Transformer Tap Adjustment

Predicted LV voltage at each MV tap, from a measurement.

EstimatePreliminary result, not a design verification.

Before you calculate

Predicted LV voltage at each MV tap

From a measured LV voltage and the nameplate tap voltages. A planning estimate; off-circuit taps are changed only de-energized under the approved procedure.

Learn more — complete guide →

Step 1

Transformer and measurement

Enter the MV tap voltages printed on the nameplate and the LV voltage you measured. The default tap list is an ILLUSTRATIVE 20 kV ± 2 × 2.5 % pattern.

Rated HV voltage (kV)

Nominal LV voltage (V, line-to-line)The voltage the tool aims for: the tap closest to it is shown.

Current MV tap voltage (kV)

Measured LV voltage (V, line-to-line)

Advanced: your nameplate tap list and three-phase readings (optional)

MV tap voltages from the nameplate (kV)Comma-separated; position labels optional as position:kV. Position numbers have no universal direction.

Acceptable deviation from the target (V)Your own tolerance; it decides whether the nearest tap counts as reaching the target.

Measured L1-L2 (V)

Measured L2-L3 (V)

Measured L3-L1 (V)

Step 2

Results

Check before relying on this result

Off-circuit (off-load) tap changers must never be operated energized. Follow the transformer manual and the switching procedure: isolation, proving dead, earthing, qualified personnel.

Nearest MV tap to the target19.5 kV-2.5 % of rated · target inside the tap span

Expected LV voltage (ratio estimate)400.0 VSigned difference from the target 0.0 V · within your ±2 V

MV tap	% of rated	Predicted LV	Difference	Note
21 kV	5.0 %	371.4 V	-28.6 V
20.5 kV	2.5 %	380.5 V	-19.5 V
20 kV	0.0 %	390.0 V	-10.0 V	current
19.5 kV	-2.5 %	400.0 V	0.0 V	nearest to target
19 kV	-5.0 %	410.5 V	10.5 V

Planning estimate, not a switching instruction. The real voltage after the change also depends on the grid provider's supply voltage, the load and its power factor, and the transformer's regulation. Off-circuit taps are changed only de-energized, under the approved procedure, by qualified personnel.

1. Predicted LV voltage at another tap

Ideal ratio model: same incoming MV voltage and similar load before and after. A lower MV tap voltage gives a higher LV voltage.

U_{LV,new}\approx U_{LV,meas}\,\frac{U_{tap,current}}{U_{tap,new}}

U_{LV,new}\approx 390\times\frac{20}{19.5}=400\ \text{V}

2. Difference from the target

The tap with the smallest absolute difference in the list is shown as the nearest.

\Delta U=U_{LV,new}-U_{target}

\Delta U=400-400=0\ \text{V}

Calculations use full precision internally; the displayed numbers are rounded for readability.