Taps sit on the HV winding; you measure on the LV side
A distribution transformer usually offers taps on its higher-voltage winding to compensate for the incoming voltage and keep a suitable LV voltage. Engineers usually measure only the LV output, while each tap is labelled on the nameplate by a rated MV-side voltage. The calculator connects the two.
1. What a tap changes
On a simplified unloaded model the voltage ratio follows the turns ratio. Choosing another HV tap changes the effective HV turns. With the same incoming MV voltage, a tap with a LOWER rated tapping voltage generally RAISES the LV output; a higher one lowers it.
Nameplates may number the positions 1 to 5 or give plus/minus percentages. A position number has no universal voltage direction: always use the rated tap voltages printed on the nameplate.
2. Why the LV measurement is enough for a first estimate
If the incoming MV voltage and the load are about the same before and after the change, the measured LV voltage already contains their effect. Only the ratio of the present and the new rated tap voltages is needed:
This does not predict a changing supply voltage from the grid provider, phase imbalance, or the transformer's regulation under a different load.
3. Worked 20/0.4 kV example
An illustrative five-position nameplate of 19.0 / 19.5 / 20.0 / 20.5 / 21.0 kV, currently on the 20.0 kV tap, with 390 V measured line-to-line and a target of about 400 V:
| MV tap | Predicted LV | Difference from 400 V |
|---|---|---|
| 21.0 kV | 371.4 V | −28.6 V |
| 20.5 kV | 380.5 V | −19.5 V |
| 20.0 kV (current) | 390.0 V | −10.0 V |
| 19.5 kV | 400.0 V | 0.0 V |
| 19.0 kV | 410.5 V | +10.5 V |
The nearest match is 19.5 kV, assuming that tap really exists on your transformer. The calculator shows every tap, not only the nearest, so the remaining difference stays visible; if the target lies outside the reachable range it says so.
4. Why real results may differ
- The supply voltage may change between measurements.
- Loading and power factor change the internal voltage drop.
- Taps can change the internal impedance and the magnetising flux.
- Temperature, unbalanced loading and instrument uncertainty change the readings.
- On-load tap changers and fixed off-circuit taps are operated in different ways.
5. Three line voltages
With L1-L2, L2-L3 and L3-L1 entered, the same ratio is applied to each. A tap change scales all three together: it does not correct phase imbalance, which needs separate troubleshooting.
6. The safety boundary
Most distribution-transformer off-circuit (off-load) tap changers must never be operated energized. The installation instructions decide the safe sequence: switching off the supply, isolation, proving the absence of voltage and earthing, and only then operating the tap changer, by qualified personnel only. KiloVolt does not authorise switching and does not decide whether the voltage stays within limits under all operating conditions.
7. Common mistakes
- Entering the measured MV supply voltage instead of the nameplate tap voltage.
- Assuming that "tap 1" always raises the LV voltage.
- Assuming every transformer has ± 2 × 2.5 % taps.
- Expecting a tap change to fix a problem on one LV phase.
- Comparing readings taken under very different loads and calling the difference a tap effect.
Related: the Transformer fundamentals guide · Schneider Electric: transformer taps