Does BS 7671 require a maximum demand calculation for a consumer unit change?

Yes. Any alteration to an installation requires re-assessment of the maximum demand per Regulation 311.1. When replacing a consumer unit, the designer should recalculate the maximum demand to confirm the existing supply capacity is adequate, especially if new circuits (such as an EV charger or electric shower) are being added as part of the work. This assessment must be documented on the Electrical Installation Certificate.

What diversity applies to multiple electric showers in a property?

IET Guidance Note 1 does not provide explicit diversity between multiple showers because it is unusual for two showers to operate simultaneously in a domestic setting. In practice, many designers take the first shower at 100% and apply a diversity of around 50–75% to any additional showers. However, this is a judgment call — in a hotel or gym with multiple shower rooms, all showers should be taken at 100% unless metered data supports a reduction.

How does BS 7671 Amendment 2 affect maximum demand for EV charging?

Amendment 2 (2022) introduced Section 722 specifically for EV charging installations. Regulation 722.311 requires the maximum demand to be assessed including the EV charging load. Where the existing supply cannot accommodate the additional demand, the amendment permits the use of an Energy Management System (EMS) to dynamically limit the EV charging current based on the real-time demand of other circuits. This allows a 7 kW EV charger to be installed on a 60 A supply by reducing the charger output when other high-demand appliances (shower, cooker) are in use.

Maximum Demand Calculator — BS 7671 🇬🇧

United KingdomEdition 2018+A2:2022Free Online Tool

Maximum demand assessment under BS 7671:2018+A2:2022 (the IET Wiring Regulations) is required by Regulation 311.1, which states that the maximum demand of every installation shall be assessed, making due allowance for diversity. However, unlike AS/NZS 3000 or NEC, BS 7671 itself does not prescribe specific demand factor tables — it relies on the designer's engineering judgment.

In practice, UK electricians and designers use the tables in the IET On-Site Guide (OSG) and IET Guidance Note 1 to determine assumed current demands for outlet points and to apply diversity percentages by circuit type. This approach gives experienced designers flexibility but can lead to inconsistency when different practitioners apply different assumptions.

This calculator implements the IET OSG methodology with full transparency, showing exactly which diversity percentages are applied to each circuit type and providing clause references throughout. This makes the calculation auditable and consistent across projects.

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How Maximum Demand Works Under BS 7671

Regulatory Basis: Regulation 311.1

Regulation 311.1 of BS 7671:2018+A2 requires that "the maximum demand (often referred to as the design current) of an installation, and of individual circuits, shall be determined". The regulation further notes that diversity may be taken into account, meaning not all loads need to be assumed to operate simultaneously. However, the standard does not define specific diversity percentages — this is left to engineering judgment supported by IET guidance publications.

Step 1: Determine Current Demand per Outlet Point

The IET On-Site Guide, Table 1A provides assumed current demands for common outlet types when the actual connected load is not known:

13 A socket outlet (ring final) — assumed at the circuit design current, not per socket
Lighting outlet — assumed at 100 W per point minimum (actual lamp wattage if known)
Cooker control unit — rated current of the connected appliance
Immersion heater — rated current (typically 13 A for a 3 kW unit)
Shower — rated current (e.g., 10.5 kW / 230 V = 45.7 A)

Step 2: Identify Circuit Types and Design Currents

Each circuit in the consumer unit is categorised by type. Common domestic circuits include:

Ring final circuits (32 A Type B MCB) — serving socket outlets
Lighting circuits (6 A or 10 A Type B MCB)
Cooker circuit (32 A or 45 A)
Shower circuit (40 A or 50 A)
Immersion heater (16 A or 20 A)
Electric vehicle charger (32 A dedicated circuit)

Step 3: Apply Diversity per IET Guidance Note 1

IET Guidance Note 1, Table 1 provides recommended diversity percentages for different circuit types in domestic and small commercial installations. The key principle is that diversity varies by circuit type, not by a single blanket factor:

Lighting — 66% of total current demand
Heating (including immersion heater) — 100% of largest appliance + 75% of remaining
Cooking appliances — 10 A + 30% of remaining connected load + 5 A for socket in cooker control unit
Motors (other than lifts) — largest at 100% + 80% of second largest + 60% of remaining
Socket outlets (13 A ring or radial) — 100% of largest circuit + 40% of remaining circuits
Showers — 100% (no diversity for single shower)

Step 4: Sum Diversified Demands

The diversified demand for each circuit type is calculated individually, then all are summed to produce the total maximum demand of the installation. This total determines the required main switch rating (typically 63 A or 100 A for domestic) and the service fuse size requested from the Distribution Network Operator (DNO).

Step 5: Verify Against DNO Capacity

The calculated maximum demand must be reported to the DNO (UK Power Networks, Western Power Distribution, etc.) using the application form for new connections or alterations. The DNO will confirm whether the existing service cable and cut-out can support the calculated demand. For demands exceeding 100 A single-phase, a three-phase supply or service cable upgrade may be required per Regulation 313.2.

Step 6: Document the Assessment

Per Regulation 132.4, the assessment of maximum demand forms part of the design documentation. It should be recorded on the Electrical Installation Certificate (EIC) and retained for the life of the installation. Appendix 6, Table 6A provides standard circuit arrangements that can simplify documentation for typical domestic installations.

Key Reference Tables

IET Guidance Note 1, Table 1 — Diversity Allowances

The primary diversity reference for BS 7671 installations. Provides percentage diversity allowances for each circuit type: lighting, heating, cooking, motors, socket outlets, and other loads. Separate columns for individual dwellings, small shops, and small hotels.

Select the diversity percentage for each circuit type from the appropriate column, apply it to the circuit's design current, then sum all diversified demands to obtain the total maximum demand.

IET On-Site Guide, Table 1A — Assumed Current Demand

Provides assumed current demand values for common outlet points when the actual connected load is unknown. Values include 100 W minimum per lighting point, rated current for fixed appliances, and the circuit design current for socket outlet circuits.

Use when the specific load connected to an outlet is not yet known at the design stage. Commonly used for speculative building projects where the end user has not specified appliance selections.

Regulation 311.1 — Assessment of Maximum Demand

The regulatory requirement for maximum demand assessment. States that the maximum demand shall be determined with due allowance for diversity. Cross-references Chapter 31 general characteristics assessment requirements.

Cited as the regulatory justification for performing the maximum demand calculation. Required reference on the Electrical Installation Certificate.

Appendix 6, Table 6A — Standard Circuit Arrangements

Defines standard circuit types for domestic installations, including ring finals (32 A), radial circuits (20 A / 32 A), lighting (6 A), cooker (varies), and immersion heater circuits. Specifies maximum floor area, cable sizes, and protective device ratings.

Reference for identifying circuit types when assessing maximum demand. Also used to verify that circuit arrangements comply with BS 7671 standard configurations.

Regulation 313.2 — Supply Characteristics

Requires that the designer ascertain the supply characteristics including the number of conductors, frequency, voltage, prospective fault current, external earth fault loop impedance, and the nature and rated current of the overcurrent protective device at the origin of the installation.

Determines whether the existing supply is adequate for the calculated maximum demand. Triggers a DNO application if the demand exceeds the existing service capacity.

Regulation 132.4 — Design Documentation

Requires that design documentation be prepared to demonstrate that the installation will comply with BS 7671. The maximum demand assessment forms a core component of this documentation package.

Ensures the maximum demand calculation is formally recorded, not just done mentally. The assessment should be retained with the installation records for future reference during alterations or additions.

Worked Example — BS 7671 Maximum Demand

Scenario

A 3-bedroom UK semi-detached house with: electric shower (10.5 kW, 230 V), cooker (13 kW), immersion heater (3 kW), three ring final circuits (kitchen, ground floor, first floor), four lighting circuits, and an EV charger (7.4 kW, 32 A dedicated circuit).

Lighting demand (IET GN1 Table 1 — 66%)

Four lighting circuits, each with approximately 800 W of connected luminaires. Total lighting current = 4 × (800/230) = 13.9 A. Apply 66% diversity per IET Guidance Note 1.

I_lighting = 13.9 × 0.66 = 9.2 A

9.2 A

Socket outlets — ring finals (IET GN1 Table 1 — 100% + 40%)

Three ring final circuits, each on a 32 A MCB. Apply diversity: 100% of largest circuit + 40% of each remaining circuit.

I_sockets = (32 × 1.0) + (32 × 0.4) + (32 × 0.4) = 32 + 12.8 + 12.8 = 57.6 A

57.6 A

Cooker demand (IET GN1 Table 1 — 10 A + 30% + 5 A)

Cooker rated at 13 kW = 13000/230 = 56.5 A. Diversity formula: 10 A + 30% of remainder + 5 A for cooker control unit socket.

I_cooker = 10 + (46.5 × 0.3) + 5 = 10 + 14.0 + 5 = 29.0 A

29.0 A

Electric shower (IET GN1 Table 1 — 100%)

Shower at 10.5 kW = 10500/230 = 45.7 A. Showers are taken at 100% demand with no diversity — the full load is assumed whenever the shower is in use.

I_shower = 10500 / 230 = 45.7 A

45.7 A

Immersion heater (IET GN1 Table 1 — 100% largest heating)

Immersion heater at 3 kW = 3000/230 = 13.0 A. As the only heating appliance in this example, it is taken at 100%.

I_immersion = 3000 / 230 = 13.0 A

13.0 A

EV charger (dedicated circuit — 100%)

EV charger on a dedicated 32 A circuit. As a single high-demand appliance on its own circuit, it is taken at full rated current. BS 7671 Amendment 2 requires dedicated EV circuits per Regulation 722.

I_ev = 32 A

32.0 A

Total maximum demand

Sum all diversified circuit demands. This determines the main switch rating and whether a DNO supply upgrade is required.

I_total = 9.2 + 57.6 + 29.0 + 45.7 + 13.0 + 32.0 = 186.5 A

186.5 A — exceeds 100 A single-phase service capacity. A three-phase supply upgrade or load management (e.g., EV charger with dynamic load balancing) is required.

The calculated maximum demand of 186.5 A exceeds the typical UK single-phase 100 A service fuse. The designer must either request a three-phase supply from the DNO, install dynamic load management on the EV charger to reduce the simultaneous demand, or reconsider the heating strategy (e.g., gas boiler instead of immersion heater). This result highlights how EV charging is significantly impacting UK domestic maximum demand calculations and why BS 7671 Amendment 2 introduced Section 722 for EV installations.

Common Mistakes When Using BS 7671

1
Applying a single blanket diversity percentage (e.g., 60%) to the total connected load. IET Guidance Note 1 specifies different diversity percentages for each circuit type — lighting, cooking, sockets, and showers all have different allowances.
2
Confusing maximum demand with design current. The design current (I_b) is the actual current a specific circuit will carry; the maximum demand is the aggregate diversified load of the entire installation at the point of supply. They are related but distinct concepts.
3
Applying diversity to individual circuits rather than to the group of circuits. Diversity in the BS 7671 context means that not all circuits will be at peak simultaneously — it applies when combining multiple circuits, not within a single circuit.
4
Forgetting that electric showers are taken at 100% demand with no diversity reduction. A 10.5 kW shower at 45.7 A represents a significant proportion of a domestic installation's maximum demand and cannot be reduced.
5
Not considering the impact of EV chargers on existing installations. A 32 A EV charger adds substantial demand and may push the installation beyond the existing 60 A or 100 A service fuse capacity, requiring a DNO upgrade or smart charging with load management per BS 7671 Section 722.

How Does BS 7671 Compare?

BS 7671 is the <strong>least prescriptive</strong> of the major standards for maximum demand. The standard itself only requires the assessment (Regulation 311.1) without specifying the method. Practitioners rely on IET Guidance Note 1 and the On-Site Guide for practical diversity tables — but these are <em>guidance</em>, not regulation. This contrasts sharply with AS/NZS 3000's mandatory Appendix C formulas and NEC Article 220's mandatory demand factor tables, where the calculation method is part of the enforceable standard itself.

Frequently Asked Questions

No. The IET On-Site Guide and Guidance Notes are published by the IET to support the application of BS 7671, but they are not part of the standard itself. The diversity percentages in these publications represent best-practice guidance that has been widely accepted by the industry. However, a designer with sufficient experience and justification could use different diversity figures if they can demonstrate compliance with Regulation 311.1.

The standard UK domestic service fuse (cut-out) is typically 60 A or 100 A for single-phase supplies (230 V, 50 Hz). Some older properties may have 30 A or 40 A cut-outs. For demands exceeding 100 A single-phase, the DNO may offer a three-phase supply (3 × 100 A). The service fuse size cannot be changed by the electrician — it is owned and maintained by the DNO.

IET Guidance Note 1 provides a separate diversity column for small shops and commercial premises. Generally, commercial diversity allowances are more conservative (less reduction) than domestic because commercial loads tend to operate at higher utilization rates. For larger commercial and industrial installations, the designer should assess actual load profiles, possibly using metered data from similar installations, rather than relying solely on tabulated diversity percentages.

Maximum Demand Calculator for Other Standards

🇦🇺AS/NZS 3000

🌍IEC 60364

🇺🇸NEC/NFPA 70