Calculate transformer tap voltage and recommended tap setting from primary and secondary voltages, tap step size, range, and desired output.
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- Tap Voltage Calculator
Tap Voltage Formula
The calculator has two modes. Each mode uses its own formula.
Voltage percentage change mode finds the new voltage at a chosen tap percentage:
Transformer tap mode finds the primary tap percentage needed to deliver a target secondary voltage:
- V_tap = voltage at the selected tap
- V_s = secondary (or base) voltage you start from
- tap% = direct voltage-change percentage in the first mode; primary winding tap percentage in the second mode
- V_pa = measured (actual) primary voltage
- V_pr = transformer rated primary voltage
- V_sr = transformer rated secondary voltage
- V_sd = desired ideal secondary voltage (load regulation excluded)
The Voltage percentage change mode applies a direct percentage increase or decrease to a base voltage. Its percentage is not the label of a primary winding tap; use Transformer tap mode for that convention.
The Transformer tap mode sizes the tap for you. You enter the measured primary voltage, the nameplate ratings, the desired secondary voltage, the step size of the tap changer, and the available tap range. The calculator computes the exact percentage and selects the available whole-step position with the smallest absolute secondary-voltage error. A higher primary tap lowers the secondary voltage. A lower primary tap raises it.
Typical Tap Settings and Voltages
Tap positions and operating limits depend on the actual transformer. Use its nameplate and manufacturer instructions. The first table illustrates direct voltage-percentage changes, not primary-tap positions.
| Tap position | Multiplier | 480 V base | 240 V base |
|---|---|---|---|
| +5.0% | 1.050 | 504.0 V | 252.0 V |
| +2.5% | 1.025 | 492.0 V | 246.0 V |
| Nominal | 1.000 | 480.0 V | 240.0 V |
| −2.5% | 0.975 | 468.0 V | 234.0 V |
| −5.0% | 0.950 | 456.0 V | 228.0 V |
The following table contains arithmetic voltage-range illustrations only; it is not a current ANSI compliance table or an equipment rating. Confirm applicable service and utilization limits separately.
| Nominal voltage | Illustration (±5%) | Illustration (about +5.8% / −8.3%) |
|---|---|---|
| 120 V | 114 to 126 V | 110 to 127 V |
| 240 V | 228 to 252 V | 220 to 254 V |
| 480 V | 456 to 504 V | 440 to 508 V |
| 4160 V | 3952 to 4368 V | 3815 to 4399 V |
Example Problems
Example 1: Quick tap voltage. A 480 V base is increased directly by 2.5%. The new tap voltage is 480 × 1.025 = 492 V. For a direct 5% decrease, the adjusted voltage becomes 480 × 0.95 = 456 V.
Example 2: Choosing a transformer tap. A 13800/480 V transformer is fed by a measured primary of 13200 V. You want 480 V at the secondary. Without any tap, the secondary would be 13200 × (480 / 13800) = 459.1 V, which is low. The required tap is [(13200 × 480) / (13800 × 480) − 1] × 100 = −4.35%. With 2.5% steps, the closest setting is −5%, giving an expected secondary of about 483.3 V. The −5% position is two 2.5% steps below neutral. This ideal ratio estimate excludes load regulation and does not authorize an adjustment.
FAQ
Why does a higher primary tap lower the secondary voltage? Raising the primary tap adds turns on the primary side, which increases the turns ratio. A larger ratio means less voltage transferred to the secondary for the same primary input.
Can I change taps under load? Only with a load tap changer (LTC). Standard de-energized tap changers (DETC) on distribution transformers must be switched with the unit fully de-energized and locked out.
What if the calculated tap is between two steps? The calculator chooses the whole-step position within the entered range that minimizes absolute secondary-voltage error. If the result sits more than 1% from your target, the status shows a warning so you can decide whether the closest step is acceptable.
What does it mean when the result is clamped? The continuous target is outside the available whole-step positions. A qualified professional must assess a suitable equipment or supply solution; the calculator cannot establish safe operation.
