EngiMetric Arena / Engineering Trade-off
Choose a transformer rating inside the loading band
Competency Constraints and trade-offs primary Design and sizing supporting
What you should take away: Evaluate a transformer rating against both an upper loading limit and a lower utilisation limit, accepting a band of defensible sizes.
The situation
What is happening
A 620 kVA connected load is reviewed at a 70 % demand factor with a 10 % growth allowance. The project wants the transformer loaded between 55 % and 85 % after growth. Specify a standard rating from 500, 630, 800 or 1 000 kVA.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Connected load | 620 | kVA | Connected equipment rating. |
| Demand factor | 70 | % | Applied to connected load. |
| Future growth | 10 | % | Applied after demand. |
| Minimum loading | 55 | % | Avoids an oversized transformer. |
| Maximum loading | 85 | % | Leaves margin for starting and growth. |
Assumptions this challenge makes
- The demand and growth factors are project assumptions.
- The candidate ratings are the four listed standard sizes.
- Loading is based on demand after growth, not on the connected load.
Limits this decision must satisfy
| Limit | Requirement | If it is breached |
|---|---|---|
| Maximum loading after growth | not more than 85 % | Above 85 % the transformer has too little spare capacity for motor starting and future load. |
| Minimum loading after growth | not less than 55 % | Below 55 % the transformer is oversized, adding no-load losses and capital cost without useful margin. |
Choose 500, 630, 800 or 1 000 kVA. More than one rating may satisfy the loading band.
Your call
Your answer
Hints
- Compute demand after the 70 % factor, then apply the 10 % growth allowance.
- For each candidate, divide demand-with-growth by the rating to get loading.
- A rating can fail at either end: too small exceeds 85 %, too large falls below 55 %.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- Demand is 620 x 0.70 = 434 kVA. With 10 % growth it is 477.4 kVA.
- At 500 kVA the loading is 95.5 %, above the 85 % limit. At 630 kVA it is 75.8 %, inside the band.
- At 800 kVA the loading is 59.7 %, also inside the band. At 1 000 kVA it is 47.7 %, below the 55 % lower limit.
- So 630 kVA and 800 kVA are both defensible. The smaller unit costs less and runs closer to its rating; the larger unit buys more growth margin at a higher capital and no-load loss cost.
- The constraints define the acceptable set. The calculation does not choose between 630 and 800 kVA without a project cost or growth preference.
What this challenge does not establish
- Loading is a steady-state screening ratio; motor starting, harmonics and ambient temperature are not modelled.
- The demand and growth factors are assumptions, not measured load data.
- The lower 55 % limit is a project preference, not a universal code requirement.
- The calculation does not compare transformer losses, impedance or fault levels.