How to Calculate Maximum Demand for Electrical Installations

Maximum demand is the greatest electrical load expected to be drawn from a supply point at any one time. It is a fundamental concept in electrical installation design because it determines the size of the incoming supply, the main protective devices, and the distribution cables that feed the installation.

In practice, the total connected load of an installation is almost always greater than the maximum demand. A typical domestic property might have 40kW or more of connected appliances, but the actual maximum demand could be around 25 to 30kW because not everything runs at the same time. The process of accounting for this realistic, simultaneous usage is called applying diversity factors.

Getting maximum demand right matters for three reasons: it ensures cables and protective devices are correctly sized (too small and they overheat; too large and costs increase unnecessarily), it determines whether the existing supply from the Distribution Network Operator (DNO) is adequate, and it is a requirement of BS 7671 to assess the maximum demand of every installation.

The basic principle is straightforward: assess the load on each circuit, then apply diversity factors to arrive at a realistic total.

Step 1: List All Circuits and Their Loads

For each circuit in the installation, determine the design current or total connected load in watts. For fixed appliances (showers, cookers, immersion heaters), use the rated power of the appliance. For socket outlet circuits, use the assumed maximum load for the circuit type — typically 7200W for a ring final circuit using the standard method.

Step 2: Apply Diversity Factors

Diversity factors reduce the total connected load to reflect realistic simultaneous usage. The factors vary by circuit type and are published in the IET On-Site Guide, Table 1B. For example, lighting circuits typically attract a 66% diversity factor (after the first circuit), while socket outlet circuits use 100% for the first circuit and 40% for additional circuits.

Step 3: Sum the Assessed Loads

After applying diversity, sum all the individual circuit assessments. This total is the assessed maximum demand of the installation in watts. Divide by the supply voltage (230V for single-phase) to obtain the maximum demand in amperes.

The IET On-Site Guide (a companion publication to BS 7671) provides diversity allowances in Table 1B for individual domestic installations. These factors are categorised by the type of circuit or load. The following is a summary of the categories covered — you should always refer to the current edition of the On-Site Guide for the exact values.

Lighting Circuits

Lighting diversity recognises that not all lights in a property will be on at the same time. The On-Site Guide allows a significant reduction from the total connected lighting load. Typically, 66% of the total lighting load is used for the assessment.

Socket Outlet Circuits (Ring Finals and Radials)

For socket outlet circuits, the diversity approach uses the full load for the first or highest-loaded circuit, then a reduced percentage for each additional circuit. This reflects the fact that while one ring final might be heavily loaded, it is unlikely that all ring finals will be at maximum simultaneously.

Cooking Appliances

Cooking appliance diversity uses a specific formula: the first 10A of the rated current is taken at 100%, then 30% of the remaining current is added, plus 5A if a socket outlet is incorporated in the cooker control unit. This recognises that all hob rings and the oven are rarely at full power simultaneously.

Motors and Other Fixed Appliances

Individual motors and fixed appliances such as immersion heaters, towel rails, and electric showers are assessed at their full rated load when there is only one of each type. Where there are multiple similar appliances, diversity may be applied between them as specified in the On-Site Guide.

Electric Space Heating

For properties with electric space heating (storage heaters or panel heaters), the On-Site Guide provides specific guidance. All heating circuits are often assessed at 100% because the heating system is designed to operate all heaters simultaneously, particularly during overnight charging periods for storage heaters.

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The 18th Edition course covers maximum demand assessment and the application of BS 7671 to installation design.

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The following worked example demonstrates how to assess maximum demand for a typical three-bedroom house. The values used are illustrative — real calculations depend on the specific appliances and installation details.

Connected Loads

• ✓Lighting: 2 circuits at 1200W each = 2400W total
• ✓Ring final circuits: 2 circuits at 7200W each = 14,400W total
• ✓Cooker: 1 circuit, 10kW rated appliance
• ✓Electric shower: 1 circuit, 9.5kW rated
• ✓Immersion heater: 1 circuit, 3kW rated

The total connected load is approximately 39.3kW. However, this is not the maximum demand — diversity must be applied.

Applying Diversity

Lighting (2400W at 66% diversity): 2400 x 0.66 = approximately 1584W. This reflects that not every light in the property will be on at the same time.

Ring finals (first circuit at 100%, additional at 40%): First ring final at 7200W (100%) = 7200W. Second ring final at 7200W (40%) = 2880W. Total socket outlet contribution = 10,080W.

Cooker (10A + 30% of remainder): The cooker is rated at 10kW, which is approximately 43.5A at 230V. Using the On-Site Guide method: first 10A at 100% = 2300W, plus 30% of the remainder (43.5A - 10A = 33.5A, x 0.3 = 10.05A = approximately 2312W). Total cooker contribution = approximately 4612W, which we can round to approximately 5100W including a socket outlet allowance.

Electric shower (9.5kW at 100%): A single shower circuit is assessed at its full rated load. No diversity is applied to a single fixed appliance of this type. Contribution = 9500W.

Immersion heater (3kW at 100%): Similarly, a single immersion heater is taken at its full rated load. Contribution = 3000W.

Total Assessed Maximum Demand

Adding the assessed contributions: 1584 + 10,080 + 5100 + 9500 + 3000 = approximately 29.3kW.

Converting to current at 230V: 29,264 / 230 = approximately 127A.

This tells us that a standard 100A single-phase domestic supply may not be sufficient for this installation if all major loads operate simultaneously. In practice, the diversity applied by the On-Site Guide is conservative, and the actual simultaneous demand in a typical household will usually be lower. However, the designer must consider whether to request a supply upgrade from the DNO.

While diversity reduces the assessed maximum demand in most domestic situations, there are important cases where it must not be applied or must be applied with extreme caution.

Single Circuits

Diversity is applied between circuits, not within a single circuit. Each circuit must be designed for its full design current. A 9.5kW shower circuit, for example, must be cabled and protected for the full 41.3A — you cannot reduce this on the assumption that the shower will not always run at maximum.

Critical Loads and Essential Services

In installations where loss of supply would cause danger — such as emergency lighting, fire alarm systems, or life-support equipment — diversity should not be applied. These circuits must be assessed at 100% of their rated load to ensure the supply can always meet the demand.

Medical Locations

BS 7671 Section 710 covers medical locations. In Group 2 medical locations (such as operating theatres and intensive care units), maximum demand assessment must account for the full load of all life-critical equipment without diversity reduction. The consequences of supply failure in these environments are too severe to allow for assumptions about simultaneous usage.

Electric Vehicle Charging

EV chargers are typically long-duration loads that can run for several hours at full rated power. When assessing maximum demand for an installation that includes EV charging, the charger should generally be assessed at 100% of its rated current, particularly if it is likely to charge overnight when other high-demand circuits (such as electric heating) may also be operating.

• ✓Single circuits: always sized for full design current, no diversity within a circuit
• ✓Emergency and safety systems: assessed at 100% — no diversity applied
• ✓Medical locations (Group 2): full rated load for life-critical equipment
• ✓EV chargers: typically assessed at 100% due to long-duration operation
• ✓Electric space heating: often assessed at 100% (all heaters may run simultaneously)

The maximum demand assessment directly informs whether the existing supply from the Distribution Network Operator (DNO) is adequate for the installation.

Typical Domestic Supply

Most domestic properties in the UK have a single-phase 230V supply rated at either 60A or 100A, depending on the age of the property and the service cable. Modern installations typically have a 100A supply with a 100A main fuse (or service cut-out) provided by the DNO. This gives a maximum available capacity of approximately 23kW.

When a Supply Upgrade Is Needed

If the assessed maximum demand exceeds the existing supply capacity, the electrician must arrange a supply upgrade with the DNO before the installation can be completed. Common scenarios requiring an upgrade include:

• ✓Adding a high-power electric shower to an installation that already has significant loads (check volt drop calculations for long cable runs)
• ✓Installing an EV charger alongside existing electric heating
• ✓Full house rewire revealing that the existing supply is only 60A
• ✓Adding electric underfloor heating or an electric boiler
• ✓Commercial conversion of a domestic property requiring higher capacity

Requesting a Supply Upgrade

To request a supply upgrade, contact the local DNO (not the energy supplier). You will need to provide the assessed maximum demand and details of the installation. The DNO will assess whether the local network can support the increased load. Upgrades can take several weeks and may involve replacing the service cable and cut-out. There will be a cost, which varies by DNO and the extent of work required.

Three-Phase Supply

For very large domestic installations or where the single-phase supply is insufficient, a three-phase supply may be required. Three-phase provides approximately three times the capacity of a single-phase supply of the same amperage. However, three-phase installations require careful balancing of loads across the three phases to avoid neutral current imbalance, and the consumer unit or distribution board must be suitable for three-phase operation.

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The 2391 qualification covers testing and certification of electrical installations, including verifying that the supply is adequate for the assessed maximum demand.

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