WiremanWireman · Changing taps for high or low voltage: troubleshooting for electricians · 2023 NEC ·
Changing taps for high or low voltage
Work out how far off you are, pick the right tap and change it safely.
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How it works
Most general-purpose dry-type transformers have taps on the primary winding, commonly in 2.5% steps (for example, two above and four below nominal). Connecting to a tap with more primary turns lowers the secondary voltage; fewer turns raises it. These are no-load taps: the transformer must be de-energized to change them, and every coil must be set to the same tap.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Test: Measure the primary and secondaryExpect: Secondary within about 2.5% of nominal (one tap step).
- Secondary highIs the primary also high by about the same percent?
- Yes, primary is highRetap for high supply voltage
- No, primary is normalTap is set wrong
- Secondary lowIs the secondary low at light load, or only under heavy load?
- Low even at light load, primary also lowRetap for low supply voltage
- Low at light load, primary normalTap is set wrong
- Only low under heavy loadVoltage drop under load
- Within about 2.5%No tap change needed
- Secondary highIs the primary also high by about the same percent?