Electrical Load Assessment: Maximum Demand Guide

An electrical load assessment is a calculation that determines the maximum electrical demand (in amps or kilowatts) that an installation will place on its supply. It is the fundamental starting point for any design decision about an electrical installation, whether you are designing a new installation from scratch or adding load to an existing one.

The assessment considers every fixed electrical load in the property: lighting circuits, socket outlets, cooker, shower, immersion heater, heating system, and any specialist loads such as EV chargers, heat pumps, or workshop equipment. Each load is recorded with its rated current, and diversity factors are applied to account for the fact that not every load operates simultaneously.

The result is the assessed maximum demand, which is compared against the available supply. For most UK domestic properties, the supply is a single-phase 100 A supply with a 60 A, 80 A, or 100 A service fuse. If the assessed maximum demand exceeds the supply capacity, the installation cannot safely operate and a supply upgrade is required before the new load can be connected.

Load assessments are not optional. Regulation 311.1 of BS 7671 requires that the maximum demand of every installation is assessed to ensure that the supply characteristics are adequate. Getting this wrong means the supply fuse blows under peak load, or worse, the supply cable overheats because the fuse rating has been increased without upgrading the cable.

A load assessment should be carried out whenever the electrical demand of a property is likely to change. The most common triggers are:

Maximum demand is calculated by listing every fixed electrical load in the installation, recording its rated current, and then applying diversity factors to produce a realistic total. Without diversity, the raw total of all loads would produce an unrealistically high figure because it assumes every load operates simultaneously at full capacity.

Step 1: List All Loads

Record every fixed electrical load with its rated current in amps. Common domestic loads include:

Step 2: Apply Diversity

Diversity factors from BS 7671 Appendix A, Table A1 are applied to each load category. These factors account for the reality that not all loads run at full power simultaneously. The diversity factor calculator in Elec-Mate applies these automatically.

Step 3: Sum the Diversified Loads

Add up all the diversified load values to produce the assessed maximum demand. Compare this figure against the available supply (typically 60 A, 80 A, or 100 A for domestic single-phase). If the assessed demand exceeds the supply, action is needed.

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Diversity is the principle that not all electrical loads in an installation will operate simultaneously at their maximum rating. A house with a 45 A cooker, a 41 A shower, a 32 A EV charger, and multiple ring circuits does not draw 150+ A because not all of these loads run at the same time at full output.

BS 7671 Appendix A, Table A1 provides standard diversity factors for domestic premises. These factors specify what proportion of each load category should be included in the maximum demand calculation. For example, the first 10 A of cooking appliance load is taken at 100%, but the remainder is taken at 30%. For socket outlets, the first ring circuit is assessed at 100% of the connected load, and additional circuits at 40%.

Diversity factors do not apply to individual circuits for cable sizing purposes. Each circuit cable must be sized for the full expected load on that circuit. The cable sizing calculator handles this separately from the maximum demand calculation.

Diversity is only used to assess the total demand on the supply. It answers the question: "Can the incoming supply handle all these loads, given that they will not all be running at full power at the same time?" If the answer is no — even after applying diversity — the supply needs upgrading.

It is important to note that diversity factors are based on typical domestic usage patterns. Properties with unusual load profiles (home workshops, multiple EV chargers, cryptocurrency mining, commercial-grade kitchens in domestic premises) may require a more conservative approach with reduced or no diversity applied to the atypical loads.

The Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002, Regulation 22, requires consumers (or their electrician) to give notice to the DNO before making any change to the electrical installation that may significantly affect the load. In practice, this means notifying the DNO when:

DNO notification is typically done via the DNO's website or through your competent person scheme registration (NICEIC, NAPIT, etc.). Some schemes include DNO notification as part of the Building Regulations notification process. The DNO will respond with either an acceptance (the existing supply is adequate) or a requirement for a supply upgrade.

Failure to notify the DNO is a breach of ESQCR 2002 and can result in the DNO refusing to supply, or the supply fuse blowing repeatedly because the load exceeds its rating. It also leaves the electrician liable if the supply cable overheats because the assessment was not done.

The two loads that most frequently trigger supply upgrade discussions in 2026 are EV chargers and heat pumps. Both are large, continuous loads that significantly increase the maximum demand of a domestic property.

The combination of EV charger plus heat pump on a single-phase domestic supply is one of the most common load assessment scenarios in 2026. Many older properties with 60 A service fuses simply cannot accommodate both without an upgrade. As a responsible electrician, identifying this early in the project saves the customer from costly surprises mid-installation.

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A supply upgrade is required when the assessed maximum demand exceeds the capacity of the existing supply infrastructure. This is not just the service fuse rating — it also includes the service cable capacity and the meter rating.

Always discuss potential supply upgrade costs with the customer early in the project. A customer who expects to pay £800 for an EV charger installation may not be prepared for an additional £2,000+ if a supply upgrade is needed. Early communication prevents disputes and lost deposits.