Calculator Inputs
Example Data Table
| Line Voltage | Target Current | Time | NGR Resistance | Power Duty | Energy Duty |
|---|---|---|---|---|---|
| 4.16 kV | 400 A | 10 s | 6.004 ohm | 960.7 kW | 9607 kJ |
| 11 kV | 10 A | 10 s | 635.085 ohm | 63.5 kW | 635 kJ |
| 13.8 kV | 400 A | 10 s | 19.919 ohm | 3187.0 kW | 31870 kJ |
Formula Used
Neutral voltage from line voltage: VN = VLL / √3
Resistance sizing: RNGR = VN / IG
Fault current from resistor: IG = VN / RNGR
Power duty: P = IG2 × RNGR = VN × IG
Energy duty: E = P × t
Tolerance range: Imax = VN / Rmin, and Imin = VN / Rmax
How to Use This Calculator
- Select whether you want resistance, current, or a combined check.
- Enter system voltage and choose the correct voltage basis.
- Enter target ground fault current or existing NGR resistance.
- Add fault clearing time, tolerance, and safety factor.
- Enter charging current when relay coordination margin matters.
- Press calculate, then export the result as CSV or PDF.
Neutral Grounding Resistor Design Notes
Why Neutral Grounding Resistance Matters
A neutral grounding resistor limits current during a single line to ground fault. It is installed between transformer neutral and earth. The resistor prevents a solidly grounded fault from becoming destructive. It also keeps the neutral reference stable enough for protection relays. A correct value can reduce damage, flash risk, and unnecessary service loss.
Important Design Inputs
The main input is the system voltage at the neutral point. When a line to line voltage is entered, the calculator converts it to phase voltage. The desired ground fault current sets the ohmic value. A lower current needs a higher resistor. A higher current needs a lower resistor. Fault duration controls energy duty. Resistor tolerance changes possible minimum and maximum current values.
Thermal Duty and Rating
Power is found from voltage and current, or from current squared times resistance. The short time energy is power multiplied by clearing time. This duty helps compare the selected resistor with its nameplate rating. The safety factor increases the suggested rating. It does not replace manufacturer data. It gives a practical allowance for relay delay, voltage variation, and heating.
Protection Coordination
The resistor current should be high enough for ground fault relays. It should also stay above system charging current by a useful margin. If it is too close, relays may become less dependable. If it is too high, equipment stress increases. The best value depends on transformer size, cable length, grounding method, and site rules.
Engineering Use
Use this tool for quick studies and preliminary sizing. Enter the actual voltage basis carefully. Use the protection pickup current and clearing time from the relay plan. Compare the result with standard resistor ratings. Confirm insulation level, enclosure class, temperature rise, and duty cycle. Final selection should follow applicable standards and supplier drawings.
Common Mistakes
Do not size the resistor from line voltage unless the neutral rating is known. Do not ignore tolerance, because current can rise when resistance is low. Do not use continuous power rating for a short time unit without checking duty. Review relay pickup, transformer connection, cable charging current, and earthing transformer limits before ordering field hardware. Keep records clear for maintenance teams and future protection studies as built.
FAQs
What is a neutral grounding resistor?
It is a resistor connected between a system neutral and earth. It limits ground fault current while keeping the neutral referenced to ground.
Which voltage should I enter?
Enter the voltage you know, then choose its basis. For most three phase studies, line to line voltage is converted to neutral voltage.
How is NGR resistance calculated?
The calculator divides neutral voltage by the desired ground fault current. This gives the required ohmic value for the resistor.
Why is fault duration important?
Duration controls energy duty. A longer clearing time increases heat energy and may require a stronger short-time resistor rating.
What does resistor tolerance affect?
Tolerance changes the actual current. A lower actual resistance raises current, while a higher actual resistance lowers current.
What is charging current ratio?
It compares ground fault current with system capacitive charging current. A low ratio may need relay coordination review.
Can this replace manufacturer selection?
No. Use it for study estimates. Confirm final duty, enclosure, insulation, standards, and thermal limits with the supplier.
Why add a safety factor?
A safety factor adds rating margin for voltage variation, relay delay, tolerance, and conservative preliminary design.