Understand diversity and maximum demand in domestic installations

Helpful video reference. John Ward (jwflame) explains "Maximum Demand & Diversity for Electrical Installations" clearly, covering how circuits rarely draw their rated maximum simultaneously and how the diversity calculation works. John is one of the most technically precise electrical educators on YouTube, and his treatment of BS 7671 concepts is consistently accurate.

1. Connected load and why it is not the same as maximum demand

Connected load is the theoretical maximum: every appliance in the property simultaneously drawing its full rated power. For a typical 3- or 4-bedroom UK home it is not hard to add up to 40 kW or more.

Consider a modest example: a 9.5 kW electric shower, a 3.5 kW cooker, a 3 kW kettle, a 2.4 kW washing machine, a 2 kW tumble dryer, and a 9 kW heating circuit. Connected load is already nearly 30 kW. Add lighting, a fridge-freezer, a dishwasher and a television, and you are over 33 kW.

But a standard UK domestic service fuse is 60A or 100A -- equivalent to about 13.8 kW or 23 kW at 230V. The fuse does not blow because the connected load never all runs at once.

2. What diversity means in practice

Diversity is simply the recognition that domestic electricity use has natural patterns. The kettle runs for three minutes. The shower runs for ten. The tumble dryer runs in the morning, the cooker in the evening. Even when several appliances overlap, the combination rarely reaches the connected load figure.

This is not guesswork. Network data from the DNOs, and decades of electrical installation experience, support the idea that domestic peak demand is a fraction of connected load. That fraction has been codified into the diversity tables in the IET On-Site Guide.

It is worth noting that diversity does not mean demand can be ignored. As homes add EV chargers (7.4 kW), heat pumps (3-10 kW) and battery storage systems, the simultaneous peak can rise substantially. An electrician assessing an installation today needs to account for future loads, not just current ones.

3. The BS 7671 approach: Chapter 31 and On-Site Guide Appendix A

BS 7671 Regulation 311.1 requires that the maximum demand of every installation is assessed before the design is finalised. The wiring regulations themselves do not prescribe a specific calculation method for domestic work -- instead, they defer to recognised guidance.

The IET On-Site Guide provides that guidance in Appendix A. It gives diversity factors by circuit type, based on the assumption that different loads coincide to different degrees in a typical home. Electricians use these factors to calculate a realistic maximum demand, which is then compared to the service fuse rating and the ratings of the consumer unit, main switch and meter tails.

The method is not the only way to assess demand -- for complex or unusual installations, a full load analysis may be needed -- but for standard domestic work, Appendix A provides a consistent and accepted approach.

4. How the diversity factors work, circuit by circuit

Lighting circuits. The On-Site Guide allows 66% of the designed load for lighting circuits, on the basis that not every light in the house runs simultaneously at full brightness. If you have three 16A lighting circuits (total 3.68 kW), the demand contribution is taken as 2.43 kW.

13A ring final circuits. The first ring circuit is taken at full demand (typically 7.2 kW for a full ring with 32A MCB protection). Each additional ring circuit is assessed at 40% of its own maximum, on the basis that a second or third ring serves rooms that are not usually all in use simultaneously.

Cooker circuits. The first 10A (2.3 kW) is taken in full. Of the remaining connected load above 10A, 30% is added. If a socket outlet is incorporated into the cooker control unit, a further 5A is added. So a 14.4 kW range cooker on a 45A circuit is assessed at: 10A + 30% of (45A minus 10A) + 5A = 25.5A, not 45A.

Water heaters. Instantaneous water heaters (such as an electric shower or combination unit) are assessed at full load for the first 3 kW, 75% for the next 3 kW, and 50% for anything above that. A 9.5 kW shower is assessed at approximately 6.6 kW, not 9.5 kW.

Space heating. Thermostatically controlled fixed electric heating is taken at full connected load. Unlike the other load types, electric heating can genuinely run at full output for extended periods on cold days, so no diversity is applied.

EV chargers. A home EV charger is typically treated at full load in a demand assessment. At 7.4 kW (32A Mode 3 charger), this is a significant addition to maximum demand. Some smart chargers can limit their output automatically to protect the installation, which may justify a lower demand figure -- but this requires engineering justification and ideally a load limiter device.

5. How maximum demand shapes the installation

Once the maximum demand figure is calculated, it is compared against three things: the service fuse, the meter tails and the consumer unit main switch.

Most UK homes have a 60A or 100A service fuse in the DNO cutout. If the calculated maximum demand exceeds this, the DNO must be contacted to upgrade the service -- a process that can take several weeks and involves a supply interruption. For homes with a 60A cutout, adding a 32A EV charger alongside a large shower circuit and a cooker often pushes the demand calculation past the limit, prompting a cutout upgrade before the charger can be installed.

Meter tails are the cables between the cutout and the consumer unit. They must be rated for the maximum demand, not just the consumer unit rating. Standard 25 mm² tails are suitable for 100A. Older 16 mm² tails may need replacing if the cutout is upgraded.

The consumer unit main switch is rated at 63A or 100A in most domestic installations. A 100A board needs 100A tails. Fitting a 100A board on a 60A service is not wrong, but fitting a 63A board on a 100A service undercuts the available headroom.

All of this matters when planning an extension, a full rewire, or the addition of large new circuits. The diversity calculation is where the numbers meet the reality of your specific home.

When to call an electrician

Any time you are adding a significant new circuit -- shower, EV charger, heat pump supply, large extension -- it is worth asking your electrician whether the existing service fuse and consumer unit can accommodate it. A brief demand assessment at the start of a project avoids unpleasant surprises when the DNO tells you your 60A service cannot take a 32A charger without an upgrade.