Transformer Current and Short Circuit Calculator
Find the full load current of a transformer on both sides and the maximum fault current at its secondary terminals from the kVA rating and percent impedance.
- IFL = S ÷ (√3 × V) = 1,000,000 ÷ 749.98 = 1,333 A
- ISC = IFL × 100 ÷ %Z = 26.67 kA (infinite source assumed)
Transformer Current Formulas
ISC = IFL × 100 ÷ %Z
This assumes an infinite (zero impedance) upstream supply, which gives the highest possible fault current and is the safe assumption for switchgear ratings.
Worked example
A 1000 kVA, 11 kV/433 V transformer with 5 % impedance: IFL = 1,333 A on the LV side and ISC = 1,333 × 100 ÷ 5 = 26.7 kA.
Frequently Asked Questions
What is the full load current of a 1000 kVA transformer?
At 433 V it is about 1,333 A; on the 11 kV side about 52.5 A.
What does transformer percent impedance mean?
It is the percentage of rated voltage needed on the primary to circulate full load current with the secondary shorted. Lower %Z means higher fault current.
Does this include motor contribution?
No. Running motors add fault current for a few cycles; include them for switchgear peak ratings.
Learn more on the blog
Cable sizing and ampacity explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article