Transformer & Busbar Sizing Calculator
Size distribution transformers, calculate fault levels, and select busbars
Reference Info & Formulas
125% Rule: Transformer rated at least 125% of the maximum demand.
Fault Level: If = FLC / (Z% / 100). Protective devices must have adequate breaking capacity.
Busbars: Rated for continuous FLC with temperature derating applied.
50, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 kVA
Per BS EN 50464-1
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Data: BS EN 50464-1, IEC 60076 — transformer and busbar sizing
For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.
Transformer Fault Level Calculation
The prospective fault current at the secondary terminals depends on the transformer rating and percentage impedance.
If = FLC / (Z% / 100)- If
- = Fault current in amps
- FLC
- = Full load current of the transformer
- Z%
- = Percentage impedance of the transformer
IEC 60076 / BS 7671 Regulation 434
This gives the maximum prospective short-circuit current assuming an infinite source (negligible supply impedance). In practice, the supply impedance reduces the actual fault current, but the transformer-limited value is used for protective device selection as a safe assumption.
Standard Transformer Ratings
Three-phase oil-filled distribution transformers
BS EN 50464-1| Rating (kVA) | Typical Z% | 400V FLC (A) | Approx. Fault (kA) |
|---|---|---|---|
| 100 | 4.0 | 144 | 3.6 |
| 250 | 4.5 | 361 | 8.0 |
| 500 | 5.0 | 722 | 14.4 |
| 1000 | 5.0 | 1443 | 28.9 |
| 1600 | 5.5 | 2309 | 42.0 |
| 2500 | 6.0 | 3608 | 60.1 |
FLC calculated at 400V three-phase. Actual fault current depends on supply impedance.
Busbar Selection Notes
Derating for Ambient Temperature
Fault Withstand
Frequently Asked Questions
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