Calculating the Premises Maximum Demand for an EV charge point or heat pump installation — the figure the ENA EVCP & HP Connections Form is built around, and often the least familiar part of an otherwise straightforward charger install.
Maximum Demand (MD) is the highest current the whole premises is realistically expected to draw at once, in amps per phase (a CT-metered supply states it in kVA) — not the sum of everything at full load; the gap between the two is what diversity captures. A domestic cut-out fuse is commonly 60, 80 or 100 A — and an EV charge point can add a continuous 32 A (a typical 7.4 kW charger) to whatever the house already draws, for hours at a time. The DNO’s connect-and-notify criteria exist to catch the cases where that pushes a supply too far.
This is not optional paperwork. The ENA EVCP & HP Connections Form’s own guidance states: “A load survey is required to calculate the Maximum Demand. This should comprise the existing Maximum Demand of the whole premises and the new equipment to be installed as well as any import or load limiting devices.”
The figure is one of the criteria deciding connect-and-notify versus apply-before-install — alongside the equipment itself (its ENA register listing, and how many pieces of EVCP/HP equipment the premises is gaining). On the demand side: the MD must stay below the cut-out fuse rating, and an EV charge point additionally needs the premises at or under 60 A per phase (13.8 kVA) to stay on the notify path. Enter the figure in CLRD’s Supply Details and it tests these criteria and routes the form automatically.
Measure it, if you can. A smart meter’s half-hourly history gives a measured answer: take the highest half-hour’s consumption in kWh, double it for kW, and divide by 0.23 for amps on a 230 V single phase (an in-home display or the supplier’s data shows the same history). On a three-phase supply the meter’s figure is the total across all three lines, and MD is the most heavily loaded one — dividing by three only holds if the load is genuinely balanced, so clamp-log each phase separately instead. Make sure the period is representative — a summer month misses the heating load — and remember the meter records net import: existing solar or a battery masks whatever demand it was covering, so at a premises with generation the history understates the true load. Some older meters carry a maximum-demand indicator, or the supply can be clamp-logged for a representative period.
Otherwise, calculate it. Itemise the property’s circuits and apply the standard diversity allowances for an individual household — the method set out in the IET On-Site Guide (Appendix A, “Maximum demand and diversity”). The typical allowances:
| Circuit / appliance | Allowance |
|---|---|
| Lighting | 66% of the total connected lighting load |
| Socket circuits (ring finals / radials) | 100% of the largest circuit + 40% of every other circuit |
| Cooker | The first 10 A of its rating + 30% of the rest (+ 5 A if the control unit has a socket) |
| Instantaneous water heaters / electric showers | 100% of the largest + 100% of the second + 25% of any further |
| Immersion heater (thermostatic) | Full load — no diversity |
| Storage heating / floor warming | Full load — no diversity |
| Other fixed heating appliances | The first 10 A + 50% of the rest |
These are the standard guidance values for an individual domestic installation; the On-Site Guide is explicit that diversity is an exercise of judgement by the surveying electrician, not arithmetic to apply blindly — if the household genuinely runs loads together, count them together.
On a three-phase supply, run the same itemisation per line: assign each circuit to the phase it sits on, apply the diversity per line, load a three-phase appliance onto every line, and take the highest line’s total as the Maximum Demand.
The proposed EV charge point or heat pump goes on top at its full rating, with no diversity — they are long-duration loads (a 7.4 kW charger is 32 A for the length of the charge). Count battery and V2G charging too: storage draws from the grid when it charges.
Do not deduct an import or load-limiting device from the figure. Declare the scheme in CLRD instead — it applies the device’s limit when testing the connect-and-notify criteria and states the limited figure wherever a form wants it, while the survey figure stays the honest unrestricted demand.
Small gas-heated house, single-phase, 100 A cut-out — adding a 7.4 kW (32 A) EV charge point:
| Load | Rating | After diversity |
|---|---|---|
| Lighting (total connected) | 1.5 kW ≈ 6.5 A | 66% → 4.3 A |
| Ring final circuit | 32 A | 100% → 32 A |
| Immersion heater | 13 A | No diversity → 13 A |
| Existing Maximum Demand | ≈ 49 A | |
| EV charge point (proposed) | 32 A | Full rating → 32 A |
| Premises Maximum Demand | ≈ 81 A |
81 A is safely below the 100 A cut-out — but above 60 A per phase, so this charge point is apply-before-install: CLRD produces the application pack and the DNO assesses the supply before the install. On the same house with a 60 A or 80 A cut-out the demand would exceed the fuse itself, and the job needs a load-limiting (curtailment) device — declared in CLRD as the import scheme — or a DNO supply upgrade first.
Put the result in the Premises Maximum Demand field in Supply Details — amps on a whole-current supply, kVA totalled across the phases on a CT-metered one — along with the cut-out rating. CLRD tests the connect-and-notify criteria, routes the EVCP & HP Connections Form onto the right pack, and prints the figure where the form wants it. Keep your itemisation with the job’s records: the MD is your professional assessment, and the itemised survey is what stands it up.