7 kW EV charger on a 60 A supply with storage heaters

A common question from landlords and installers is whether a 7 kW charger can go on a 60 A single-phase supply that already has storage heaters and an immersion heater on an off-peak tariff. Usually it can, with load management. This worked example shows the numbers and the DNO route.

The numbers

Take a flat with a 60 A main fuse and 8 kW of storage and water heating, all on Economy 7.

8 kW at 230 V is about 35 A.
A 7 kW (7.4 kW) charger draws 32 A.
With the heating and the charger on together, that's about 67 A before any other load, which is more than the 60 A fuse.

BS 7671 (regulation 722.311.201) doesn't allow diversity on an EV charging circuit unless load control is fitted, so without load management the charger has to be counted at its full 32 A. On these figures, a charger without load management doesn't fit.

With load management, the charger only uses what's spare. With all the heating on, a 60 A supply leaves about 25 A, around 5.5 to 6 kW, for the car before other loads are counted. The immersion usually switches off once the cylinder is hot, and storage heaters don't always charge for the whole off-peak period, so on many nights there's more than that.

How load management works here

A current clamp on the meter tails measures the whole property's load. The charger turns itself down so the total stays under a set limit, and turns back up when other loads switch off. The tenant simply gets whatever capacity is spare.

Set the limit a little below the fuse rating, say 55 A on a 60 A fuse, rather than right at it. That covers the few seconds the charger takes to turn down when a heater switches on, and any small measurement error.

Pick a charger that fails safe. If the clamp is disconnected or the signal is lost, the charger should drop to a low rate rather than carry on at full power. EREC G100 expects load management schemes to fail safe, and in a rental nobody may notice a loose clamp for months.

What to tell the DNO

The ENA's process for EV chargers and heat pumps has a "connect and notify" route. The installer fits the charger and notifies the DNO within 28 days, without applying first. It applies when the property's maximum demand is 60 A per phase or less, the cut-out rating is known and adequate, there are no safety concerns, and the form's other criteria are met. If a load management scheme keeps the whole property at 60 A or less whenever the charger runs, and fails safe, the property can use this route even though its demand without the scheme would be higher.

If something doesn't fit those conditions, such as a looped supply, an unknown fuse rating or a cut-out in poor condition, the installer applies first and waits for the DNO's answer. Keep the notification or the DNO's reply with the property records.

Check the fuse before relying on "60 A"

Don't go on what you've been told. Check the rating on the fuse itself where it's visible, or ask the DNO. The marking on the fuse carrier is often the rating of the holder (for example 100 A), not the fuse inside, which may be 60, 80 or 100 A. Never open or tamper with the sealed cut-out.

What Gridcap does for a job like this

Gridcap's free check works out the headroom on each phase from the fuse rating and the property's peak demand, with and without G100 load management, and shows the likely DNO route. The optional £49 report sets out those figures for the DNO notification or application. The DNO still makes its own assessment, and the installer remains responsible for the BS 7671 design.

Installer checklist

Confirm the main fuse rating from the fuse or the DNO.
Add up the fixed loads that can run during charging hours, including storage heaters and the immersion.
Fit a charger with load management and a defined fail-safe, and set the limit a little below the fuse rating.
If the property meets the connect and notify conditions, notify the DNO within 28 days. Otherwise, apply first.
Tell the tenant that charging will slow down when the heating is on, especially in winter.