To size a dry type transformer for a generator, I first calculate the connected and demand load in kVA, then compare that requirement with the generator’s continuous kVA rating. I also verify the generator and transformer voltage, phase, frequency, power factor, motor-starting demand, transformer inrush current, protection settings, and site conditions. As a practical starting point, I select a transformer with enough capacity for the calculated operating load while keeping the generator within its permitted continuous loading range, then confirm the result with a transient and protection review.
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The transformer must be sized as part of the complete generator system, not as an isolated component. A transformer that appears adequate during steady-state operation may still cause excessive voltage dip during energization or motor starting. The final selection should therefore be based on equipment data, the generator manufacturer’s limits, and the actual electrical design.
The first objective is to determine how much apparent power the downstream load requires. I normally collect the rated power, power factor, starting method, duty cycle, and operating schedule for each load connected to the transformer. This information is more reliable than simply adding nameplate kW values, because transformers and generators are commonly rated in kVA.
For a three-phase load, the basic relationship is kVA = kW ÷ power factor. For example, a 400 kW load operating at a 0.8 power factor requires 500 kVA before applying any demand or future-growth assumptions. If the load includes variable-speed drives, rectifiers, UPS systems, or other nonlinear equipment, I also review harmonic current and input characteristics rather than relying only on the nominal kW.
Next, I separate continuous loads, intermittent loads, standby loads, and loads that may operate simultaneously. A load schedule should identify which equipment starts together and which equipment is intentionally sequenced. If the calculated diversified load is 500 kVA, I do not automatically select a 500 kVA transformer; I check operating margin, ambient conditions, future expansion, and the generator’s ability to supply the transformer.
Generator nameplates commonly show standby, prime, or continuous ratings, and these ratings are not interchangeable. I use the rating that matches the project duty and operating schedule, then confirm whether the manufacturer specifies a minimum or maximum loading range. The transformer’s expected demand must remain within the generator’s continuous operating capability, with additional allowance for starting and transient events.
For example, if a generator is rated at 800 kVA for the required duty, a calculated transformer load of 700 kVA may leave limited headroom for motor starting, voltage regulation, and load growth. A conservative project may target approximately 80% of the available generator rating for normal steady-state loading, but the correct value depends on the generator manufacturer and the system design. I treat this percentage as a design checkpoint, not a universal rule.
I select the transformer based on the maximum expected simultaneous load, not merely the sum of every connected device. The selected kVA should cover normal demand and a documented allowance for future expansion where required. At the same time, oversizing should be considered carefully because a substantially oversized transformer can increase purchase cost, physical footprint, no-load losses, and generator energization inrush.
As an example, if the diversified operating load is 500 kVA and the project requires a 20% capacity allowance, the preliminary transformer size is 600 kVA. I then check whether a standard rating near that value is available and whether the generator can support the transformer’s magnetizing inrush. The final rating may change after reviewing motor starting and the manufacturer’s transient data.
The transformer primary voltage must match the generator output voltage, while the secondary voltage must match the distribution system and connected equipment. I verify line-to-line voltage, line-to-neutral voltage where applicable, frequency, phase sequence, and the required neutral arrangement. A voltage mismatch can prevent correct operation even when the kVA rating is adequate.
I also confirm whether the generator output is three-phase or single-phase and whether the transformer should be delta-wye, wye-wye, or another connection. The winding connection affects grounding, neutral availability, phase shift, and fault-current behavior. These details should be shown on the single-line diagram and confirmed before manufacturing.
Motor starting is often the most important sizing issue in generator applications. Large motors, pumps, compressors, fans, and fire-protection equipment can draw several times their running current during direct-on-line starting, although the actual value depends on the motor and starting method. I request starting-current data and determine whether motors start across the line, through a soft starter, or through a variable-frequency drive.
Transformer energization creates a separate transient called inrush current. Even when downstream loads are disconnected, the transformer may briefly draw a high magnetizing current when energized. I therefore ask the transformer manufacturer for suitable inrush information and coordinate the energization sequence, generator excitation response, breaker settings, and any automatic transfer controls.
Dry type transformer capacity depends on the specified installation conditions. I review ambient temperature, elevation, ventilation, enclosure type, humidity, dust, corrosive exposure, indoor or outdoor placement, and available clearance. Where the installation differs from the manufacturer’s reference conditions, a derating or alternative cooling arrangement may be required.
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The enclosure must also suit the environment and personnel-access requirements. A ventilated enclosure may be appropriate for a clean indoor electrical room, while a more protective enclosure may be necessary in dusty, wet, or outdoor locations. I avoid assuming that a standard indoor design is suitable for every generator enclosure or power plant environment.
Generator fuel use, alternator current, and voltage performance are affected by power factor, so I review the expected operating power factor at the generator terminals. Capacitor banks, UPS equipment, and drives may also influence harmonics or switching behavior. Where harmonic content is significant, I discuss an appropriate transformer design, thermal capability, and system mitigation with the electrical engineer and supplier.
Transformer impedance affects voltage drop and available fault current. A lower impedance may improve voltage regulation but can increase secondary fault current, while a higher impedance can limit fault current but create more voltage drop. I verify that the proposed impedance works with the generator subtransient reactance, breaker interrupting ratings, cable sizes, and protection coordination.
Primary and secondary overcurrent protection must be selected as a coordinated system. I check transformer protection, generator breaker settings, secondary feeder protection, grounding, and transfer-switch behavior. Protection settings should be confirmed by the responsible electrical designer rather than copied from a previous installation.
Generator systems may run for short emergency periods or for extended prime-power operation. A transformer selected for occasional standby duty may require a different thermal and maintenance review than one operating continuously. I ask whether future loads, parallel generators, additional transfer switches, or expansion feeders must be included in the initial design.
| Design Item | Information to Confirm |
|---|---|
| Load demand | Running kW, power factor, diversified kVA, and duty cycle |
| Generator capacity | Standby, prime, or continuous rating and transient limits |
| Electrical interface | Primary voltage, secondary voltage, phase, frequency, and neutral |
| Transient performance | Motor starting current, transformer inrush, and energization sequence |
| Installation conditions | Ambient temperature, elevation, enclosure, ventilation, and clearances |
One common mistake is sizing from the generator kW rating without converting the load to kVA. Another is selecting a transformer equal to the connected load while ignoring motor starting, future expansion, or nonlinear current. I also see projects where voltage and phase are assumed from a previous system instead of being checked against the actual generator and distribution equipment.
Oversizing can also be a problem. A very large transformer may operate at a low load level, increase no-load losses, require more space, and create a larger energization event for the generator. Conversely, undersizing can lead to thermal stress, excessive voltage drop, nuisance trips, and insufficient capacity during simultaneous operation.
Before approval, I prepare or review a load schedule and single-line diagram showing the generator, transformer, transfer equipment, feeders, and major loads. I compare the calculated demand with the proposed transformer rating and the generator’s applicable rating. I then request a technical submittal covering winding connection, impedance, insulation system, enclosure, cooling, dimensions, weight, sound expectations, taps, and required accessories.
For systems with large motors or sensitive electronic loads, I recommend a documented voltage-drop and transient review. The review should consider generator voltage recovery, transformer inrush, motor starting sequence, protection coordination, and the worst credible operating condition. If the result is uncertain, the design team can consider staged starting, reduced-voltage motor starting, pre-energizing the transformer, or a different transformer and generator arrangement.
At BTW, we support buyers by converting application information into a practical dry type transformer specification. We can review the required kVA, voltage ratio, phase, frequency, winding connection, impedance, enclosure, cooling arrangement, installation environment, and delivery requirements. The final configuration should be based on confirmed project data rather than a generic catalog selection.
When requesting a quotation, I recommend sending the generator datasheet, load list, single-line diagram, motor-starting information, site conditions, required standards, and target delivery date. This allows the supplier to identify missing information before production and helps prevent changes after order placement. BTW can also clarify technical questions, provide a project-specific offer, and coordinate documentation for engineering review.
The correct dry type transformer size for a generator is the result of load calculation, generator-capacity verification, voltage matching, transient analysis, and protection review. I would use the diversified kVA requirement as the starting point, then confirm that the generator can handle both normal operation and the largest credible starting or energization event. A final selection should also reflect site conditions, future demand, enclosure requirements, and the transformer manufacturer’s technical data.
Your next step is to assemble the generator rating, load schedule, voltage details, motor-starting data, site conditions, and single-line diagram. Send these details to BTW for a technical review and quotation based on your actual application. This process gives your engineering and purchasing teams a clearer basis for selecting a reliable dry type transformer without depending on unsupported assumptions.
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