Maximum Demand Calculator: After-Diversity Totals for Domestic Properties

Summary

One of the most common misconceptions in domestic electrical design is that the consumer unit and supply fuse must be capable of simultaneously supplying every installed appliance at full load. In practice, no household ever runs every appliance simultaneously at maximum rating. After Diversity Maximum Demand (ADMD) accounts for this statistical reality, producing a realistic figure for the actual peak load the supply must be able to deliver.

ADMD calculation is required to verify that the existing supply is adequate for an installation, to size a new supply, to size cable from the electricity meter to the consumer unit, and to assess whether a property can support additional loads such as an EV charger or heat pump without upgrading the supply.

UK domestic supplies are typically rated at 60A, 80A, or 100A at 230V (single phase). A 100A supply provides 23kW of continuous capacity. Given typical ADMD for a modern 3–4 bedroom house is 15–25kW, this is usually adequate — but as electric heating, EV charging, and heat pumps become more common, ADMD is increasing and supply upgrades are becoming more frequent.

Key Facts

• ADMD definition: After Diversity Maximum Demand — the realistic peak simultaneous load accounting for statistical probability that not all loads operate simultaneously
• BS 7671 Appendix 1: Contains diversity allowances for domestic installations — these are the basis for ADMD calculation
• Standard domestic supply: 60A, 80A, or 100A single-phase at 230V (13.8kW, 18.4kW, or 23kW respectively)
• Three-phase supply: 400V three-phase available in some areas; 3 × 100A = 69kW — needed for very high demand properties
• Electric shower diversity: Electric showers are typically rated at 7.5kW–12kW; no diversity applied (100% of largest shower load is included)
• Cooking appliance diversity: 10A for first 12A of cooker circuit, plus 30% of remaining current, plus 5A if socket outlet in cooker circuit — per BS 7671
• Motor loads diversity: 100% of the largest motor + 80% of others (commercial) or 100% first + 50% subsequent (domestic appliances)
• Lighting diversity: 66% of total connected lighting load in domestic premises
• Socket outlet circuits diversity: First socket outlet assumed at full load (3kW ring final circuit = 30A); additional rings at 40% of full load
• Consumer unit rating: Consumer unit must be rated to carry the total ADMD of the installation; typically 63A, 80A, or 100A main switch
• Meter tails: Sized to carry the ADMD continuous; minimum 16mm² for 80A, 25mm² for 100A supplies (check BS 7671 for specific installation conditions)
• EV charger ADMD impact: A 7.4kW (32A) EV charger at 100% increases ADMD significantly — consider load management systems (dynamic load balancing)
• Heat pump ADMD: Air source heat pump typically 3–6kW continuous consumption when running — lower than many assume due to COP > 1

Diversity Allowances — BS 7671 Appendix 1 (Domestic)

Worked Examples

Example 1: Standard 3-Bedroom Semi-Detached House (Gas Heating & Cooking)

Installed loads:

• Lighting: 2kW total (all circuits combined)
• Ring final circuits: 2 × 30A rings
• 10.5kW electric shower
• Immersion heater: 3kW
• Tumble dryer: 2.5kW
• Washing machine: 2.5kW

ADMD Calculation:

Step 1 — Lighting:

• 2,000W × 66% = 1,320W = 5.7A

Step 2 — Cooker:

• Gas cooker — no electrical cooker circuit

Step 3 — Electric shower:

• Shower rated 10.5kW = 45.7A
• Diversity: 100% of largest shower = 45.7A

Step 4 — Immersion heater:

• 3,000W ÷ 230V = 13A
• Diversity: 100% of largest = 13A (only one immersion; it's thermostatically controlled, so only this one taken at 100%)

Step 5 — Tumble dryer:

• 2,500W = 10.9A; as a motor load, 100% = 10.9A (largest)

Step 6 — Washing machine:

• 2,500W = 10.9A; 50% of second motor load = 5.4A

Step 7 — Ring final circuits:

• First ring: 30A × 100% = 30A
• Second ring: 30A × 40% = 12A

Total ADMD (BS 7671 Appendix 1): 5.7 + 45.7 + 13.0 + 10.9 + 5.4 + 30 + 12 = 122.7A

This is the single-pass ADMD calculated by applying BS 7671 Appendix 1 diversity factors once per load type — no secondary diversity factor is applied. The result indicates this property requires a 125A or larger supply, or a discussion with the DNO about the actual metered supply rating. In practice, many properties with 10kW+ showers are served by 100A supplies with the understanding that peak demand rarely reaches the theoretical maximum.

Consumer unit main switch: 80A or 100A main switch (100A preferred with a 10.5kW shower). Confirm with the DNO if upgrading the supply is required.

Example 2: All-Electric Modern Property with EV Charger

Installed loads:

• Lighting: 1.5kW (LED throughout)
• Ring final circuits: 2 × 30A rings
• 10kW electric shower
• 7kW air source heat pump
• 7.4kW EV charger (32A)
• 7kW induction hob (electric cooker)
• 3kW oven
• 2.5kW tumble dryer

This property has very high potential demand. Calculate ADMD:

Step 1 — Lighting: 1,500W × 66% = 990W = 4.3A

Step 2 — Cooker (induction hob + oven on one circuit or separate?):

• If on one cooker circuit: 10A + 30% × (43.5A - 10A) = 10 + 10.05 = 20.05A + 5A (socket) = 25.1A
• Oven on separate circuit: 3,000W = 13A at 100%

Step 3 — ASHP: 7,000W = 30.4A at 100% (heating load, continuous when running)

Step 4 — EV charger: 7,400W = 32.2A at 100% (no diversity for EV chargers without DLM)

Step 5 — Shower: 10,000W = 43.5A at 100%

Step 6 — Rings: First 30A + 40% × 30 = 12A

Step 7 — Dryer: 2,500W = 10.9A at 100%

Raw total: 4.3 + 25.1 + 13 + 30.4 + 32.2 + 43.5 + 30 + 12 + 10.9 = 201.4A

This clearly exceeds any standard domestic supply. Apply diversity:

Practical interpretation: Not all loads will run simultaneously. The shower is typically short-duration; the EV charger runs overnight; the heat pump runs during the day; the cooker runs at meal times. An appropriate ADMD for engineering purposes might consider:

Evening peak (most likely simultaneous demand):

• Lighting: 4.3A
• Cooker: 25.1A
• Oven: 13A
• EV charger: 32.2A (overnight, but EV DLM reduces this — assume 16A with DLM)
• Ring finals: 42A (both rings)
• Dryer: 10.9A
• ASHP: 30.4A (may run in early evening)
• Shower: 43.5A (100% per BS 7671 — no additional period diversity factor applied)

Evening peak ADMD: 4.3 + 25.1 + 13 + 16 + 42 + 10.9 + 30.4 + 43.5 = 185.2A

This requires a three-phase supply or load management. Conclude: this property cannot be served by a standard single-phase 100A supply without demand management.

Recommendation: Install a three-phase supply (3 × 100A) or implement comprehensive load management (smart charger, smart ASHP controls, timed immersion heater, Zappi/Ohme charger with load balancing).

Example 3: Checking Supply Adequacy Before Installing an EV Charger

Existing 3-bed property, 80A supply. Current ADMD assessed at 52A. Adding a 7.4kW (32A) EV charger.

New ADMD = 52 + 32 = 84A

80A supply has a rating of 80A continuous. 84A exceeds this.

Options:

1. Apply for supply upgrade to 100A (contact DNO — typically no cost for standard upgrade)
2. Install a smart charger with dynamic load management — charger reduces output when other loads are high, keeping total demand within 80A
3. Limit EV charger to 3.7kW (16A) — reduces ADMD to 52 + 16 = 68A, within the existing 80A supply

Most installers choose option 2 (DLM smart charger) as it avoids the delay of supply upgrade while maintaining the fastest practical charge rate.

Frequently Asked Questions

Does the main fuse (supply cutout) limit the current permanently?

Yes — the distributor's cut-out fuse (the sealed fuse before the meter) is the absolute limit for the installation. Drawing more than its rating continuously will cause it to operate. Brief surges (motor starting) are tolerable; sustained overload will blow the fuse. Upgrading the cut-out requires a DNO (Distribution Network Operator) visit — this is not something an electrician can do independently.

Can I add a 10kW electric shower to a property with an 80A supply?

Calculate the existing ADMD first. If the existing ADMD is, say, 45A, adding a 10kW shower (43.5A) would give a theoretical simultaneous load of 88.5A — slightly over the 80A supply. In practice, full simultaneous load is unlikely, and a DNO may confirm the 80A supply is adequate given diversity. Apply the diversity table: if the realistic ADMD including the shower is assessed at under 80A, the existing supply is sufficient. If over, request a DNO supply upgrade (80A to 100A) which is typically straightforward.

What is the difference between ADMD and connected load?

Connected load is the sum of the rated kW of every installed appliance — this is always the larger number. ADMD is the realistic peak demand after diversity factors are applied. For a typical domestic property, ADMD is typically 40–70% of connected load. Using connected load for supply sizing would require a much larger (and more expensive) supply than is actually needed.

Where do I find the diversity allowances in BS 7671?

Appendix 1 of BS 7671:2018+A2:2022 (the IET Wiring Regulations 18th Edition with amendments) contains the diversity tables for domestic and commercial premises. The IET On-Site Guide (OSG) also contains the domestic diversity table in a simplified format useful for everyday calculation.

Regulations & Standards

• BS 7671:2018+A2:2022 (IET Wiring Regulations, 18th Edition) — Appendix 1 contains diversity factors; Section 311 covers maximum demand

• IET On-Site Guide — simplified diversity tables and worked examples for domestic installations

• ENA (Energy Networks Association) G83/G98/G99 — connection requirements for generators and EV chargers on domestic supplies

• PD IEC TS 62898-3-1 — microgrids and load management relevant to EV charger installations

• IET BS 7671 Wiring Regulations — 18th Edition with amendments; Appendix 1 diversity factors

• IET On-Site Guide — simplified domestic diversity calculation reference

• OZEV EV Charger Installation Guidance — EVHS grant and technical requirements

• Energy Networks Association Supply Connection Guidance — DNO supply upgrade procedures

• cable sizing — cable sizing for meter tails and sub-mains

• consumer units — consumer unit sizing and configuration

• part p notifications — notifiable electrical work including new consumer units