How to Calculate a Neutral Grounding Transformer Current

Written by dwight chestnut
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How to Calculate a Neutral Grounding Transformer Current
NGTs protect circuits from large fault currents. (Hemera Technologies/Photos.com/Getty Images)

Neutral grounding transformers, or NGTs, provide a low impedance path to ground for high-voltage electrical power systems. This low impedance path protects sensitive circuits from high-fault currents. An NGT consists of a transformer and a neutral ground resistor, or NGR, packaged inside of a metal or stainless steel enclosure. Calculate the NGT current using Ohm's Law -- or I = V/R -- by dividing the line to ground voltage of the grounding transformer by the NGR.

Skill level:
Moderately Easy

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Instructions

  1. 1

    Determine the line to ground voltage of the NGT. Find this information in the NGT specification sheet. For example, assume the line to ground voltage is 120 volts alternating current, or VAC.

  2. 2

    Determine the resistor value of the NGR. This information is in the NGT specification sheet. For example, assume the NGR is 150 ohms.

  3. 3

    Divide the line to ground voltage by the NGR to determine the NGT current. Using the values from the examples in Steps 1 and 2, 120 VAC/150 ohms equals 0.80 amperes.

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