Sheet CALC-19 · Design Guide & Calculator
EV Charging Design: Will Your Supply Take It?
The question that decides most EV charging projects is not which charger to buy — it is whether the existing supply can carry them, and what load management can do before a utility supply upgrade becomes necessary. Model the scheme below, then read the guide for what the numbers mean in Kenyan practice.
The Guide
Why Coincidence and Load Management Decide the Supply
Add up the nameplate ratings of ten 7.4 kW wallboxes and you get 74 kW. That is the connected load, but it is not automatically the site’s coincident EV demand. Vehicles arrive with different states of charge, dwell for different periods and stop drawing power when charging is complete. The design therefore needs an explicit coincidence assumption or measured charging profile.
There is no universal IEC diversity factor that turns every residential, workplace or public charging scheme into one correct demand figure. The presets in this calculator are planning assumptions only. Replace them with measured data, a fleet charging schedule, charger-management data or another project-specific basis whenever one is available. Rapid and fleet charging often require a more conservative assumption than long-dwell parking.
The Design Sequence
Establish the existing demand
Not the nameplate of the transformer — the actual measured maximum demand. A week of logging at the incomer is worth more than any assumption.
Set the connected EV load
Charger type and quantity per parking group. Decide this from dwell time and user expectation, not from what the client saw elsewhere.
Apply diversity
Use measured profiles or a documented project assumption. The calculator offers editable planning presets, not normative diversity factors.
Test against the supply
Existing demand plus assessed EV load, against supply capacity with a spare margin kept for growth. This is the go or no-go moment.
Add load management
If it does not fit, a static cap or a dynamic system that watches the incomer will almost always be cheaper than a supply upgrade.
Design the circuits
Each point on its own final circuit with its own RCD protection and DC fault detection, cable sized for continuous duty and volt drop over long car park runs.
Charger Types and Where They Fit
| Charger | Typical rating | Supply | Where it fits |
|---|---|---|---|
| AC dedicated EVSE | 3.7 kW | Single phase 16 A | Low-power overnight charging where long dwell time makes speed unnecessary |
| AC wallbox | 7.4 kW | Single phase 32 A | Homes and long-dwell parking; useful overnight charging for many vehicles |
| AC three phase | 11 - 22 kW | Three phase 16 - 32 A | Workplaces, hotels, retail; a useful top-up in a few hours |
| DC rapid | 30 - 60 kW | Three phase, dedicated | Forecourts and fleet depots; 30 to 60 minutes to most of a battery |
| DC ultra rapid | 120 kW and above | Three phase, often own transformer | Highway corridors; needs serious supply capacity |
The temptation is always to specify faster. Resist it where dwell time and vehicle onboard-charger capability do not justify the cost. A 22 kW AC point can add little practical benefit over 11 kW for many all-day parking applications, while increasing connected load and potentially cable and supply requirements.
Protection
What EV Circuits Need That Ordinary Circuits Do Not
- Dedicated EV supply requirements. EV charging circuits are a special installation covered by IEC 60364-7-722. Final protective arrangements must be coordinated with the actual EVSE and its manufacturer’s instructions.
- Residual-current and DC fault protection. For AC charging, verify the required RCD arrangement and protection against DC residual current. IEC 62955 covers residual direct current detecting devices (RDC-DD) used for Mode 3 charging. Do not assume an ordinary Type AC RCD is suitable.
- Continuous loading. EVSE can draw close to rated current for long periods. Verify cable current-carrying capacity, protective-device rating, terminals, grouping and ambient correction factors for the actual installation.
- Volt drop and long routes. Car parks can create long cable runs. The calculator gives a screening result, but the final design must use the selected cable construction and applicable installation data.
- Earthing and bonding. Confirm the supply earthing arrangement, protective bonding and automatic disconnection. Where an open-PEN or neutral condition is relevant, use an engineered protective solution appropriate to the supply system rather than assuming an earth electrode alone solves it.
- DC rapid chargers. Treat high-power DC EVSE as manufacturer-engineered equipment and apply the relevant IEC 61851-23 requirements. Its internal protection cannot be reduced to the same rule used for a small AC wallbox.
Standards & Scope
What This Calculator Does, and What It Does Not
This is a feasibility and preliminary-design calculator. It screens connected load, an editable coincidence assumption, load-management headroom, apparent-power loading, rated charger current, indicative conductor size, volt drop, annual energy and simple commercial metrics.
Final electrical design should be checked against the standards and requirements applicable to the project. Key references include IEC 60364-7-722 for low-voltage supplies to electric vehicles, the IEC 61851 series for conductive charging equipment, IEC 61851-23 for DC EV supply equipment, the IEC 62196 series for EV couplers and EPRA’s E-Mobility Charging Infrastructure Guidelines. Cable ampacity, protection coordination, fault level, selectivity, earthing, surge protection, metering, accessibility, fire strategy, civil works and utility connection requirements remain project-specific.
In Kenyan Practice
What Actually Decides the Project
A supply upgrade can dominate the budget. A new transformer, utility connection work and associated civils can cost more than the chargers themselves. Proper load management can therefore avoid or defer an upgrade where the existing supply has usable headroom.
Use the current e-mobility tariff, not a generic building tariff. Kenya Power stated on 4 June 2026 that the e-mobility tariff is KSh 16/kWh during peak hours and KSh 8/kWh off-peak. Confirm the applicable tariff, taxes, levies, time bands and any demand/network charges for the actual account. Model the peak demand as well as the kilowatt-hours.
Solar pairs well, but only with honest timing. Workplace charging matches the solar day almost perfectly; residential overnight charging does not, and needs storage or grid to serve it. Test the combination in the hybrid sizing calculator before promising a client that solar will cover their fleet.
Metering and revenue. If you intend to resell energy, the metering arrangement and the regulatory position need settling early with EPRA, not after the chargers are installed.
Design for the second phase. Fleets grow. Lay containment and leave board capacity for the points the client will want in three years, because the cost of doing it now is a fraction of the cost of doing it twice.
Related Tools
Where This Goes Next
The assessed EV demand belongs in the load schedule, and feeds transformer sizing if a supply upgrade proves unavoidable. Final circuits are confirmed in the cable sizing and circuit breaker tools. All the tools sit in the calculator suite.
Beyond the Calculator
Planning an EV Charging Installation?
We assess supply capacity, design charging schemes with load management, and prepare the drawings, schedules and specifications for approval and tender — or teach your team to do it.