Maximum Demand Calculator: After-Diversity Totals for Domestic Properties

Quick Answer: Maximum demand for a domestic property is calculated using After Diversity Maximum Demand (ADMD) — not the simple sum of all installed loads. BS 7671 (IET Wiring Regulations) and Appendix 1 of the On-Site Guide provide diversity allowances for domestic loads. A typical 3-bedroom house with a 10kW electric shower, gas cooking, and standard appliances has an ADMD of approximately 17–20kW, typically served by a 80–100A supply fuse. Always calculate per-circuit as well as total ADMD to verify consumer unit sizing.

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

Diversity Allowances — BS 7671 Appendix 1 (Domestic)

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Load Type Diversity Rule
Lighting 66% of total connected load
Heating and air conditioning 100% of largest item + 40% of remaining items
Cooker (household) 10A + 30% of remaining current over 10A + 5A (if socket outlet on circuit)
Water heater (instantaneous, e.g. shower) 100% of largest + 100% of second largest + 25% of remaining
Water heater (thermostatically controlled, e.g. immersion) 100% of largest only (50% if multiple, only one likely on at any time)
Motor (domestic appliance) 100% of largest + 50% of remaining
Socket outlet circuits — ring final First ring: 100% of full load (30A); each additional ring: 40% of full load
Socket outlet circuits — radial 100% of first + 40% of remaining
Electric vehicle charger 100% (no diversity applied; or use dynamic load balancing if fitted)
Heat pump 100% of rated input when running

Worked Examples

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

Installed loads:

ADMD Calculation:

Step 1 — Lighting:

Step 2 — Cooker:

Step 3 — Electric shower:

Step 4 — Immersion heater:

Step 5 — Tumble dryer:

Step 6 — Washing machine:

Step 7 — Ring final circuits:

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:

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?):

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):

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