Transformer Capacity and Loading Guide

In short. A transformer’s rating is its kVA at defined temperature rise. Size it for the maximum load with margin, check the loading under failure conditions, and consider impedance, losses and fault level. Transformers can carry temporary overloads, but only within the limits of the loading guide and the ambient temperature.

Basic sizing

Example (illustrative): demand of 880 kVA with a target loading of 80 per cent needs at least 1,100 kVA, so a 1,250 kVA transformer is a sensible standard size. Target loading should leave room for growth and for uneven loading.

Loading and efficiency

• Transformers are often most efficient at moderate loading, commonly around 40 to 60 per cent, where load losses balance no-load losses.
• No-load (iron) losses run all the time; load (copper) losses rise with the square of current.
• Over a year, lightly loaded oversized transformers can waste energy. Consider loss evaluation.

Impedance

A 1,250 kVA, 11 kV/415 V transformer has a rated secondary current near 1,739 A. With impedance of 5 per cent, the secondary fault level from the transformer alone is about 34.8 kA, ignoring the source. Higher impedance lowers fault current but raises voltage drop and affects motor starting. See transformer impedance and fault contribution.

Overload capability

Standards such as IEC 60076-7 give loading guides that allow short overloads depending on ambient temperature, cooling and prior load. Overload accelerates ageing, so use it for contingencies, not normal operation. Ask the manufacturer for permitted overload times.

Double-ended and N+1 arrangements

When one transformer fails, the other must carry the load of both. Check against the full load, not half. See main-tie-main distribution.

Generator connection

• Check that the generator can energise the transformer without excessive inrush. See inrush screening.
• Transformer impedance and vector group affect fault current and earthing when on generator supply.
• Step-up transformers for medium-voltage generation need their own protection scheme.

Harmonics

Non-linear loads cause extra losses in transformers. Standard transformers may need derating, or a K-rated or harmonic-mitigating design. See harmonic screening.

Worked example (illustrative)

A site has two 1,000 kVA transformers feeding a main-tie-main board, with 1,300 kVA total demand. Each carries 650 kVA, 65 per cent normally. On failure of one, the other carries 1,300 kVA, which is 130 per cent. The overload may be tolerable for a short time per the loading guide, but not for hours. The design either shedding 300 kVA on a transformer failure, or choosing 1,600 kVA units. The decision comes from checking the failure state, not the normal state.

Loading a transformer

Worked example (illustrative)

A 1,000 kVA, 11 kV/415 V transformer carries 720 kVA. Loading = 72 percent. Secondary current ≈ 1,002 A. Primary current ≈ 38 A. A 1,000 kVA unit at 72 percent is healthy; sustained operation above 100 percent shortens insulation life.

With generators

• Where a transformer sits between the generator and load, include its impedance in voltage dip estimates.
• Energising causes inrush; see inrush screening.

Parallel transformers

Parallel units share load by impedance. Mismatched impedance or ratio overloads one unit. Check the vector group and tap positions.

See transformer protection and transformer impedance and fault contribution.

Common mistakes

• Sizing for normal state only.
• Ignoring harmonic heating.
• Forgetting the effect of impedance on voltage regulation and fault level.

What to send us

Send the load, voltage levels, redundancy target and any existing transformer data. Send your application details.

Questions engineers ask

Figures are illustrative. Use manufacturer data and loading guides for design.