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How to Size Transformer Neutral Resistor

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    How to Size Transformer Neutral Resistor

    Can someone explain how to size a transformer neutral resistor for limiting fault current? For example: What size neutral resistor would be required to limit the fault current of a 13,800-480Y/277V 1500kVA Transformer to 200A?
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    This question comes from the level 2 practice exam..!?

    im curious my self

    Dan
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    Quote Originally Posted by AlexT90 View Post
    What size neutral resistor would be required to limit the fault current of a 13,800-480Y/277V 1500kVA Transformer to 200A?
    Simple ohms law. Use line-to-neutral voltage: 277 volts / 200 amps = 1.385 ohms
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    Neutral Ground Resistor

    Quote Originally Posted by AlexT90 View Post
    Can someone explain how to size a transformer neutral resistor for limiting fault current? For example: What size neutral resistor would be required to limit the fault current of a 13,800-480Y/277V 1500kVA Transformer to 200A?
    277 V/200 A=1.385 OHMS to limit the ground fault current to 200 A on the 480Y/277 V system.

    Some related information in case you are designing a resistance grounded system as opposed to answering a test question:

    1. A 200 A max ground current limit would be more appropriate for a MV system (e.g., 4.16Y/2.4 kV).

    2. In a LV system, a high resistance grounding scheme is employed with a resistor typically sized to limit ground fault current to < 10 A (the current must be > the system capacitive charging current e.g., cables, motors, transformers,.. capacitances).

    3. The minimum size of the conductor from the supply transformer neutral to the neutral grounding resistor is stated in the NEC.

    4. When investigating HRG system viability there are some application considerations such as:
    • line to neutral loads like 277 V lighting must first be supplied via an isolation transformer's
    • overvoltage protection of adjustable speed drives can be problematic


    Several manufacturers have NGR whitepapers on the internet.
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