EV chargers on a 100 A three-phase commercial supply

A 100 A three-phase supply is common on small offices, shops, workshops and landlord supplies. Adding even two or three chargers can take it past what the supply can carry without help. This guide shows how to work out the supply capacity phase by phase, which DNO route applies, and how G100 load management fits the chargers within the 100 A, with a worked example.

What each charger adds, phase by phase

On a three-phase supply, what matters is the current on each phase, not the kW total.

• An 11 kW three-phase charger draws about 16 A on each phase.
• A 22 kW three-phase charger draws about 32 A on each phase.
• A 7 kW single-phase charger puts about 32 A on one phase only.

That last point is why balancing matters. Three 7 kW chargers spread one per phase add about 32 A to each phase. Put two of them on the same phase and that phase takes about 64 A, another 32 A and the third none. On a 100 A supply, that can be the difference between fitting and not fitting.

Work out headroom from the busiest phase

Headroom is worked out per phase, starting from the busiest phase's measured peak. Never use the average of the three phases. Commercial loads such as fridges, lighting and single-phase machines are rarely spread evenly, and the busiest phase is the one that reaches the fuse rating first.

Getting a real peak figure

Most commercial sites can get half-hourly consumption data from their energy supplier. It shows the actual peak over months, including the busy days, and it beats estimating every time.

Where there's no measured data, the diversity method in the IET On-Site Guide is the fallback for a maximum demand calculation. The result is an estimate, and it should be called one on any paperwork, including the DNO application.

Which DNO route applies

The ENA's connect-and-notify route (fit first, then notify the DNO) only applies where the total demand is up to 60 A per phase. A 100 A site with chargers is almost always over that, so in practice it's an application to the DNO before installing.

That's where EREC G100 load management comes in. Rather than asking for a bigger supply, the application shows how the chargers will be kept within the 100 A.

How G100 load management keeps the site within 100 A

A current transformer (CT) clamp measures the site's load. The load management device turns the chargers down when the rest of the site is busy, and back up when there's spare capacity, so the total stays under a set limit.

Two things matter for the application:

1. The load management device must meet EREC G100. On a three-phase supply it needs a CT on each phase, so it can manage the busiest one.
2. It must fail safe. If the CT clamp or the communications link is lost, it drops the managed load rather than carrying on at full power.

Worked example: a heat pump and two 22 kW chargers at a small office

An illustrative example. Real sites need measured data and an installer's design.

A small office has a 100 A three-phase supply. It wants a heat pump with a 9 kW electrical input and two 22 kW chargers for staff.

1. Existing demand: half-hourly data shows the busiest phase peaks at 52.0 A.
2. Heat pump: counted at its full rating and not managed, about 13.0 A per phase. That takes the busiest phase to 65.0 A.
3. Chargers unmanaged: about 31.9 A per phase each, so 63.8 A for the pair. The unmanaged peak is 128.8 A on a 100 A fuse. Without load management, it doesn't fit.
4. With G100 load management: An operating limit is set a little below the fuse, at 95.0 A (the DNO confirms the final setting), so the chargers share the 30.0 A that's left on the busiest phase. That's one car at about 30 A, or both at about 15 A each.
5. DNO route: even when managed, the demand is above 60 A per phase, so it's an application to the DNO before installing, with the load management details included.

Result: amber. Potentially workable with load management, subject to the DNO's assessment.

What Gridcap can and can't tell you

Gridcap checks the incoming supply only. It works out the headroom on each phase from the main fuse rating and the site's peak demand, with and without G100 load management, and shows the likely DNO route. The optional £49 report sets out those figures to support the DNO application.

It doesn't check the cut-out, the earthing or the BS 7671 design. Those stay with the installer, and the DNO still makes its own assessment.

FAQ

How many EV chargers can a 100 A three-phase supply take? It depends on the busiest phase's measured peak, not on the 100 A alone. With G100 load management, several chargers can often share whatever is spare on that phase. Without it, each charger has to be counted at its full current.

Can I use the average of the three phases? No. Headroom is worked out per phase from the busiest phase's measured peak. An uneven site can have plenty spare on two phases and very little on the third.

Does a commercial EV charger install need a DNO application? Usually, on a 100 A site. Connect-and-notify only covers total demand up to 60 A per phase, and a 100 A site with chargers is almost always over that.

What if we don't have half-hourly data? Ask the energy supplier first, as most commercial sites can get it. If that isn't possible, use the IET On-Site Guide diversity method and call the result an estimate.

Installer checklist

1. Ask the customer to request half-hourly data from their energy supplier.
2. Find the busiest phase's measured peak. Don't average the phases.
3. Add each charger per phase: about 16 A for 11 kW, about 32 A for 22 kW, and about 32 A on one phase for a 7 kW single-phase unit.
4. Balance single-phase chargers across the three phases.
5. If the total is over 60 A per phase, apply to the DNO before installing, with G100 load management details.
6. Specify a G100-compliant load management device that drops the managed load if the CT or comms is lost. On a three-phase supply it needs a CT on each phase, so it can manage the busiest one.