Electrical reference chart
NGR Sizing Chart
Use this worksheet after the calculator result to record system voltage, line-to-neutral voltage, target ground-fault current, neutral grounding resistor value, duty time, thermal energy, charging current, and relay follow-up.
Quick reference table
An NGR sizing chart is a calculator-led planning worksheet. It connects neutral grounding resistor ohms, fault-current limit, thermal duty, and charging-current checks before protection settings or equipment selection are finalized.
NGR sizing worksheet
| Item | Record from calculator | Follow-up |
|---|---|---|
| System basis | System voltage and grounding point | Confirm transformer or generator connection |
| Fault-current target | Desired ground-fault current | Compare with detection and coordination needs |
| Resistance and power | Ohms, kW, and thermal energy | Verify resistor duty and enclosure data |
| Charging current | Estimated or measured system charging current | Check detection margin |
| Protection | Relay or alarm follow-up | Coordinate settings and maintenance plan |
NGR application review lanes
| Application issue | Record on worksheet | Why it changes the resistor package |
|---|---|---|
| Grounding point | Transformer or generator neutral location | The system neutral source controls NGR placement |
| Duty rating | 10-second, extended-time, or continuous note | Thermal duty changes resistor construction |
| Protection interface | Ground-fault relay, CT, alarm, trip setting | The NGR current must coordinate with detection |
| Environment | Enclosure, ventilation, temperature, corrosion note | Installation conditions affect manufacturer selection |
How to use this chart
Record the grounding point
Identify the transformer or generator neutral, system voltage, and line-to-neutral basis.
Document duty assumptions
Keep current limit, resistor ohms, time duration, power, and thermal energy together.
Route protection review
List relay pickup, alarm, trip, monitoring, and manufacturer follow-up items.
Formula basis
NGR resistance = line-to-neutral voltage / target ground-fault current. NGR power = current squared x resistance.
- Line-to-neutral voltage is derived from the system voltage and grounding point.
- Target ground-fault current is the current limit selected for planning.
- Resistance is the neutral grounding resistor ohm value.
- Thermal duty depends on current, resistance, and the allowed fault duration.
Worked examples
Medium-voltage NGR record
Record system voltage, target ground-fault current, calculated NGR resistance, thermal duration, charging current estimate, relay pickup basis, and manufacturer follow-up.
Generator neutral grounding package
Keep generator voltage, neutral point, target ground-fault current, resistor duty, monitoring relay, enclosure, and alarm or trip plan in one worksheet.
Related calculators
NGR Calculator
Calculate neutral grounding resistor (NGR) sizing per IEEE 142 for medium and high voltage systems
Ground Fault Loop Calculator
Calculate ground fault loop impedance (Zs) and prospective fault current. Verify protective device coordination against the adopted NEC edition, equipment documentation, and AHJ requirements.
Protection Coordination Calculator
One-point screening tool for coordination time margin, published selective current, and instantaneous pickup checks.
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Protection Coordination Chart
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Short Circuit Current Chart
Plan short-circuit current checks from utility data, transformer kVA and impedance, voltage, conductor path, motor contribution, and equipment SCCR/AIC.
Frequently asked questions
These answers explain how to use the chart without turning a quick reference into a final design decision.