EngiMetric Arena / Spot the Error
Did the transformer review forget future growth?
Competency Quantitative reasoning primary Design and sizing supporting
What you should take away: Apply demand and future-growth allowances in the right order and distinguish the demand figure from the transformer rating that must carry it.
The situation
What is happening
A plant review starts with 520 kVA of connected load, applies a 75 % demand factor and then adds a 10 % future-growth allowance. The reviewer writes down a 400 kVA transformer because 520 x 0.75 is 390 kVA. What standard rating does the complete calculation support?
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Connected load | 520 | kVA | Sum of connected equipment ratings. |
| Demand factor | 75 | % | Applied to the connected load. |
| Future growth | 10 | % | Allowance applied after demand. |
| Reviewer note | 400 | kVA | The rating written in the review. |
Assumptions this challenge makes
- The demand factor and growth allowance are project assumptions, not measured values.
- The transformer is selected from the standard rating table in the calculator.
- No spare capacity beyond the stated growth allowance is added.
Your call
Your answer
Hints
- Apply the demand factor to the connected load first. Keep the result separate from the rating you will order.
- The future-growth allowance applies after demand, not to the connected load a second time.
- Find the first standard rating at or above the demand-with-growth figure.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- Demand is 520 x 0.75 = 390 kVA.
- With 10 % growth, the load becomes 390 x 1.10 = 429 kVA.
- The first standard rating at or above 429 kVA is 500 kVA. Its loading after growth is 429 / 500 = 85.8 %.
- The reviewer's 400 kVA is below the demand-with-growth figure, so it is not a defensible selection even though 400 kVA is a standard size.
- The calculation establishes a minimum standard rating. Motor-starting, harmonic loading, ambient temperature and redundancy are separate checks.
What this challenge does not establish
- The demand factor and growth allowance are inputs; the calculation does not verify them against the actual load profile.
- Transformer impedance, motor-starting voltage dip, harmonics and ambient derating are not modelled.
- The standard rating table is a screening list, not a manufacturer quotation.
- The result is a minimum rating, not a complete transformer specification.