This is the most common question in commercial charging and the one most often answered with a calculation that has no relationship to the building.
The supply rating tells you what the connection is rated for. It does not tell you what the building already draws at its busiest, which is the number that determines how many vehicles can charge.
Start from measured load, not design load
Buildings rarely draw what their documentation predicts, and the error runs in both directions. Design figures reflect assumptions made years ago about occupancy and equipment that have since changed.
Where recorded data exists, use it. Where it does not, a temporary logger at the supply point across a period covering the seasonal and weekly peak is the first step, and skipping it means designing against a guess.
Find the peak that actually binds
Coincidence is the factor that changes the answer most. Overnight charging at a commercial site sits outside the peak and is close to free capacity. Daytime workplace charging lands directly on it.
- Seasonal peak, which for most buildings is cooling or heating driven.
- Weekly pattern, and whether the peak is occupancy or plant driven.
- Coincidence between that peak and the intended charging window.
- Any contracted demand limit, which may bind before the physical rating.
Managed and unmanaged give different answers
An unmanaged site must be designed for every bay drawing full rate simultaneously, because nothing prevents it. That is the number most quotes are based on, and it is the smallest possible answer.
A managed site can be designed against a controlled allowance, because the system enforces the total. The difference is frequently large enough to change the project from unviable to viable.
Diversity has to be defensible
Assuming not every bay draws at once is reasonable and must be justified rather than assumed. The right assumption depends on fleet composition, dwell time and whether charging is managed, and it changes as adoption grows.
A diversity factor chosen to make a project work, rather than derived from the site, is a design risk that surfaces as nuisance tripping once occupancy rises.
Protection and distribution can bind before the supply
A site with supply headroom may still be limited by the existing distribution board, cable routes or protection arrangement. Capacity at the incomer is not capacity at the parking bays.
Assessing the path from supply to bays, rather than only the incoming capacity, is what turns a theoretical bay count into an installable one.
Design the full build now, install in phases
The second phase is always harder than the first, because the easy capacity was spent. Establishing at the outset what the site supports at full build, and what triggers a supply upgrade, prevents an expansion that requires redoing the first installation.
Sizing containment and distribution for the eventual position while populating charge points incrementally is usually the cheapest path to a fully charged car park.