WiremanWireman · Transformer gives the wrong secondary voltage: troubleshooting for electricians · 2023 NEC ·
Transformer gives the wrong secondary voltage
Check the primary, the taps, the load and the secondary bond to find out why the output is off.
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How it works
A transformer’s secondary voltage is its primary voltage times the turns ratio, so a primary 5% high gives a secondary 5% high. Taps on the primary adjust the ratio in small steps. The secondary also drops a little under load (regulation), and its line-to-neutral readings float or look strange if the secondary isn’t bonded to ground the way it should be.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What’s wrong with the secondary?
- All phases high or low by about the same amountTest: Measure the primaryExpect: Within a few percent of the voltage for the tap in use.
- Primary off by the same percentSupply voltage is off
- Primary is normalIs the secondary wrong only under heavy load, or with light load too?
- Only under heavy loadVoltage drop under load
- Wrong even at light loadWrong tap or wrong connection
- Phases unequal, or one very lowTest: Measure all three primary phasesExpect: Three equal readings.
- One primary phase missing or lowLost primary phase
- Primary balancedTransformer or secondary connection problem
- Line-to-neutral unstable or odd, line-to-line fineTest: Check the secondary bondExpect: One system bonding jumper at the transformer or at the first disconnect, plus a grounding electrode conductor.
- No bond anywhereSecondary not bonded
- Bonded correctlyLoose neutral or ghost readings
- One leg reads about 208 V to neutral (240 V delta)Normal high-leg delta
- All phases high or low by about the same amountTest: Measure the primaryExpect: Within a few percent of the voltage for the tap in use.