Auxiliary Transformer Sizing Calculation – Complete Engineering Guide for Accurate Load & Capacity Selection

Auxiliary transformer sizing calculation is a critical step in power system design for industrial plants, substations, commercial buildings, and renewable energy projects. An accurately sized auxiliary transformer ensures reliable operation of control systems, lighting, HVAC, protection relays, battery chargers, and other essential loads. If the transformer is undersized, it will overheat and fail prematurely. If oversized, it increases capital cost and reduces efficiency.

This complete engineering guide explains the practical approach to auxiliary transformer sizing calculation, including load assessment, demand factors, diversity, short circuit considerations, voltage regulation, and future expansion planning. The aim is to help engineers select the correct transformer rating with technical accuracy and long-term reliability in mind.

Understanding the Purpose of an Auxiliary Transformer

Before starting any auxiliary transformer sizing calculation, it is important to understand the role of the auxiliary transformer in the system.

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An auxiliary transformer typically steps down medium voltage to low voltage levels such as 415 V, 400 V, or 230 V to supply auxiliary loads. These loads may include:

• Motor control centers
• Plant lighting
• Ventilation systems
• Battery chargers and UPS
• Control panels and PLC systems
• Cooling systems for main transformers

In substations and power plants, auxiliary power is vital for operational continuity. A failure in auxiliary supply can result in a total plant shutdown. Therefore, correct transformer capacity selection is not only an electrical design task but also a reliability decision.

Step 1: Identify and Categorize All Connected Loads

The first step in auxiliary transformer sizing calculation is to prepare a detailed load list. Every connected load must be identified along with its rated power, voltage, and operating characteristics.

Loads are generally categorized as:

• Continuous loads
• Intermittent loads
• Motor loads
• Resistive loads
• Critical loads

Below is a typical load schedule format used in engineering design.

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Table 1: Sample Load Data for Auxiliary Transformer

This connected load is only the starting point. The actual transformer rating must account for demand and diversity factors.

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Step 2: Apply Demand Factor and Diversity Factor

In practical systems, not all loads operate simultaneously at full capacity. This is where demand factor and diversity factor are applied during auxiliary transformer sizing calculation.

Demand factor is defined as:

Demand Factor = Maximum Demand / Total Connected Load

For industrial auxiliary systems, a typical demand factor ranges from 0.7 to 0.9 depending on the process.

Assume a demand factor of 0.8.

Maximum Demand = 186 kW × 0.8 = 148.8 kW

Now convert kW to kVA using average power factor. Assume overall power factor of 0.9.

• kVA = kW / Power Factor
• kVA = 148.8 / 0.9 = 165.3 kVA

This gives the preliminary transformer capacity requirement. Find more Transformer calculators here

Step 3: Consider Motor Starting Current

Motor loads significantly affect auxiliary transformer sizing calculation because starting current can be 5 to 7 times the full load current.

If multiple motors start simultaneously, the transformer must withstand voltage dip and thermal stress. In auxiliary systems, sequential starting is often implemented to reduce impact.

For example, if the largest motor is 18 kW:

Full load current at 415 V:

• I = (18 × 1000) / (√3 × 415 × 0.85)
• I ≈ 29.4 A

Starting current at 6 times:

I_start ≈ 176 A

The transformer must handle this inrush without excessive voltage drop. A typical acceptable voltage dip is less than 15 percent for motor starting.

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Step 4: Add Future Expansion Margin

A professional auxiliary transformer sizing calculation always includes future load growth. Industrial facilities expand, and auxiliary systems often require additional capacity over time.

Standard practice is to add:

• 10 to 20 percent margin for commercial installations
• 20 to 25 percent margin for industrial plants

If calculated demand is 165 kVA, adding 20 percent margin:

Required Capacity = 165 × 1.2 = 198 kVA

The nearest standard rating above this value should be selected. Standard transformer ratings are:

Table 2: Common Transformer Ratings

In this case, 200 kVA would be an appropriate selection.

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Step 5: Check Transformer Loading Percentage

After selecting a 200 kVA transformer, verify loading:

• Actual Demand = 165 kVA
  Transformer Rating = 200 kVA
• Loading Percentage = (165 / 200) × 100 = 82.5 percent

This is within the ideal operating range of 70 to 85 percent. Operating within this range improves efficiency and extends insulation life.

Step 6: Evaluate Voltage Regulation and Impedance

Another important aspect of auxiliary transformer sizing calculation is transformer impedance and voltage regulation.

Typical impedance values:

• 100–250 kVA transformer: 4 to 6 percent

Voltage drop under load should remain within acceptable limits, typically below 5 percent for distribution systems.

Voltage regulation ensures stable performance of sensitive loads such as PLCs, relays, and control circuits.

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Step 7: Consider Short Circuit Level

The transformer must withstand fault current until protection devices operate.

Short circuit current is calculated as:

Fault Current = (Rated Current) / (Impedance in per unit)

For a 200 kVA transformer at 415 V:

Full load current:

I = (200 × 1000) / (√3 × 415) ≈ 278 A

If impedance is 5 percent (0.05 per unit):

Fault Current ≈ 278 / 0.05 = 5560 A

Switchgear and circuit breakers must be rated above this value.

Step 8: Cooling Method and Installation Type

Cooling method affects capacity and performance. Common types include:

• ONAN (Oil Natural Air Natural)
• Dry type air cooled
• Cast resin transformers

For indoor installations such as control rooms, dry type transformers are often preferred due to fire safety. For outdoor substations, oil immersed transformers are common.

Ambient temperature also affects auxiliary transformer sizing calculation. High temperature reduces effective capacity. Correction factors must be applied if ambient exceeds 40°C.

Step 9: Efficiency and Loss Evaluation

Transformer losses include:

• Core loss (no load loss)
• Copper loss (load loss)

Selecting a transformer operating near optimal load improves efficiency and reduces energy cost over its lifetime.

For example:

Table 3: Typical Loss Data for 200 kVA Transformer

Energy efficiency becomes more critical in continuous operation facilities.

Step 10: Compliance with Standards

Auxiliary transformer sizing calculation must comply with international standards such as:

• IEC 60076
• IEEE C57
• Local utility regulations

These standards define temperature rise limits, insulation class, testing requirements, and permissible overload conditions.

Practical Engineering Checklist

Before finalizing the transformer rating, confirm:

• All loads are listed and verified
• Demand factor is realistic
• Motor starting conditions are considered
• Future expansion margin is included
• Short circuit level matches switchgear rating
• Installation environment is evaluated
• Cooling type is selected appropriately

A systematic auxiliary transformer sizing calculation ensures that the selected transformer performs reliably throughout its service life.

Final Example Summary

• Connected Load: 186 kW
• Demand Factor: 0.8
• Maximum Demand: 148.8 kW
• Power Factor: 0.9
• Required kVA: 165 kVA
• Future Margin: 20 percent
• Final Selection: 200 kVA Transformer

This method provides a balanced solution between technical reliability and cost efficiency.

Conclusion

Auxiliary transformer sizing calculation is not just about adding loads and selecting the next higher standard rating. It requires careful evaluation of demand patterns, motor starting currents, voltage regulation, short circuit strength, thermal limits, and future expansion.

A well executed auxiliary transformer sizing calculation ensures system stability, operational safety, and long equipment life. Engineers who follow a structured calculation method can avoid common mistakes such as oversizing, undervaluing motor inrush, or ignoring environmental conditions.

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