How many EV chargers can your building’s supply actually run?

The first question we get on almost every commercial survey is “how many chargers can we have?” The honest answer is that it depends far less on the chargers than on the electricity supply coming into the building, and on how cleverly the chargers share it.

Here is how we work it out, and why a site that “cannot take any more” can often take eight.

Start with the supply, not the chargers

Every building has a maximum supply, set by the fuse the network operator fitted. A small office might have a 100 amp single-phase supply. A warehouse or school is more likely to have a three-phase supply of 100, 200 or 400 amps per phase. That fuse is the hard ceiling for everything in the building: lights, heating, kitchen, machinery and chargers together.

A 7kW charger draws about 32 amps on one phase. A 22kW charger draws 32 amps on each of three phases. So a 100 amp three-phase supply could, on paper, run three 22kW chargers flat out and nothing else. In practice the building already uses a chunk of that, which is why the next step matters more.

Measure what the building actually uses

The supply rating tells you the ceiling. What matters is the gap between the ceiling and your real peak load. On a survey we look at the main board, check the incoming fuse, and where the answer is not obvious we fit a logger for a week or two to record the actual demand through the working day. Most sites use far less than their rating, and that spare capacity, the headroom, is what the chargers get to use.

A typical finding: a 100 amp per phase supply peaking at 55 amps leaves 45 amps per phase of headroom. That is two 22kW chargers at full power, or four 7kW chargers, with nothing else changed.

Load balancing: the part that changes the answer

That is where most people stop, and it is why so many sites end up with two chargers and a queue. Modern commercial chargers can share capacity dynamically. A current transformer on the incoming supply tells the chargers how much headroom exists at any moment, and they divide it between whichever cars are plugged in, backing off when the kitchen fires up at lunchtime and ramping up again when it quietens down.

With dynamic load balancing, the same 45 amps of headroom can feed eight or ten 7kW bays. Not all at 7kW at once, but staff cars are parked for eight hours and most need two or three hours of charging, so they take turns without anyone noticing. The number of bays goes up by a factor of two to three for the cost of a sensor and some configuration.

When you really do need more supply

Load balancing stretches what you have; it does not create capacity. Sites that need every bay charging at full power all day, depots turning vans round between shifts, or anywhere planning rapid DC chargers, usually need a supply upgrade from the network operator. That is a DNO application, it takes six to twelve weeks at best, and the cost ranges from a few hundred pounds to tens of thousands depending on where the nearest capacity is. We will tell you on the survey whether you are in that category, and we handle the application if you are.

What to ask any installer

• What is my incoming supply rating, and have you measured my actual peak load?
• How many bays can I run today without a supply upgrade?
• Are you quoting dynamic load balancing, and across which chargers?
• If I add more bays in two years, does the design allow for it?
• Do I need a DNO application, and who submits it?

If an installer quotes you a number of chargers before asking about your supply, be careful. The chargers are the easy bit. Whether they all work at once is the engineering.

Related reading: why your car, not the charger, sets your AC charging speed, and our guide to commercial EV charger installation.