Maximum demand calculator

Will the supply take the charger?

Assess maximum demand with diversity applied, then add the EV charger or heat pump and see what is left against the main fuse. Every diversity allowance is yours to change, and the arithmetic is shown line by line.

An estimate to sanity-check a survey, not a substitute for design. Nothing is sent anywhere.

2. Existing load

Lighting and socket circuits are assessed from circuit ratings, which is how the On-Site Guide does it. Fixed appliances are entered in kW. Every diversity allowance is yours to change; check the defaults against the current On-Site Guide.

CategorykWDiv %

Lighting circuitsHow many, not the connected load

watts assumed per circuit1200 W is the usual figure

Ring final circuits32 A each

Radial socket circuits20 A each. Largest circuit taken at 100%, rest at the % shown

CookerFirst 10 A at 100%, remainder at the % shown

Electric showerLeave at 0 if the property has none

Immersion heater

Other fixed loads

of which the largestTaken at 100%

Socket outlet in the cooker control unit (adds 2 A)

3. What you are adding

EV charge points are a continuous load and normally take no diversity. Lower it only if load curtailment or a smart charging limit is actually being fitted.

New loadkWDiv %

EV charge point7.4 kW is a typical single-phase unit

Heat pumpRunning load, not peak start current

Assessed maximum demand

After diversity

Lighting

Socket outlets

Cooker

Shower

Immersion

Other fixed

Existing

New load

EV charge point

Heat pump

Total demand

Per phase

Supply selected

Smallest supply that carries it

Currents are calculated at 230 V. This is an estimate for sizing up a survey, not a design. The diversity that applies depends on the installation and how the property is used, and the network operator has the final say on what the supply will carry.

Maximum demand assessment

Estimate with diversity applied. Not a design document.

Reference: Date: Supply:

Load	As entered	After diversity (A)
Lighting
Socket circuits
Cooker
Electric shower
Immersion heater
Other fixed loads
Existing installation
EV charge point
Heat pump
New load added
Total maximum demand
Assessed against the supply
Smallest supply that carries it

Currents calculated at 230 V. Diversity allowances are those selected by the person running this calculation and are a judgement about how the property is used, not a fixed standard. Authoritative figures are in the current IET On-Site Guide and BS 7671. This sheet is an estimate for sizing up a survey. The distribution network operator has the final say on what the supply will carry, and final design remains the responsibility of the installation designer.

The method

How this maximum demand calculator applies diversity

Seven lines. Every one of them is a number you can change.

LightingNumber of circuits × the watts you assume per circuit, × the allowance you set. Three circuits at 1200 W and 66% is the usual domestic starting point.

Socket circuitsAssessed from the circuit ratings, not from an estimate of connected load: 32 A per ring final, 20 A per radial. Largest circuit at 100%, the remainder at your allowance. This is how the On-Site Guide does it, and it is why the tool asks how many circuits rather than how many kW.

CookerFirst 10 A at 100%, the remainder at your allowance, plus 2 A if there is a socket in the control unit.

Shower and immersionNo diversity by default. These are short, heavy and often coincide with everything else.

EV charge pointNo diversity by default. Charging is a continuous load. Reduce it only if load curtailment or a smart charging limit is genuinely being fitted.

Heat pumpRunning load at 100%. Start current is a separate question and is not modelled here.

Three phaseThe balanced total divided by three, compared against the per-phase rating.

Why the allowances are editable. Diversity is a judgement about how a property is actually used, not a constant. A family of five with an electric shower running every morning does not behave like a couple who are out all day. The defaults here are the commonly used domestic figures to start from; the authoritative ones for your installation are in the current IET On-Site Guide and BS 7671, and this maximum demand calculator deliberately does not reproduce them. The network operator has the final say on what the supply will carry.

The answer to this usually decides the paperwork

If the supply takes it, the connection still needs notifying. If it does not, you are into an application and a wait. installHUB captures the connection data during the survey and files it through ENA Connect Direct without re-keying, then tracks the response against the job.

G98 or G99? Check in 3 questions →Is it notifiable under Part P? →How DNO submissions work →

Questions

Maximum demand and diversity, answered

What is maximum demand?+

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.

What is diversity in an electrical installation?+

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.

Does an EV charger get diversity?+

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.

My calculation says the supply will not take it. What now?+

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.

Why does this not give me a cable size?+

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.

Is this good enough to put in front of a customer?+

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.

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