Adding electric vehicle (EV) chargers to an existing factory transformer requires a capacity review of the complete supply path. The difference between nameplate MVA and average factory demand is not automatically available charging capacity. Use measured plant profiles, charger alternating current (AC) input data and credible coincidence cases, then check cables, switchgear, cooling and protection alongside the transformer.
Recover the existing operating picture
Collect time-stamped current, active and reactive power, voltage and relevant temperature or cooling records. Include seasonal production, different shifts, major motor starts and planned process expansion. Identify missing periods rather than assuming that a short survey captured the annual peak.
Keep measured demand separate from connected-load totals. Both are useful, but they answer different questions. A factory may have low average utilization and still experience a brief or sustained high-demand process cycle.
General transformer application guidance supports reviewing ratings and operating conditions. It does not establish spare capacity without the site’s actual duty and equipment records. [1]
Place charging at the correct AC boundary
Obtain charger input current and power characteristics rather than adding direct current (DC) output ratings directly to factory demand. Include cooling, lighting and other charging-area auxiliaries if they share the transformer.
IEC 61851-23 addresses DC charging equipment within its scope, not the factory’s combined supply capacity. [2] The site assessment must determine whether charging is uncontrolled, scheduled or limited dynamically according to factory load.
An illustrative factory drawing 1.4 MVA from a 2 MVA transformer does not establish a universally available 0.6 MVA charging allowance. The snapshot may omit a later production peak, reactive changes, harmonic effects or a contingency restriction. It is a starting observation, not a capacity certificate.
Compare coincident cases and the rest of the path
| Case | What to include |
|---|---|
| Normal production with charging | Actual simultaneous factory and charger duty |
| Peak process interval | Motor or process demand plus permitted charging |
| Seasonal extreme | Ambient conditions and changed auxiliary loads |
| Demand-control failure | Defined fallback charger limit |
| Future production | Committed additions to the factory load |
Review cable and bus current ratings, voltage behavior, switchgear duties and protection coordination. Increasing the transformer’s allowable loading does not automatically increase the capacity of every downstream component.
Where nonlinear currents are significant, obtain the relevant spectra and an assessment within the applicable transformer scope. IEEE C57.110 is not a blanket rule for every specialized converter arrangement. [3]
Choose a bounded connection strategy
The outcome may be unrestricted charging within a demonstrated envelope, scheduled charging, dynamic demand management, a separate supply or equipment upgrades. Each option should state its operating restrictions and the service delivered to vehicles.
Thermal assessment should use the appropriate transformer model and actual chronology. Existing equipment condition and maintenance history are separate considerations from whether the proposed load fits a theoretical rating. [4]
The handover should preserve the baseline measurements, charger data, control settings and approved combined-duty cases. Adding more chargers or changing factory production should trigger review when it alters those assumptions.
The useful answer is not “there is spare MVA.” It is a documented statement of how much charging can operate, at what times and under which controls, without exceeding the demonstrated limits of the existing electrical system.
Core procurement checkpoint: A capacity review is not authorization to alter the core
Adding chargers to an existing factory transformer calls for an equipment assessment before any replacement-core proposal. The manufacturer should establish whether the unit can be retained and what data or tests are required. Where a new core is genuinely specified, use a complete dimensional and interface drawing rather than inferring a suitable replacement from nameplate MVA or apparently unused capacity.
For the core enquiry: Existing-unit design reference; approved replacement decision; complete core geometry; matching interfaces; material and inspection requirements.