NGR Calculator

How to Size a Neutral Grounding Resistor (NGR) per IEEE 142

This calculator helps you determine the correct NGR resistance, power rating, and thermal capacity for medium and high voltage systems using high-resistance grounding (HRG). It applies the IEEE 142 methodology used by electrical engineers worldwide.

Quick Reference: Common NGR Values for 10A Fault Limit

Step 1: Select Calculation Mode

• NGR Resistance Sizing – Primary mode: enter voltage and desired fault current to get NGR resistance.
• Fault Current Analysis – Compare NGR-limited current to bolted fault current.
• Power Rating – Calculate thermal capacity needed based on fault duration.
• Charging Current Analysis – Verify NGR current exceeds system charging current (critical for HRG).
• Complete Design – Full NGR specification with all parameters.

Step 2: Enter System Parameters

• System Voltage (L-L) – Select your line-to-line voltage (480V to 69kV).
• Desired Fault Current – Typical values: 5-25A for generators, 25-400A for transformers.
• Fault Duration – For power rating: 10 seconds is standard, 60 seconds for extended.

Step 3: Interpret Results

• NGR Resistance – Calculated as R = V_LN / I_fault per IEEE 142.
• Power Rating – P = I² × R, must be rated for specified duration.
• Thermal Capacity – Energy in kJ or kW-seconds the NGR must absorb.
• Charging Current Ratio – Must be <100% for proper HRG operation.

Core Formula (IEEE 142)

R_NGR = V_LN / I_fault
 = (V_LL / √3) / I_fault

Where:
 V_LL = Line-to-line system voltage
 V_LN = Line-to-neutral voltage (phase voltage)
 I_fault = Desired ground fault current limit

Example (13.8kV, 10A limit):
 R = (13,800 / √3) / 10
 R = 7,967 / 10
 R = 797 Ω