How to Calculat the Size and Capacity of a Power Tranformer？

When selecting a transformer, determining the appropriate size is crucial to ensure efficient and reliable operation. An undersized transformer may overheat and fail prematurely, while an oversized one leads to unnecessary costs and inefficiencies. The calculation of transformer size depends on various factors, including the total load demand, voltage levels, power factor, and safety margins. Whether for industrial, commercial, or residential applications, accurately sizing a transformer is essential for optimal performance and energy efficiency. This article provides a step-by-step guide to calculating the correct transformer size, helping you make informed decisions based on load requirements and electrical specifications. Understanding these principles will not only enhance system reliability but also improve overall power distribution efficiency.

1. Identify Load Types
Continuous Load: Loads that operate continuously for 3hours up (e.g., lighting, HVAC).
Intermittent Load: Loads that operate for shorter periods (e.g., elevators, pumps).
Standby Load: Loads that are not normally in use but are available when needed (e.g., backup generators, emergency lighting).

2. Determine Individual Loads
List all the loads in your system and categorize them into continuous, intermittent, and standby. Note the power rating (in kW or kVA) of each load.

3. Apply Demand Factors
Demand Factor (DF): This is the ratio of the maximum demand on the system to the total connected load. It accounts for the fact that not all loads operate at the same time or at their maximum capacity.
For continuous loads, the demand factor is typically 1 (or 100%) since they are always on. For intermittent and standby loads, the demand factor is less than 1, depending on the usage pattern.

Calculate the demand load for each category: Demand Load = Connected Load x Demand Factor

4. Apply Diversity Factor
This accounts for the fact that not all loads will be operating simultaneously. It is the ratio of the sum of individual maximum demands to the maximum demand of the entire system.

Diversity Factor = Sum Of Individual Maximum Demands \ Maximum Demand of the System.

Diversity factor is typically greater than 1.
A higher diversity factor indicates a lower maximum demand on the transformer.

Calculate the diversified load:

Diversified Load = Sum Of Demand Loads \ Diversity Factor.

5. Sum the Load: Sum the diversified loads for all categories (continuous, intermittent, and standby) to get the total load on the transformer.

6. Select Transformer Size: Choose a transformer with a capacity equal to or greater than the total calculated load. It’s common practice to add a safety margin (e.g., 10-20%) to account for future expansion or unforeseen increases in load.

E.G
Assume the following loads:
Continuous Loads: 200 kW (DF = 1)
Intermittent Loads: 100 kW (DF = 0.5)
Standby Loads: 50 kW (DF = 0.2)
Diversity Factor: 1.2
Efficiency: 90%

1. Calculate Demand Loads:
Continuous: 200kW x 1 = 200kW
Intermittent: 100kW x 0.5 = 50kW
Standby: 50kW x 0.2 = 10kW

2. Sum of Demand Loads:
200kW + 50kW + 10kW = 260kW

3. Apply Diversity Factor:
Diversified Load = 260kW / 1.2 = 217kW

Add 10% safety margin:
217kW x 1.1 = 239kW (output power).

Add Efficiency 90% = (0.9)
239kw / 0.9 = 266kw (Input power)

KVA = kw /pf. KVA = 266kw / 0.8
KVA = 333Kva

Choose Transformer capacity at least 333kVA OR NEXT.

By following these steps, you can accurately size a transformer considering demand factors and diversity factors for continuous, intermittent, and standby loads.
Base on NEC & IEC Standard.