Putting Charge Points Into a Shared Car Park

Three Residents Asked, and the Quote Came Back at Six Figures

A managing agent forwards a connection quotation for a new substation because three leaseholders in a forty-bay undercroft asked about charging. Nobody in that email chain has established what the existing landlord's supply would carry tonight, unmodified, and the honest answer usually sits between the four bays the caretaker assumed and the forty the sales deck promised.

The question a block manager, a facilities lead or a site owner is really asking is not whether a charge point can be installed. It is how many bays can be served before somebody has to pay a network operator, and that is a subtraction followed by a division. Both can be done from twelve months of metering data and a schedule of the car park's own loads, weeks before anyone prices a trench.

The individual unit is the same job it is on a driveway — the device that has to detect smooth direct residual current, the earthing arrangement where the supply earth is combined with the neutral, the configured output on the certificate — and the charge point installation guide covers all of it. What changes when the bays are shared is everything upstream of the point, everything about containment, and the fact that there is now a bill to divide.

Spare Capacity Is a Subtraction, and It Happens at Two in the Morning

Start by being precise about which supply is being assessed. In a residential block the flats are individually metered and the landlord's supply carries only the communal load: stair and car park lighting, the lifts, the sump and booster pumps, the gate motor, door entry, smoke extract. In an office or a depot one supply usually carries the whole site. The two produce different spare figures and different arguments about who pays, so settle which you are in before reading a meter.

For a half-hourly metered supply, ask the supplier for a full year of consumption data. That file is the most valuable document in the whole exercise, and it is free. It gives the maximum demand actually reached, when it was reached, and — the part almost nobody uses — what the site was drawing during every half hour of every night. This is where a car park scheme stops looking like an upgrade and starts looking affordable.

The building's peak and the car park's peak are not the same hour. An office peaks mid-morning on the coldest working day and idles from seven in the evening; a block's communal load peaks on a winter evening and drops when the lifts stop. Charging lands in the window nobody else uses. So the subtraction is not the annual maximum taken off the agreed capacity once — it is done per half hour across the window, and the constraint is the worst half hour inside it.

A site whose annual peak leaves 30 kVA spare can leave six times that between midnight and six, and whether the scheme can be told to use it is a later question. The number comes first, because it is what decides whether there is a reinforcement conversation at all.

Where the supply is not half-hourly metered, a logger is the substitute on the same conditions the single charge point guide sets out. One car-park warning on top: the loads living in your charging window are the ones people diversify away on paper. Lighting runs all night, the sump pump runs when it rains, the gate motor runs at six — read them off the distribution schedule instead.

1. Identify which meter feeds the bays, and whether the flats or tenancies sit behind it or beside it.
2. Request twelve months of half-hourly data for that meter, not a summary of it.
3. Fix the charging window from the site's own occupancy: when cars arrive, when they leave.
4. Compute headroom for every half hour inside that window across the year, and take the worst one.
5. Add anything committed but not yet connected — a heat pump, a new lift, a kitchen fit-out.

The Resource Is Dwell Time, Not Power

Here is the shift that changes the answer: a bay does not need 7.4 kW, it needs a quantity of energy delivered before the driver leaves, and in a car park you know to within an hour when that is. Work it from the site's own mileage rather than a brochure. A car covering 8,000 miles a year at 3.5 miles per kilowatt-hour uses about 2,300 kWh, a little over 6 kWh a night — fifty-one minutes at 7.4 kW. The bay is occupied for thirteen hours, and for the other twelve the point holds a share of capacity the network was asked to reinforce.

So the bay count is not spare capacity divided by 7.4 kW. It is the energy available inside the charging window divided by the energy each car needs, provided something exists to spread the cars across the window. Those are very different divisions: the first says six bays, the second says forty, and both are arithmetically correct answers to different questions. Which question applies to your site depends on dwell time and nothing else.

The exception is real and not rare. Where dwell is short — visitor bays, retail, a customer car park — power is the product, and a driver offered 3 kW for ninety minutes has been sold something they will complain about. Where a departure time is fixed and the round is long, as with a van fleet leaving at six, headroom cannot be borrowed from a neighbour who has not turned up. On those sites the arithmetic reverts to the first division and the reinforcement conversation is genuine.

Every figure below is worked from stated assumptions, not surveyed. Substitute the site's own: the residents' mileage, the fleet's telematics, the shift pattern on the gate records.

The Count Falls Out of the Floor You Guarantee

A load-management scheme across many points is not the single-point arrangement repeated: its controller allocates current between bays as well as holding a total, and two properties of that allocation decide whether it survives contact with residents.

The first is that the controller must know which phase every point is on. A scheme balancing the sum while three of its points sit on one leg holds a comfortable total and overloads a third of the supply, and because the total looks healthy nobody investigates until a fuse operates. Phase allocation belongs on the schedule, in the controller's configuration and on a label inside the enclosure — three places, because the point moved during a repair is the one that breaks it.

The second is that there is a floor below which throttling stops being throttling. The control pilot signalling in IEC 61851-1 conveys an available current down to 6 A and no lower, so a scheme allocating less does not slow a car, it stops it — and a vehicle that stops at three in the morning does not always resume. Set the guaranteed minimum above that floor and build the scheme around it: guarantee 8 A and a supply that would carry six bays at 32 A carries twenty-four of them.

Which leads to the recommendation that surprises people. On a long-dwell site a static scheme — every point permanently configured to a modest current, no controller, no current transformer, no communications — delivers as much energy per night as a dynamic one, because the constraint was never power, and it keeps working when the controller's network does not. Reach for dynamic management where dwell is short or the bay mix is uneven; reach for static where forty cars have thirteen hours each.

Where a dynamic scheme is used, the current transformer's position and polarity and the behaviour on loss of signal are settled exactly as on a single installation. The multi-point addition is that the failure now hits every bay at once, so the fallback current must be one the supply carries with every point sitting at it.

1. State the guaranteed minimum per occupied bay, in amps, and check it against the signalling floor.
2. Multiply that minimum by the maximum bays that can be occupied at once, per phase.
3. Confirm the result sits inside the worst half hour of headroom, not the average one.
4. Write the phase allocation for every point on the schedule before the first cable is pulled.
5. Specify the fallback current on loss of communications, then prove it by disconnecting the link.
6. Agree in writing what happens when demand exceeds the scheme — queue, share or refuse.

Duct Every Bay the First Time the Surface Comes Up

The irreversible half of this job is the route, not the equipment. Cores through a suspended deck, a trench across tarmac, a tray fixed to a soffit and firestopped where it crosses a compartment line: disruptive once, expensive once, and cheap to extend afterwards if somebody left a duct and a draw rope. Size the containment for the bay count you might reach in ten years and the switchgear for the bays being energised now.

In several jurisdictions this is no longer a judgement call. Approved Document S in England sets out where new and majorly renovated car parks must provide cable routes to bays receiving no charge point, and Article 8 of the Energy Performance of Buildings Directive as amended by Directive (EU) 2018/844 does the equivalent across the EU, on thresholds tied to space count and building type. Confirm which applies first — the passive provision required may already cover what a phased scheme wanted.

Coring a parking deck brings its own conditions. A post-tensioned slab must be scanned and the penetration agreed with somebody competent to say where the tendons are; a crossing through a compartment floor or wall is reinstated to the building's fire strategy; and a deck is laid to fall, so a surface duct must not dam water into a bay. None of these are electrical questions and every one will stop the works.

Record the route while it is open: dimension the duct from two fixed points on the structure, photograph it before backfill, leave the draw ropes in, label the spare ways at both ends. Otherwise phase two proves cheaper to dig beside your trench than to find it.

It Is Still a Car Park

The scheme has to survive people who are not thinking about it. Leads dropped across a footway, a unit at the head of a bay that a bumper eventually reaches, a tethered lead long enough to reach a badly parked car and therefore long enough to lie in a puddle. Cable management and impact protection sized for the vehicles actually in that car park are worth more to the scheme's reputation than the last kilowatt of allocation.

Accessible bays need designing rather than nominating. PAS 1899 sets out what an accessible charge point has to deliver — reach ranges, the space kept clear around the bay and its transfer zone, kerb upstands, connector weight and handling — and BS 8300-1 governs the bay and the route from it. The failure is banal: the accessible bay lands wherever the cabling was easiest, or a bollard protecting the unit blocks the transfer zone. Both are settled at layout stage and neither is fixable without lifting the surface.

In covered and underground car parks the fire strategy governs, and it is the building's strategy rather than a manufacturer's leaflet. NFPA 88A covers parking structures in the US; elsewhere it is the code the building was designed to, together with whatever position the site's insurer takes — and insurers do take positions here, sometimes before the authority does. Ask early and in writing, because a scheme designed and priced before the insurer is consulted carries a very expensive open question.

Finally, phase the works around people who still need to park: bays lost to a compound, a coring programme that stops before the lifts get busy, a notice period the block will accept. Hand over a schedule naming each bay, its phase, its configured current, its circuit and its meter identity — the person adding bays nine through twenty is either building on that document or repeating this whole assessment.