Introduction: A 1000 kVA distribution transformer in a factory normally steps down 10 kV, 11 kV, 20 kV, or 35 kV to 0.4 kV, 0.415 kV, or 0.433 kV, and the tap range and vector group then follow the plant’s single-line diagram.
Factory project engineers usually reach this question after the load schedule is complete and the utility connection point is known. The answer is not one voltage number. It defines the transformer ratio, tap range, vector group, impedance, cooling, and the information sent to suppliers. A clear definition early avoids rework, because the transformer must match the grid supply, motor control center, fault level, and local code. 1000 kVA is a common reference capacity in factory distribution, but the load profile and design margin decide whether it is the right size.
1000 kVA sits in a practical range for many factory distribution projects. A production building with mixed motor loads, lighting, HVAC, compressed air, and office demand often falls near this size after diversity and future growth are applied. Engineers use it because it is a standard capacity class that fits common switchgear, cable sizes, and protection settings without forcing the site into a larger substation layout. It also provides useful design margin. If calculated demand is around 700–800 kVA, a 1000 kVA transformer can operate below its nameplate limit and leave room for a new line or additional motor loads. That margin matters when production equipment changes faster than the building. The number is a reference point rather than a rule. A plant with heavy continuous motor load, high starting current, or long low-voltage cable runs may need a different capacity. Match the transformer to the load schedule rather than forcing the load into a 1000 kVA template. For a factory project, review capacity class and voltage ratio together. A 1000 kVA unit can be built as an oil-immersed, three-phase transformer within a 30 kVA to 2500 kVA production range, but the final rating still follows actual demand, ambient conditions, and future expansion plans.
The voltage ratio comes from two fixed points: the site’s incoming grid voltage and the plant’s low-voltage distribution system. Everything between them is engineering judgment. Start with the utility connection letter, single-line diagram, and voltage ratings of the largest motor and distribution board. Then choose a high-voltage primary and low-voltage secondary that keep the plant within acceptable voltage drop and fault current limits. For a 1000 kVA factory transformer, common high-voltage options are 10 kV, 11 kV, 20 kV, and 35 kV. The low-voltage side is typically 0.4 kV, 0.415 kV, or 0.433 kV. The selected pair becomes the first line in any inquiry to a distribution transformer manufacturer.
The local utility decides whether the factory receives 10 kV, 11 kV, 20 kV, or 35 kV. In many industrial parks, 10 kV and 11 kV are common for medium-size factory loads. A 20 kV connection appears where the utility standard uses it, and 35 kV is used for larger sites or where distribution line length makes a higher voltage more practical. Incoming voltage affects switchgear, cable, and protection costs. A 35 kV supply may reduce line current and cable size, but it requires higher insulation levels and more expensive switchgear. Order the transformer with the primary voltage that matches the connection, not a generic “medium voltage” label. Fault level and utility connection conditions belong in the same review because they influence transformer impedance and the downstream protection study.
On the secondary side, 0.4 kV, 0.415 kV, and 0.433 kV are all practical for factory distribution. The difference looks small, but it matters to motors, contactors, drives, and distribution boards. A 0.4 kV system is a common nominal value for 400 V equipment. A 0.415 kV secondary suits plants with 415 V motors and equipment commonly used in 50 Hz regions. A 0.433 kV secondary gives a little more voltage headroom for long cable runs or motor starting, where voltage dip is a concern. Select the low-voltage output with the motor control center rating, cable voltage drop, and available fault current in mind. If the plant has many direct-on-line motors, check starting voltage dip and transformer impedance together. The secondary voltage determines whether the plant’s equipment operates inside its designed voltage window.
Once the ratio is clear, the next inquiry details are tap range, vector group, impedance, and cooling. A standard off-circuit tap changer is normally ±2 × 2.5% or ±5%. This range lets the transformer be adjusted during commissioning or a planned shutdown to match actual grid voltage. Off-circuit means the tap change is made with the transformer de-energized, which suits factory distribution where supply voltage is reasonably stable and the tap setting is not changed under load. Where utility voltage swings widely, discuss the tap range and the need for on-load adjustment with the supplier and the utility. Dyn11 is the standard vector group for this class of oil-immersed distribution transformer. It provides a delta primary and a star secondary with a neutral point, which is useful for mixed factory loads that include single-phase lighting, control circuits, and small equipment. The neutral path helps handle unbalanced loads, and the delta primary provides a path for triplen harmonics. Custom vector groups are available for projects with special paralleling or protection requirements, but Dyn11 covers most factory distribution designs. Impedance usually sits between 4% and 6%. That range balances voltage regulation against short-circuit current. Lower impedance improves voltage regulation but raises fault current; higher impedance reduces fault current but increases voltage drop. Check downstream breaker and cable ratings against the chosen impedance. ONAN cooling uses natural oil circulation and natural air circulation, so there are no fans or pumps to maintain in normal service. The sealed corrugated tank design removes the conservator and lets the tank walls flex as oil volume changes, keeping the mineral oil isolated from air and moisture. The transformer uses a wound core, copper or aluminum windings as specified, and IEC 60296 mineral oil. Top oil temperature rise is limited to 60°C and average winding rise to 65°C. Routine and type test reports are available for the project file. A 1000 kVA unit within the 30 kVA to 2500 kVA range can be configured with a 6 kV to 35 kV primary and a 0.4 kV to 0.433 kV secondary. New Trans Tech (Newtranstech) builds these oil-immersed distribution transformers to IEC 60076 and provides a 12 to 18 month standard warranty.
For a factory project, the 1000 kVA transformer decision is complete when you can write four lines: primary voltage, secondary voltage, tap range, and vector group. Add impedance, cooling, and capacity class, and you have a technical package for internal approval and supplier inquiry. Keep the final voltage ratio tied to the site grid connection, load equipment, fault level, and local code. Treat 1000 kVA as a common reference capacity, not a universal size for every plant. When you contact distribution transformer suppliers, send the single-line diagram, load schedule, motor starting data, and required standards. Ask for a formal RFQ that confirms price, lead time, tap range, test reports, and warranty start date.
A：It should use the voltage pair that matches the site. Common high-voltage inputs are 10 kV, 11 kV, 20 kV, and 35 kV, and common low-voltage outputs are 0.4 kV, 0.415 kV, and 0.433 kV. The utility connection and the motor and distribution equipment ratings decide the final ratio.
A：Dyn11 is the standard and practical choice for most factory distribution because it provides a neutral for unbalanced loads and a delta path for harmonics. An off-circuit tap changer with ±2 × 2.5% or ±5% is also common, and it lets you adjust the ratio during commissioning or a planned shutdown to match actual grid voltage.
A：Start with the load schedule and the 10 kV utility connection. Confirm the low-voltage equipment is rated for 0.4 kV, check motor starting and voltage drop, then select transformer impedance, tap range, and Dyn11 vector group. Send those details with the single-line diagram to a distribution transformer manufacturer for a formal quote.
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