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Anonymous asked in Science & MathematicsPhysics · 2 months ago

Theoretically, if a center tap on a transformer is not connected to ground, what would the voltages be for that wire?

For a transformer where the secondary side is giving 240V, the two hot wires will each be carrying 120V and the center tapped neutral wire is carrying 0V because it cancels out with the ground wire.

But what if the neutral wire is not connected to ground; would there be some voltage coming from it?

If there is voltage, then what happens to the voltage in the hot wires?


When I measure the hot leg to ground I get 120V, so why wouldn't I get the same reading for neutral to ground?

2 Answers

  • Dixon
    Lv 7
    2 months ago
    Favorite Answer

    If the secondary is not grounded at all then the whole secondary is floating and nothing is at a defined potential with respect to Earth, all you have is the potentials between the various taps off the secondary. 

    The readings you get when measuring are not totally predictable due to the floating but using a DVM set to AC, we might expect that by symmetry, the two hots would both read 120V and the center would be at roughly zero. Essentially, the center tap is like the middle of a seesaw, with each end flapping up and down and out of phase, while the center is actually stationary - which is zero AC.

  • 2 months ago

    you need two points to make a voltage measurement. In other words, a voltage by itself has little meaning, you need to state the reference. In other words, a voltmeter has two leads, you have to connect both to get a reading.

    so with respect to ground, you have zero volts at the 3 points you mention. They are all "floating" with respect to ground. (in an actual measurement, you may read a small varying voltage due to leakage in the transformer or AC pickup)

    with respect to the center tap, each leg is 120 volts.

    "because it cancels out with the ground wire" that has zero meaning.

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