Maximum demand calculator with diversity

Maximum demand is the current an installation is assessed to draw at its busiest, after diversity, the allowance for loads that are never all on at once. Regulation 311.1 requires it to be assessed, and it is what the supply, the main switch and the meter tails are sized against.

List the circuits and what they draw. The usual household allowances are applied, every one of them shown and changeable, and the total is set against the supply fuse.

Regulation 311.1 of BS 7671 asks for the assessment; the household allowances are the conventional ones from Appendix A of the IET On-Site Guide, every one changeable. Checked against BS 7671:2018+A4:2026, updated 3 September 2026. Free, and nothing to sign in to.

Circuits and loads

Add a load

A kilowatt rating on the plate is amps ÷ 230: a 9.5 kW shower is 41.3 A, a 7 kW charger 30.4 A.

Supply fuse

Maximum demand after diversity

133 A

Over a 100 A supply

133 A after diversity is more than the 100 A supply fuse, at 133% of it. Talk to the DNO, look at load management, or check the assumptions.

Lighting circuit 12 A × 66% = 7.9 A
7.9 A
Socket circuit (ring or radial) 32 A in full + 32 A × 40% = 44.8 A
44.8 A
Cooker or hob 10 A + 35 A × 30% + 5 A for the socket = 25.5 A
25.5 A
Electric shower 41.3 A in full
41.3 A
Immersion heater 13 A in full
13.0 A

How is maximum demand assessed?

Regulation 311.1 requires the maximum demand of an installation to be assessed, and allows diversity to be taken into account, the fact that a house never has every circuit drawing its full rating at once. The assessment is what the supply, the main switch and the meter tails are sized against, and what the distribution network operator is told when a supply is applied for.

The allowances applied here are the conventional ones for an individual household, as Appendix A of the IET On-Site Guide sets them out:

  • Lighting circuit: Two thirds of the connected lighting load.
  • Socket circuit (ring or radial): The largest circuit in full, every other at 40%.
  • Cooker or hob: The first 10 A in full, 30% of the rest, plus 5 A if the cooker unit has a socket.
  • Electric shower: An instantaneous water heater draws its full rating for as long as it runs.
  • Immersion heater: Thermostatically controlled water heating takes no diversity.
  • Fixed space heating: Storage and underfloor heating take no diversity.
  • EV charger: Regulation 722 allows no diversity on a charging point unless it has load curtailment.
  • Other fixed load: Counted in full unless you know better and change it.

Appendix A is explicit that these are a guide and that the designer's judgement comes first. A house with two electric showers and a hot tub is not a typical house, and the tool shows every allowance it applied so you can see where to disagree with it. Shops, offices and small industrial premises take different allowances and are outside what this tool claims.

Worked example

A three-bedroom house: two lighting circuits at 6 A, two ring finals, a 45 A cooker circuit with a socket in the control unit, a 9.5 kW shower and a 3 kW immersion heater, on a 100 A supply.

  1. Lighting: 12 A × 66% = 7.9 A.
  2. Sockets: 32 A in full + 32 A × 40% = 44.8 A.
  3. Cooker: 10 A + 35 A × 30% + 5 A for the socket = 25.5 A.
  4. Shower: 41.3 A in full. Immersion: 13 A in full.
  5. Total after diversity: 132.5 A, over a 100 A supply, from 188 A connected.

The honest answer is that the allowances count the shower and the cooker together; the practical one is that a house like this is usually fine on 100 A, and would not be on 60 A. Add a 7 kW EV charger at 100% and the case for load management makes itself.

Questions the trade asks

The current an installation is assessed to draw at its busiest, after allowing for the fact that not everything runs at once. Regulation 311.1 requires it to be assessed, and it is what the supply, the main switch and the tails are sized against.

The allowance for loads that are not all on at the same time. A house with 60 A of lighting fittings never has them all lit, and a 32 A ring rarely carries 32 A. Diversity is the set of percentages that turn connected load into a realistic demand.

They are the conventional allowances for an individual household set out in Appendix A of the IET On-Site Guide, which the trade has applied for decades: lighting at two thirds, the largest socket circuit in full and the others at 40%, a cooker at 10 A plus 30% of the rest, and water heaters and EV chargers in full. Appendix A itself says they are a guide and the designer's judgement comes first, which is why every one of them can be changed here.

No. Regulation 722.311.201 requires a charging point to be counted at its full rating unless it has load management that limits it, so a 7 kW charger is 32 A on top of everything else. That is what pushes a lot of houses past a 60 A or 80 A supply.

Talk to the distribution network operator about the supply before the work starts, look at load management for the EV charger, and check the assumptions: a shower and a cooker are rarely on together in one household, but the allowances count them both. Do not fit a 100 A main switch to a supply the DNO has fused at 60 A and hope.

The figures go straight onto the certificate

In Pascal the maximum Zs is filled in for every circuit as you pick the device, the schedule of test results carries all 32 columns, and the observation library suggests the wording, code and regulation as you type. EICs, EICRs and minor works, from £25 a month.

See the certificate software

A calculator applies a method to the numbers you give it. It does not know the installation in front of you, and it is no substitute for BS 7671, the guidance or the judgement of the person signing the certificate. Check anything you rely on.