Enter the tap-up percentage (%) and the secondary voltage (volts) into the calculator to determine the Tap Voltage.
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Tap Voltage Formula
The calculator has two modes. Each mode uses its own formula.
Tap voltage mode finds the new voltage at a chosen tap percentage:
V_tap = V_s * (1 ± tap% / 100)
Transformer tap mode finds the primary tap percentage needed to deliver a target secondary voltage:
tap% = [(V_pa * V_sr) / (V_pr * V_sd) - 1] * 100
- V_tap = voltage at the selected tap
- V_s = secondary (or base) voltage you start from
- tap% = tap step as a percent, plus for up and minus for down
- V_pa = measured (actual) primary voltage
- V_pr = transformer rated primary voltage
- V_sr = transformer rated secondary voltage
- V_sd = desired secondary voltage at the load
The Tap voltage mode is for quick checks. You give it a base voltage and a percentage, and it returns the voltage at that tap. Use it when you already know the tap setting and want the resulting voltage.
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, rounds it to the nearest available step, and clamps it to the maximum range. A higher primary tap lowers the secondary voltage. A lower primary tap raises it.
Typical Tap Settings and Voltages
Most distribution transformers use a five-position de-energized tap changer at ±2.5% steps. Load tap changers (LTCs) on power transformers usually offer ±10% in 32 steps of about 0.625% each.
| 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 |
ANSI C84.1 sets the acceptable service voltage range. Use it as a check on whether your tap selection is needed.
| Nominal voltage | Range A (±5%) | Range B (+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 secondary is set to a +2.5% tap. The new tap voltage is 480 × 1.025 = 492 V. If the same transformer is moved to the −5% tap, the secondary 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. Drop the primary tap one step to bring the secondary back into range.
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 rounds to the nearest available step. 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 required tap exceeds the range you entered. You will need a transformer with a wider tap range, a different rated voltage, or upstream voltage correction.
