.8A + 12.8A = 57.6A. This acknowledges that heavy loads are rarely drawn from all ring circuits simultaneously.
Immersion Heaters
Immersion heaters are assessed at no diversity — 100% of the connected load. When an immersion heater is on, it draws its full rated current continuously until the thermostat switches it off. There is no statistical basis for reducing this demand.
Electric Showers
A single instantaneous electric shower is also assessed at 100% of its rated load. Electric showers draw their full current whenever in use. Where two showers are installed, it is reasonable to apply some diversity on the basis that both are unlikely to be used at exactly the same time — but the designer must exercise judgement based on the specific installation.
Space Heating
For thermostatically controlled space heating, the On-Site Guide allows diversity on the basis that not all heaters will be calling for heat simultaneously. The first kilowatt is typically taken at full load, with a reduced percentage applied to the remainder. The exact approach depends on the type of heating system and its controls.
Refer to the Source
The values above describe general principles. Always refer to the current edition of the IET On-Site Guide (Table 1B) for the exact diversity allowances. The table includes additional circuit types and notes that affect how diversity is applied.
Worked Example: Domestic Dwelling
Consider a typical three-bedroom house with the following circuits. We will assess the maximum demand with and without diversity to show the practical impact.
Connected Loads (Without Diversity)
- ✓Lighting circuit 1: 1,200W (approximately 5.2A)
- ✓Lighting circuit 2: 1,000W (approximately 4.3A)
- ✓Ring final circuit 1 (downstairs sockets): rated 32A
- ✓Ring final circuit 2 (upstairs sockets): rated 32A
- ✓Cooker circuit: 10.5kW (approximately 45.7A)
- ✓Immersion heater: 3kW (13A)
- ✓Electric shower: 9.5kW (41.3A)
Simply adding these together: 5.2A + 4.3A + 32A + 32A + 45.7A + 13A + 41.3A = 173.5A total without diversity. This would require an enormous supply and main cable — far beyond the standard 100A domestic supply.
With Diversity Applied
Now applying the diversity principles from the IET On-Site Guide:
- ✓Lighting (66% of total): 0.66 x (5.2A + 4.3A) = 6.3A
- ✓Socket outlets (100% of largest + 40% of remainder): 32A + (0.4 x 32A) = 44.8A
- ✓Cooker (10A + 30% of remainder): 10A + 0.3 x (45.7A - 10A) = 10A + 10.7A = 20.7A
- ✓Immersion heater (100%): 13A
- ✓Electric shower (100%): 41.3A
Total assessed maximum demand: 6.3A + 44.8A + 20.7A + 13A + 41.3A = 126.1A. This is a significant reduction from 173.5A, and it reflects a realistic assessment of the actual load the supply will experience. This would typically be served by a 100A supply with appropriate main cable sizing — the designer must verify this is adequate and consider whether a supply upgrade is needed.
Design Decision
In this example, the assessed demand of 126.1A exceeds the standard 100A domestic supply. The designer must either request a supply upgrade from the DNO (Distribution Network Operator), review whether additional diversity is justifiable, or consider whether loads can be managed (for example, immersion heater on a timer that avoids peak periods). This is a real-world design consideration, not just a calculation exercise.
When NOT to Apply Diversity
Diversity is a powerful design tool, but it is not appropriate in every situation. There are important circumstances where diversity must not be applied or where a different assessment method is required.
Individual Circuit Calculations
When sizing the cable and protective device for a single circuit, you must use the full design current of that circuit. Diversity applies to the aggregate demand at the origin of the installation, not to individual circuits. A 32A ring final circuit must be designed to carry 32A, regardless of how much diversity is applied at the distribution board level.
Commercial and Industrial Installations
The diversity values in Table 1B of the IET On-Site Guide apply specifically to small domestic and similar installations. Commercial and industrial premises have fundamentally different load profiles. A factory production line, a commercial kitchen, or a data centre may well run all connected equipment at full load simultaneously. Maximum demand assessment for these installations requires specific engineering knowledge of the processes involved, often supported by actual load measurements or design data from equipment manufacturers.
Medical Locations
BS 7671 Section 710 covers electrical installations in medical locations. Life-support equipment and critical medical systems must be assumed to operate at full load. Diversity must not compromise the reliability of supply to these critical loads. The design of medical location supplies is a specialist area requiring additional expertise.
Testing and Verification
When carrying out testing — for example, measuring earth fault loop impedance (Zs) or prospective fault current — diversity is irrelevant. The consumer unit must be adequately sized regardless of diversity assumptions. These measurements relate to fault conditions on individual circuits and the ability of the protective device to operate within the required disconnection time. Test values must comply with the limits specified in BS 7671 regardless of any diversity assumptions used in the design.
Critical and Continuous Loads
Any load that runs continuously at its rated current — such as an immersion heater, a storage heater during its charge period, or a continuously-running motor — should be assessed at 100%. Diversity is based on the statistical probability that loads will not coincide. Where a load is known to operate continuously, there is no statistical basis for reduction.
Designer Responsibility
The application of diversity is a design decision. The designer signs the Electrical Installation Certificate (EIC) and takes professional responsibility for the assessment of maximum demand. If the supply is undersized because diversity was applied too aggressively, the designer is accountable. When in doubt, err on the side of caution — it is always safer to overestimate demand than to underestimate it.
Common Mistakes
Several errors are frequently made when applying diversity factors. Being aware of these will help you avoid problems in practice and in examinations.
Applying Diversity to Individual Circuits
Diversity is applied to the total demand at the origin of the installation (or at a distribution board supplying multiple final circuits). It is not applied to reduce the design current of a single final circuit. Each circuit must still be designed for its full rated load.
Double-Counting Diversity
If a sub-distribution board feeds several circuits and diversity has already been applied at that board, do not apply further diversity to the same circuits at the main distribution board. Diversity should be applied once — at the point where the total demand needs to be assessed.
Using Diversity in Zs and Fault Current Calculations
Earth fault loop impedance (Zs) and prospective fault current (PSCC/PEFC) calculations are based on the circuit parameters under fault conditions. Diversity has no relevance here. The protective device must disconnect within the required time for the actual fault current on that specific circuit.
Applying Domestic Values to Non-Domestic Installations
Table 1B values are for small domestic and similar installations only. Applying these values to a commercial kitchen, workshop, or industrial unit would likely underestimate the maximum demand, potentially resulting in an undersized supply and overloaded cables.
Ignoring Future Load Growth
While diversity reduces the assessed current demand, a prudent designer also considers future load growth. The increasing adoption of electric vehicles, heat pumps, and battery storage means domestic maximum demand is trending upwards. Leaving some spare capacity in the design is good practice.
Related Course
18th Edition (2382)
The 18th Edition course covers maximum demand assessment and the correct application of diversity as part of installation design.
Frequently Asked Questions
Where are diversity factors published?
Can I apply diversity to a single circuit?
Do I apply diversity when calculating Zs values for testing?
Is diversity the same for commercial and domestic installations?
Who is responsible if diversity is applied incorrectly?
Ready to Start Training?
Browse our City & Guilds accredited courses and take the next step in your electrical career.
Related Guides
How to Calculate Maximum Demand for Electrical Installations
A practical guide to calculating maximum demand — formulae, diversity factors, worked examples, and BS 7671 requirements.
Read guideVolt Drop Calculations: BS 7671 Limits & Worked Examples
How to calculate voltage drop in electrical circuits — formulae, BS 7671 limits, cable selection tables, and worked examples.
Read guideZs Values & Earth Fault Loop Impedance Explained
What Zs values are, how earth fault loop impedance works, the Zs = Ze + R1+R2 formula, and maximum permitted values from BS 7671.
Read guideThree-Phase Power for Electricians: Fundamentals & Applications
Three-phase power explained — how it works, calculations, star and delta configurations, and when electricians encounter it.
Read guide