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Transformer size for four blocks

Short answer

Max demand per block: 250×0.9=225, 200×0.8=160, 150×0.75=112.5, 400×0.85=340 kVA. Sum = 837.5 kVA. Plant max demand = 837.5/1.5 = 558.33 kVA. Next available size is 600 kVA. Select 600 kVA transformer.

There are four blocks in an industry having connected load of 250 kVA, 200 kVA, 150 kVA and 400 kVA and demand factor of 90%, 80%, 75% and 85% respectively. Use a diversity factor of 1.5. Calculate the size of transformer for the industry if transformer available in the market are 50kVA, 100kVA, 250kVA, 400kVA, 600kVA and 750kVA.

The transformer is sized on the maximum demand of the industry, found by applying demand factor to each block and then the diversity factor for the whole plant.

Step 1: maximum demand of each block

Max demand = connected load × demand factor
Block 1 = 250 × 0.90 = 225.0 kVA
Block 2 = 200 × 0.80 = 160.0 kVA
Block 3 = 150 × 0.75 = 112.5 kVA
Block 4 = 400 × 0.85 = 340.0 kVA
Sum of individual max demands = 837.5 kVA

Step 2: maximum demand of the industry

Diversity factor = sum of individual max demands
                   / max demand of whole system
Max demand = 837.5 / 1.5 = 558.33 kVA

Step 3: select the transformer

Available sizes: 50, 100, 250, 400, 600, 750 kVA. The smallest size ≥ 558.33 kVA is 600 kVA.

Loading at peak = 558.33 / 600 = 93 %, which is acceptable.

Answer: maximum demand = 558.33 kVA; select one 600 kVA transformer. (If a future expansion margin of 20 % were added, 670 kVA would be needed and the 750 kVA unit would be chosen.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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