How to Calculate Transformer Load?

Knowing how heavily loaded your transformer actually runs is the basis for every decision that follows: whether you have capacity for a new line, whether losses are reasonable, and when to plan replacement.

This article shows how to calculate load from simple measurements you can take at the LV board.

Knowing how heavily loaded your transformer actually runs is the basis for every decision that follows: whether you have capacity for a new line, whether losses are reasonable, and when to plan replacement.

This article shows how to calculate load from simple measurements you can take at the LV board.

Three phase: kVA = (√3 × V × I) ÷ 1000, using the line-to-line voltage and the average of the three phase currents.

Example: measured currents of 620 A, 640 A and 630 A at 400 V give an average of 630 A, so kVA = 1.732 × 400 × 630 ÷ 1000 = 436 kVA.

Percentage load = (calculated kVA ÷ rated kVA) × 100. On a 1000 kVA transformer, 436 kVA is 43.6% loaded.

Most distribution transformers are sized so normal operation sits between 40% and 80% of rating, efficient, with headroom for peaks.

Load factor is average load divided by peak load over a period. A plant that peaks at 1600 kVA but averages 800 kVA has a load factor of 0.5.

Load factor matters for loss evaluation: no-load loss dominates at low load factor, load loss at high load factor.

Measure over a representative period, ideally a week including the busiest shift. Record all three phases and the neutral current. Unbalance above 10–15% should be corrected by redistributing single phase loads.

Not sure which rating fits your project? Send us your load list and we will size it for you, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.