Maximum demand calculator

Maximum demand is the highest current an installation is realistically expected to draw at one time. It is not the sum of everything connected, because in practice not everything runs together. Applying diversity to the connected load is how you get from one to the other, and it is what decides whether a supply can take another circuit.

Diversity is the allowance made for the fact that loads do not all run at full output simultaneously. Lighting might be assessed at 66% of connected load, socket circuits at the largest plus 40% of the rest. The allowances are judgements about how a property is used, which is why this calculator makes every one of them editable rather than fixing them.

Normally no. An EV charge point draws close to its rated current continuously for hours, which is the opposite of the intermittent behaviour diversity assumes. The exception is where load curtailment or a smart charging limit is actually fitted and will reduce the charger under load, in which case lower the allowance to whatever the device genuinely limits to.

Three routes, roughly in order of cost. Load management or curtailment on the new circuit, which keeps the existing supply. A supply upgrade, which means the network operator and a wait. Or reconsidering the specification, for instance a single-phase 7.4 kW charger rather than something larger. All three involve talking to the DNO, so start that conversation early.

Because doing it properly needs the tabulated current-carrying capacities from BS 7671 Appendix 4, which are copyright and which we are not going to reproduce. A calculator that guessed at them would be worse than useless on a job. This tool covers the load assessment, which is the part most often got wrong; take the resulting design current to your own tables for the cable.

No. A heat pump runs for long periods at close to its rated electrical input, so like an EV charge point it is treated as a continuous load with no diversity. Use the "typical heat pump" quick-fill to drop in a 5, 8.5 or 12 kW class, or type the figure from the datasheet.

Roughly its electrical input in kW times 1000, divided by 230 on single phase, so about 37 A for an 8.5 kW unit. Confirm it from the datasheet rather than the model name, because the electrical input a heat pump draws is not the same as the heat output it delivers.

Often it is a G98 connect-and-notify, where you tell the network operator after connecting. A larger unit, a supply that is already tight, or a heat pump added alongside an EV charger can push it into a G99 application before you connect, or load management. The calculator shows the likely route, but the decision also depends on any existing generation or storage and on DNO policy, so confirm it with your DNO.

Treat it as a sanity check while you are on site or sizing up a quote, not as a design document. The numbers depend on assumptions you have chosen, the property may not behave the way the defaults assume, and the network operator has the final say on the supply.