Third year Transformers · tool
Transformer full-load current and ratio
Enter the kVA and the two line voltages. You get the full-load current on each side, the line and turns ratios, and — for a three-phase bank — the voltage and current in each winding for the connection you pick.
- Primary full-load current—
- Secondary full-load current—
- Line voltage ratio—
- Turns ratio (per winding)—
- Primary winding—
- Secondary winding—
- Secondary line to neutral—
Show the working
Full-load current from the nameplate kVA — arithmetic only. Conductor and overcurrent sizing come from your code book.
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How it works
A transformer’s kVA is the same on both sides (less a little loss), so each side’s full-load current is just the kVA divided by that side’s voltage — with the √3 when it is three-phase. Inside a three-phase bank, a wye winding sees the line voltage ÷ √3 and carries the line current; a delta winding sees the full line voltage and carries the line current ÷ √3.
- I = kVA × 1000 ÷ (√3 × V)three-phase, each side
- I = kVA × 1000 ÷ Vsingle-phase
- Wye winding: V ÷ √3, I lineper winding
- Delta winding: V line, I ÷ √3per winding
Worked: 75 kVA, 480 V delta to 208Y/120 V
- Primary: 75,000 ÷ (1.732 × 480) = 90.2 A.
- Secondary: 75,000 ÷ (1.732 × 208) = 208.2 A.
- Line ratio: 480 ÷ 208 = 2.31 : 1. Turns ratio: 480 ÷ 120 = 4 : 1 — the delta winding sees 480 V, the wye winding 120 V.
- Current in each primary (delta) winding: 90.2 ÷ 1.732 = 52.1 A.
Questions people ask
Why is the secondary current bigger than the primary?
Power in is roughly power out. A step-down transformer lowers the voltage, so to pass the same kVA the current has to go up by the same ratio. A 480-to-208 V transformer carries about 2.3 times the current on its 208 V side — that side is the heavy-current side.
What is the difference between the line ratio and the turns ratio?
The line ratio is line voltage in ÷ line voltage out, what you would read with a meter between lines on each side. The turns ratio is the voltage across one winding on each side — and in a delta-wye bank a wye winding only sees the line voltage ÷ √3. So a 480 V delta to 208Y/120 bank has a line ratio of about 2.31 but a turns ratio of 4:1 (480 across a delta winding, 120 across a wye winding).
Is full-load current the number I size the conductors to?
It is the starting point, not the answer. The conductor, the overcurrent device and any adjustments to them are sized by the rules in your code book, which say what percentage of full-load current to use and in what circumstances. This tool gives you the current; the book turns it into a size.
Related
- Three-phase transformer connections — delta-wye, wye-wye and the rest
- Transformer basics — turns ratio and kVA on one pair of coils
- Transformer fault current (infinite bus)
- Three-phase power calculator
- Rusty on transformer connections?
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