Sizing of Transformer capacity is based on the quantity and type of the load.
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TRANSFORMER |
Step 1:
- Calculate the total connected load from the given load list.
- Add 10% or Whatever the requirement in percentage of load will add up in future, with the total connected load.
- Calculate maximum Demand based on the demand factor (Individually or Totally).
- Convert calculated maximum demand in KW to KVA at given power factor
- Apply de-rating factor for transformer (90% Efficiency) - Calculate peak demand in KVA with consideration of De-rating factor.
- Consider Maximum Demand calculated by above procedure as Solution - A
Example : Power Factor – 0.89,
Overall Demand Factor – 0.9, Motor Efficiency – 90%, Star-Delta Starter – 0.87
Pf, Tr Derating Factor – 85%
S.No |
Name of the load |
Load in (KW) |
Demand Factor |
1 |
Lighting |
200 |
0.7 |
2 |
HVAC |
750 |
0.7 |
3 |
Plumbing |
150 |
0.4 |
4 |
Lift |
60 |
0.8 |
5 |
Fire Fighting |
15 |
|
6 |
External Lighting |
25 |
0.3 |
7 |
Others |
65 |
0.3 |
|
Total |
1265 |
|
Total Connected Load as per the given list = 1265 KW
Future consideration of 15% = 1265 + 15%
= 1454.75 KW
Total Maximum Demand (in KW) = 1454.75 x 0.9
= 1309.28 KW
Total Maximum Demand (in KVA) = 1309.28 / Power Factor
= 1309.28 / 0.89
= 1471.10 KVA
Maximum Demand after Derating factor = 1471.10 / 0.85
= 1730.70 KVA
Consider the above Demand 1730.70 as Solution - A
Step 2:
- Check the largest capacity of the motor to be started.
- Calculate the full load (in KVA) at given power factor and efficiency
- Calculate the starting current of the motor.
- To arrive the base load of the transformer, subtract the actual KVA of motor with the peak demand.
- To arrive the load on transformer during starting of motor, add the base load of transformer with starting KVA of motor
- Refer the standards for over load factor of transformer to arrive rated continuous capacity of transformer capacity to meet starting rating. Compare the value with rated continuous capacity with rated factor.
From the Same Example :
TRANSFORMER SIZE
AVAILABLE IN MARKET (KVA) |
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25 |
63 |
100 |
150 |
200 |
250 |
315 |
400 |
500 |
630 |
800 |
1250 |
1600 |
2000 |
2500 |
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Select the nearest capacity of the transformer capacity which should be higher than the Solution A.
From the table, We can select 2000KVA Transformer.
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