Neutral Grounding Resistor (NGR) Sizing and Selection: A Step-by-Step Calculation Guide

A neutral grounding resistor (NGR) is a resistive impedance connected between a power system’s neutral point and earth, engineered to limit single-line-to-ground fault current to a controlled, predetermined magnitude while still allowing protective relays to detect the fault. Sizing an NGR correctly is one of the most consequential decisions in medium-voltage (MV) system design: too low a resistance value lets fault current climb toward damaging levels, while too high a value can make ground faults invisible to standard overcurrent protection. This guide walks through the governing formulas, a fully worked numerical example, a comparative selection table, and the practical pitfalls engineers encounter when specifying an NGR for transformers, generators, or switchgear neutral points. It is written for procurement engineers and design teams sourcing equipment for grids across the Middle East and Gulf (including Egypt and Saudi Arabia), North Africa, the CIS region, and Sub-Saharan Africa, where resistance-grounded MV distribution is the dominant practice for industrial and utility networks.

For related equipment context, see our hub pages on [medium-voltage switchgear](https://powersolutionshub.com/what-is-mv-switchgear-medium-voltage-switchgear/) and [protection relays](https://powersolutionshub.com/what-is-a-medium-voltage-protection-relay-mv-protection-relay/), and our transformer technical resources.

The Method: Core NGR Sizing Formulas

NGR sizing starts from the system’s line-to-neutral (phase) voltage and the target ground-fault current the designer wants to permit. The fundamental relationship is Ohm’s law applied at the neutral point:

R = V_LN / I_G

Where: – R = required neutral grounding resistance (ohms) – V_LN = system line-to-neutral voltage = V_LL / √3 (volts) – I_G = desired maximum single-line-to-ground fault current (amperes)

The resistor’s continuous or short-time power rating is then derived from:

P = I_G² × R (for the resistor’s thermal duty), typically expressed as a kW rating for a specified time duration (e.g., 10 seconds or 1 minute), since NGRs are usually rated for short-time duty rather than continuous duty.

A second, equally important formula checks the resistor against the system’s zero-sequence charging capacitance, because the whole point of resistance grounding is to make the resistive fault current dominate over the capacitive charging current:

I_R ≥ I_C (approximately), where I_C is the total per-phase capacitive charging current of the network (cables, windings, surge capacitors). Low-resistance grounding schemes typically target I_R several times larger than I_C to ensure effective, low-transient-overvoltage fault clearing, while high-resistance grounding schemes deliberately size I_R close to or just above I_C to limit fault current to only a few amperes for continued (non-tripping) operation on the first ground fault.

Finally, the resistor must be checked against the transient recovery voltage and the arc-flash energy that protection settings will allow — this is a coordination step performed together with the protection relay engineer, not a standalone calculation.

Worked Example: Sizing an NGR for an 11 kV Transformer Neutral

Step 1 — Establish system parameters. Assume a distribution transformer feeds an 11 kV system and the design target is to limit ground-fault current to a low-resistance grounding value of 400 A (a common utility target for feeder protection coordination).

Line-to-neutral voltage: V_LN = V_LL / √3 = 11,000 / 1.732 ≈ 6,351 V

Step 2 — Calculate required resistance. R = V_LN / I_G = 6,351 / 400 ≈ 15.9 Ω

Step 3 — Calculate the resistor’s thermal (power) rating. For a short-time rating of, say, 10 seconds: P = I_G² × R = (400)² × 15.9 ≈ 2,544,000 W ≈ 2,544 kW for 10 seconds

Because this is a short-time duty, the resistor element is designed to absorb this energy thermally over the rated time and then cool — it is not a continuous 2,544 kW load. The design engineer specifies the resistor’s time-current withstand curve so it survives the maximum expected fault duration set by the upstream protection (including breaker failure backup time), with margin.

Step 4 — Cross-check against high-resistance grounding, for comparison. If instead the design goal were high-resistance grounding to hold ground-fault current to a much smaller value — for example 25 A, chosen to exceed the system’s estimated charging current I_C and allow alarm-only operation on the first fault:

R = V_LN / I_G = 6,351 / 25 ≈ 254 Ω

P = I_G² × R = (25)² × 254 ≈ 158,750 W ≈ 159 kW

This comparison shows how dramatically the required resistance and let-through current change depending on whether the grounding philosophy is low-resistance (fast trip on first fault, larger resistor current, lower ohmic value) or high-resistance (alarm on first fault, tiny resistor current, much higher ohmic value).

Step 5 — Verify protection coordination. The ground-fault relay’s pickup setting must sit comfortably below I_G (400 A in the low-resistance case) with margin for CT accuracy and downstream fault contribution, while remaining above normal system unbalance and CT/CBCT error currents. This step is typically performed jointly with the protection relay engineer using the specific relay’s ground-fault element characteristics.

Selection & Comparison Table

Selection Criteria and Common Pitfalls

1. Match the resistor to the protection philosophy, not just the transformer size. The kVA rating of the connected transformer determines available fault current capability, but the actual I_G target is a protection-coordination decision made jointly with the relay engineer — sizing the NGR in isolation is the most common design error.

2. Confirm the time rating, not just the ohmic value. A resistor sized correctly in ohms but under-rated in short-time thermal withstand will fail during a sustained or repeated fault, especially where breaker failure or backup clearing times are long.

3. Check I_R versus I_C for high-resistance schemes. If the resistor’s rated current is not adequately above the system’s total charging current, transient overvoltages during arcing ground faults can exceed equipment insulation withstand — defeating the purpose of resistance grounding.

4. Verify voltage rating and BIL match the connected system. The NGR’s insulation must be rated for the full line-to-neutral voltage plus a margin, since during a ground fault the resistor briefly sees near-full phase voltage across it.

5. Coordinate with generator neutral grounding practices separately. Generators often use different grounding philosophies (frequently higher-resistance, lower let-through current) than distribution transformers, because generator windings are more sensitive to fault-induced iron damage; a single sizing rule should not be applied uniformly across generator and transformer neutrals on the same system without review.

6. Account for multiple grounding points. On systems with more than one transformer or generator neutral, only one NGR (or one grounding path) should normally be active at a time to avoid unintended parallel paths that change the effective fault current — this is managed through neutral grounding switchgear and interlocking schemes.

7. Environmental and duty-cycle derating. Ambient temperature, altitude, and enclosure ventilation affect the resistor’s thermal withstand and must be checked against the manufacturer’s correction factors for the specific installation site.

Standards Reference

NGR application and sizing practice is guided primarily by IEEE Std 32 (IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices), which defines resistor rating conventions, time-current withstand testing, and terminology for neutral grounding resistors and reactors. System grounding philosophy and its effect on protection coordination is further addressed in IEEE Std 142 (the “Green Book” on grounding of industrial and commercial power systems). Where MV switchgear housing the grounding resistor or neutral grounding cubicle is involved, the switchgear itself is covered by the relevant IEC 62271 series switchgear standards, but the resistor element’s own rating and test methodology follows the IEEE 32 framework referenced above. Designers working in IEC-based jurisdictions typically apply these same first-principles calculations, since resistance-grounding sizing methodology is standard practice rather than a code-mandated single value.

Related guides

See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.