---
title: "Comparing two 750 kVA transformers"
subject: "Industrial Power Distribution and Illumination"
chapter: 10
key: "semester_6/industrial_power_distribution_and_illumination:transformer-purchase-comparison"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/transformer-purchase-comparison
site: BCT Engineering
---

# Comparing two 750 kVA transformers

Industrial Power Distribution and Illumination · Chapter 10: Electrical Energy Audit in Industry

- Asked in: 2075 Bhadra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/transformer-purchase-comparison
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-10

## Question

The 750 kVA transformer (iron loss 1500 W and copper loss 11500 W) is estimated to cost approximately Rs. 7,21,577/00. An equivalent transformer of another manufacture is quoted at Rs 7,00,767. The iron loss of the second transformer are 1430 W and copper loss are 9845 W. Is it worth considering the purchase of second transformer instead? Assume per unit cost of electricity be Rs. 7.5.

## Short answer

Transformer B costs Rs 20,810 less and has lower iron and copper losses (70 W and 1655 W at full load). At 50% loading, annual energy saving is 4238 kWh (Rs 31,782). Second transformer is worth purchasing.

## Answer

**Given:**

| Transformer | Cost (Rs) | Iron loss | Cu loss (full load) |
|---|---|---|---|
| A (750 kVA) | 7,21,577 | 1500 W | 11 500 W |
| B (750 kVA) | 7,00,767 | 1430 W | 9845 W |

Energy rate Rs 7.5/kWh. **Assumption:** the transformer is energised for the whole year (8760 h); copper loss depends on loading, so it is worked out for a typical average load of 50% and also for other loadings.

#### Saving in initial cost
```text
Cost difference = 7,21,577 − 7,00,767 = Rs 20,810 (B cheaper)
```

#### Saving in losses with transformer B
```text
Iron-loss difference   = 1500 − 1430  = 70 W
Copper-loss difference = 11 500 − 9845 = 1655 W (at full load)
```
At 50% average load, Cu loss difference = 0.5² × 1655 = 413.75 W.
```text
Energy saved/yr = (70 + 413.75) × 8760 / 1000
                = 4237.7 kWh
Money saved/yr  = 4237.7 × 7.5 = Rs 31,782
```

| Average load | Loss saving (W) | Rs saved/yr |
|---|---|---|
| Iron only (no load) | 70 | 4,599 |
| 50% | 483.75 | 31,782 |
| 75% | 1000.9 | 65,762 |
| 100% | 1725 | 1,13,333 |

#### Decision
Transformer B is **both cheaper (by Rs 20,810) and has lower losses**, so it saves money from the first day: there is no extra investment to pay back. Even at no load it saves Rs 4,599/yr, and at 50% load about Rs 31,782/yr. Over a 25-year life this is several lakh rupees.

**Answer: Yes, the second transformer should be purchased** (provided its quality, guarantee, impedance and after-sales service are equivalent).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the difference in initial cost between the two transformers?
   A. Rs 20,810, with B cheaper
   B. Rs 20,810, with A cheaper
   C. Rs 31,782, with B cheaper
   D. No difference
   Answer: A. 7,21,577 - 7,00,767 = Rs 20,810, so B is cheaper. (Key point: Initial cost)

2. Which loss of a transformer varies with loading?
   A. Iron loss
   B. Copper loss, which varies as the square of the load
   C. Neither
   D. Both stay constant
   Answer: B. Iron loss is constant when energised; copper loss is proportional to load squared. (Key point: Loss dependence)

3. What is B's total loss saving at 50 % average load?
   A. 70 W
   B. 1655 W
   C. About 483.75 W
   D. 1725 W
   Answer: C. Iron difference 70 W plus 0.5^2 x 1655 = 413.75 W gives 483.75 W. (Key point: Loss saving at 50 %)

4. Why should transformer B be purchased?
   A. It has higher losses
   B. It is cheaper and has lower losses, so it needs no payback period
   C. It is more expensive but efficient
   D. Its iron loss is higher
   Answer: B. B costs less and loses less, saving money from day one, provided quality and service are equivalent. (Key point: Decision)

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