Transformer Sizing and Selection
Step 1: Find the Total Connected Load
Formula:
Total Connected Load (kW) = Sum of
all individual loads (kW)
Example Calculation:
Lighting load: 30 kW
HVAC load: 50 kW
Machinery load: 70 kW
Other equipment load: 20 kW
Total Connected Load = 30 + 50 + 70 + 20 = 170 kW
Step 2: Add Extra Load for Future Expansion
Formula:
Total Load with Future Expansion
(kW) = Total Connected Load x (1 + Future
Expansion Percentage)
Example Calculation:
Assume a 15% future expansion load
Total Load with Future Expansion = 170 x (1 +
0.15) = 170 x 1.15 = 195.5 kW
Step 3: Calculate the Maximum Demand Load
Formula:
Maximum Demand (kW) = Total Load
with Future Expansion (kW) x Demand Factor
Example Calculation:
Assume a demand factor of 80%
Maximum Demand = 195.5 x 0.8 = 156.4 kW
Step 4: Calculate the Maximum Demand in
kVA
Formula: Maximum Demand (kVA) = Maximum Demand (kW) / Power Factor (PF)
Example Calculation:
Assume a power factor of 0.85
Maximum Demand (kVA) = 156.4 / 0.85 ≈ 184
kVA
Step 5: Determine the Transformer Capacity
with Efficiency Considerations
Formula:
Required Transformer Capacity (kVA)
= Maximum Demand (kVA) / Transformer
Efficiency
Example Calculation:
Assume a transformer efficiency of 95%
Required Transformer Capacity (kVA) = 184 /
0.95 ≈ 194 kVA
Step 6: Select the Appropriate Transformer
Size
Select the closest standard transformer size
that can handle the calculated load.
Example Calculation:
The calculated transformer capacity is 194 kVA.
The next standard size available is 250 kVA.
Thus, a 250 kVA transformer would be chosen.
Summary:
Total Connected Load: 170 kW
Total Load with Future Expansion: 195.5 kW
Maximum Demand (kW): 156.4 kW
Maximum Demand (kVA): 184 kVA
Required Transformer Capacity: 194 kVA
Selected Transformer Size: 250 kVA