How to Configure Electrical Grounding for Unit Substations: Service vs SDS & NEC Inputs

This technical guide explains NEC-compliant grounding configurations for unit substations and service disconnect inputs clearly.

Scope includes electrode systems, GEC sizing, bonding methods, fault calculations, and real-world implementation examples today.

Grounding configuration and minimum grounding conductor sizing for service vs separately derived unit substation (thermal method)

System type Service entrance (utility-supplied)Separately derived system (unit substation transformer)

Grounding point location Service main disconnect / switchboardUnit substation LV switchgear busTransformer secondary neutral bushing

Available 3‑phase fault current at grounding point (kA)

Grounding conductor material CopperAluminum

Advanced options

Fault clearing time (s)

Design factor on fault current (multiplier)

Thermal constant k (A·s^0.5/mm²)

Secondary line-to-line voltage for transformer-based estimate (V)

Transformer rating for fault estimate (kVA)

Transformer impedance (%Z)

You may upload a nameplate or one-line diagram photo to suggest grounding and fault parameters.

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Enter available fault current or transformer data to obtain a grounded system configuration note and minimum grounding conductor cross-section.

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Formulas used (thermal sizing of grounding conductor):

1) If available 3-phase symmetrical fault current at the grounding point is provided:
Ifault(sym) [A] = Ifault_input [kA] × 1000

2) If fault current is estimated from transformer data (when Ifault_input is not provided):
Ifl [A] = (S [kVA] × 1000) / (√3 × VLL [V])
Ifault(sym) [A] ≈ Ifl [A] × (100 / %Z)

3) Asymmetrical design current including margin:
Idesign [A] = Ifault(sym) [A] × Fdesign
where Fdesign is a user-selected multiplier (typically 1.1–1.3) to account for DC offset and engineering margin.

4) Minimum grounding conductor cross-sectional area based on thermal withstand (IEC/IEEE method):
Smin [mm²] = (Idesign [A] × √t [s]) / k
where t is fault clearing time in seconds, and k is the thermal constant in A·s^0.5/mm² (typical values: copper 115, aluminum 76, for 90 °C to 250 °C).

5) The resulting Smin is rounded up to the next standard conductor size (approximate equivalence to AWG or kcmil). NEC grounding and bonding articles (e.g., 250.24 for services, 250.30 for separately derived systems, 250.66 for grounding electrode conductors) must also be checked for prescriptive minimum sizes.

Material	Thermal constant k (A·s^0.5/mm²)	Example conductor	Approx. area (mm²)	Approx. allowable I for 0.25 s (kA)
Copper	115	16 mm² (≈ 6 AWG)	16	≈ 7.4 kA
Copper	115	35 mm² (≈ 2 AWG)	35	≈ 16.1 kA
Copper	115	70 mm² (≈ 2/0 AWG)	70	≈ 32.2 kA
Aluminum	76	50 mm² (≈ 1/0 AWG Al)	50	≈ 13.2 kA
Aluminum	76	95 mm² (≈ 250 kcmil Al)	95	≈ 25.1 kA

How does this tool distinguish service vs separately derived system grounding?

The system type selection only affects the qualitative grounding configuration note. For a service, the calculator states that the main bonding jumper and grounding electrode conductor connection are at the service disconnecting means in accordance with NEC service grounding rules. For a separately derived system, it indicates that the system bonding jumper must be at the first disconnecting means or at the transformer, and that the grounded conductor must be isolated downstream, consistent with NEC separately derived system grounding.

What engineering method is used to size the grounding conductor?

The sizing is based on the thermal short-circuit withstand relationship S = I × √t / k, widely used in IEC and IEEE practice. The user inputs or estimates the available fault current and the protective device clearing time; the calculator applies a material-dependent k constant and rounds the resulting cross-section up to the next standard conductor size. Users must still verify NEC minimum sizes and any utility or project standards.

Can this calculator replace a detailed NEC or short-circuit study?

No. The calculator provides an engineering estimate of the minimum grounding conductor cross-section for thermal withstand and a qualitative indication of the grounding configuration (bonding point) for service vs separately derived systems. It does not replace a full NEC compliance review, does not model all conductor installation conditions, and does not replace a formal short-circuit and coordination study.

Which input values are most critical for accurate results?

The two dominant parameters are the available 3-phase short-circuit current at the grounding point and the actual fault clearing time of the protective device. If these are not directly known, using realistic transformer kVA, voltage, and nameplate impedance and typical clearing times is recommended, and then cross-checking with a detailed protection study before final design decisions.

Regulatory framework and core definitions

Grounding and bonding for unit substations and service disconnect inputs must be designed to satisfy the National Electrical Code (NEC), applicable local amendments, and recognized engineering standards. Key NEC sections to consult include NEC 250 (Grounding and Bonding), NEC 230 (Services), NEC 408 (Switchboards, Switchgear, and Panelboards), and the rules addressing separately derived systems in NEC 250.30. Complementary guidance: IEEE Std 142 (Green Book) and IEEE Std 80 for grounding of substations.Important definitions used throughout:

• Service: The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises (NEC art. 100).
• Service disconnecting means (SDS): The disconnect or set of disconnects required at the service point to de-energize the premises.
• Service equipment: The equipment, such as meter and main service disconnect, where the grounded conductor is connected to the grounding electrode system.
• Unit substation: A factory-assembled packaged assembly that typically includes a transformer and switchgear, provided as a single unit for building or site distribution.
• Separately derived system: A system where the neutral is not connected to the supply-side neutral (e.g., a transformer with a separately grounded neutral) and that requires its own grounding/bonding arrangements per NEC 250.30.

Fundamental grounding principles for unit substations versus SDS

A clear engineering decision: where to bond the grounded conductor (neutral) to the grounding electrode system. This determines fault return paths, equipment grounding conductor routing, and protective device coordination.

Service equipment bonding location

- When the unit substation is configured as service equipment (the utility service terminates at the substation with main overcurrent protection and service disconnecting means located in the substation), the neutral-to-ground bond must be made at that service equipment location (single bonding point for the service). NEC 250.24(A)(1) requires the grounded conductor be connected to the grounding electrode system at the service equipment. - From that service bonding point, equipment grounding conductors (EGCs) must be run with the associated circuits back to the bonded enclosure. Do not establish multiple neutral-to-ground bonds downstream in the same service system.

Separately derived system (SDS) considerations

- If the unit substation contains a transformer that establishes a separately derived system (the secondary neutral is not directly connected to the utility neutral and has a separate neutral conductor established by the transformer), then the bonding of the neutral to the grounding electrode system must occur at the SDS neutral bonding location (NEC 250.30). - For SDS installations the transformer neutral is normally bonded to the grounding electrode conductor (GEC) at the transformer enclosure and the grounding electrode system must be sized and connected per NEC 250.30(A) and 250.32. The service-side bonding point and SDS-bonding are two distinct bonding points separated by the service point or transformer, per NEC rules.

Components of the grounding electrode system and typical sizing

The grounding electrode system for unit substations may include one or more of the following (NEC 250.50–250.68):

• Driven ground rods
• Grounding electrode conductor (GEC) connecting to electrodes
• Concrete-encased electrodes (Ufer grounds)
• Ground ring or grid
• Metal underground water piping in direct contact with earth for 10 ft or more
• Plate electrodes

Note: NEC imposes minimum material and size requirements for each electrode type (see NEC 250.52 and 250.66). Always confirm local code and utility requirements.

Sizing GEC and EGC: methods, formulas, and explanations

Two separate conductor sizing concerns:

• GEC (Grounding Electrode Conductor): connects service or transformer neutral bond to the grounding electrode system (NEC 250.66, 250.64).
• EGC (Equipment Grounding Conductor): carries fault current from equipment to the overcurrent device; sized per NEC 250.122 and run with circuits.

Basic fault current estimate (prospective short-circuit current)

Explanation of variables:

• V_LL — nominal line-to-line voltage on the system secondary (e.g., 480 V for 480Y/277 V systems).
• Z_s — total system impedance seen by a fault at the point of interest (ohms), including transformer impedance, conductor impedance, and utility source impedance.

Example of transformer-limited fault at secondary:

Formula: I_sc ≈ (Transformer kVA × 1000) / (√3 × V_secondary × Z_percent/100)

Variables:

• Transformer kVA — transformer rated apparent power in kVA.
• V_secondary — line-to-line voltage on the secondary (V).
• Z_percent — transformer percent impedance (typical 4–8% for medium- to large-power transformers).

This gives a first-order estimate of available fault current into the transformer secondary before considering utility contribution or feeder impedances.

GEC sizing methodology (procedural)

- Step 1: Identify whether the connection is at a service equipment or an SDS (NEC 250.24, 250.30). The GEC originates at the bonded enclosure. - Step 2: Use the largest ungrounded service-entrance conductor or equivalent to determine minimum GEC per NEC 250.66 (NEC provides a table based on cross-sectional area). When conductors are paralleled, calculate equivalent area accordingly. - Step 3: Provide a GEC conductor continuous, with listed connectors, to the grounding electrode system. Increase conductor size where required by mechanical protection or multiple electrodes. - Step 4: Where the ground electrode conductor must be run to remote electrodes, increase size for length and potential short-circuit heating concerns, and consider parallel electrodes to reduce resistance.Important: Tables above show common engineering practice and typical selections. NEC tables and local requirements control final sizing; always verify using NEC 250.66 and 250.122 for legal minimums.

Equipment grounding conductor sizing and time‑current withstand

The EGC must be sized to safely carry fault current long enough to trip the overcurrent device without damage. NEC 250.122 provides the minimum EGC sizes for different OCPD ratings. Use the NEC table directly for compliance. Where conductors will see high fault currents or prolonged thermal stress, select larger EGCs or use parallel bonding paths (bonding jumpers, ground rings) as required.

Ground grid and electrode resistance calculations

Two useful empirical formulas appear in engineering practice:

Approximate resistance of a single driven rod (ohms):

R_rod ≈ (ρ / (2 × π × L)) × (ln(4 × L / d) − 1)

Variables:

• R_rod — approximate resistance of single rod (Ω).
• ρ (rho) — soil resistivity (Ω·m or Ω·ft, keep units consistent).
• L — length of rod (m or ft, consistent with ρ).
• d — diameter of rod (m or ft).

Typical values and units:

• ρ: common soils: 100 Ω·m (sandy loam) to 1000 Ω·m (dry rock). In US customary units: 300–3000 Ω·ft.
• L: 2.4 m (8 ft) typical rod length.
• d: 0.016 m (5/8 in) for typical rods.

For multiple rods spaced at least one to two rod lengths apart, approximate reduction in resistance follows empirical rules (two rods reduce resistance to roughly 1/3–1/2 of a single rod, spacing and soil layering affect actual values). For accurate substation design, perform soil resistivity testing (Wenner or Schlumberger methods) and use specialized software or IEEE Std 80 methodologies to size a grid and model step-and-touch potentials.These numbers are approximate. Effective substation grounding design requires layered soil modeling and iterative grid sizing to achieve acceptable touch and step potential limits per IEEE Std 80.

Installation, testing and commissioning steps

Follow a defined sequence:

1. Survey soil resistivity (Wenner test) at proposed substation site and produce layered resistivity profile.
2. Determine system configuration: service neutral bonding vs SDS neutral bond at transformer.
3. Design grounding electrode system (grid, rods, Ufer, ring) sized to meet target resistance and step-and-touch criteria.
4. Select GEC and EGC sizes according to NEC tables and engineering analysis; coordinate with overcurrent protection device ratings and prospective fault currents.
5. Install mechanical protections for exposed conductors (conduit, sleeves), and use listed connectors and bonding jumpers as required (NEC 250.8 & 250.70).
6. Perform clamp torque checks, continuity verification, and megger testing on electrodes constructed as needed.
7. Measure final ground resistance (fall-of-potential) and document step-and-touch potentials, retaining test reports and drawings.

Practical bonding, conductor routing, and common pitfalls

Key installation notes:

• Avoid multiple neutral-to-ground bonds on the service side of the SDS; this can create parallel neutral paths and nuisance currents on grounding system components.
• Run the GEC with as straight and short a route as practicable; avoid splicing unless using listed connectors and protective enclosures.
• Use exothermic welds or listed compression connectors for permanent electrode bonding when practicality allows.
• Bond structural steel and metallic enclosures to the grounding electrode system to prevent elevated potentials during fault events (NEC 250.104).
• Provide adequate mechanical protection for GEC and EGCs where subject to mechanical damage (NEC 250.64, 250.66).
• Document labeling: identify neutral-ground bond location and provide one-line diagrams showing the bonding arrangement and electrode connections.

Real-world example 1: Unit substation as service equipment — 1000 kVA, 480Y/277V secondary

Project data:

• Transformer: 1000 kVA, 480Y/277 V secondary, transformer percent impedance Z% = 5.75% (typical).
• Service configuration: utility primary to transformer primary — unit substation is service equipment; neutral bond at transformer switchgear enclosure.
• Design target: provide robust grounding for personnel safety and fault clearing, target grounding electrode resistance <10 Ω, prefer <5 Ω.

Step A — calculate full-load current on secondary: Calculation:Step B — transformer-limited short-circuit current at secondary:

Formula: I_sc ≈ (Transformer kVA × 1000) / (√3 × V_secondary × Z_percent/100)

Calculation:

I_sc ≈ 1,000,000 / (831.4 × 0.0575) ≈ 1,000,000 / 47.79 ≈ 20,935 A

Interpretation: prospective fault current available at the secondary connection, before feeder impedance, about 21 kA from transformer.Step C — GEC and EGC sizing approach:

• Identify largest ungrounded conductor (secondary bus feeding switchgear designed for 1203 A). Using NEC 250.66 or equivalent, select minimum GEC. For engineering selection, choose copper GEC not smaller than #2 AWG for high-amperage feeders commonly encountered — but refer to local NEC table for legal minimum.
• Equipment grounding conductor sizing for feeders: use NEC 250.122 table relative to the OCPD protecting the feeder conductors. For a 1600–2000 A main, typical practice is to provide large copper conductor (e.g., 4/0 Cu or larger) sized per NEC and mechanical/thermal requirements.

Step D — electrode system:

• Soil resistivity test: assume layered soils: top layer 200 Ω·m, deeper 500 Ω·m. Use a ground ring with multiple rods and Ufer where possible.
• Design a perimeter ground ring (2 AWG Cu) plus two 8 ft rods and a Ufer connection to foundation rebar to achieve <5 Ω in typical conditions.

Commissioning:

• Measure fall-of-potential and document final ground resistance. If results exceed targets, add rods or extend grid.
• Test continuity between neutral bond and each major conductive structure and record results.

Notes: This example illustrates vector methods to determine prospective fault current and a conservative engineering approach to GEC/EGC sizing. Final conductor sizes must be derived from NEC tables and local code verification.

Real-world example 2: Separately derived system inside unit substation feeding a building SDS

Project data:

• Transformer: pad-mounted 750 kVA with high-voltage primary and 208Y/120 V secondary feeding building distribution via a unit substation interior switchboard configured as a separately derived system (neutral created at transformer).
• SDS location: building main service and utility connection are upstream; transformer secondary is separately derived and must have its neutral bonded at the transformer or the first disconnecting means at the SD system per NEC 250.30.

Step A — full-load current on secondary:Step B — transformer-limited secondary short-circuit: Assume Z% = 5.5%

I_sc ≈ 750,000 / (360.3 × 0.055) ≈ 750,000 / 19.816 ≈ 37,840 A

Interpretation: transformer provides significant available fault current at the secondary.Step C — bonding strategy:

• Because the transformer secondary is a separately derived system, bond the secondary neutral at the transformer enclosure to a dedicated grounding electrode conductor which ties into the grounding electrode system serving the building per NEC 250.30(A).
• Ensure the neutral is isolated from the service neutral upstream — do not create parallel neutral-to-ground bonds downstream of the service point.

Step D — GEC and electrode connection:

• Run a GEC from the transformer neutral/bond point to the electrode system: use listed connectors and protect the conductor mechanically where exposed.
• Size the GEC according to NEC 250.66 for the transformer secondary conductor size; for high-current systems, use a larger conductor (for example, 4/0 Cu or larger) as required by both code and thermal fault analysis.

Step E — verify protective device coordination:

• Confirm that the feeder and branch protective devices will operate to clear the calculated fault currents without allowing excessive thermal stress on the EGCs. If necessary, select inverse-time coordination curves and verify clearing times.

Commissioning and documentation:

• Confirm neutral-to-ground bond is made only at the SDS bond point and record the physical location.
• Document GEC routing and verify electrode resistance by fall-of-potential testing.
• Prepare one-line diagrams showing bonding arrangement to satisfy inspection and maintenance requirements.

Testing, inspection and record-keeping

Essential tests and records:

• Soil resistivity test report with raw data and interpreted layered model.
• Fall-of-potential results and instrument calibration certificate.
• Continuity and clamp torque checklist for all bonded connections.
• Short-circuit calculations and protective device coordination study showing clearing times and I^2t exposures for GEC/EGC selection.
• As-built drawings with conductor sizes, materials, and bond point locations labeled.

References and external authoritative resources

• NFPA 70, National Electrical Code (NEC) — official code (see especially Article 250, Article 230, Article 250.30). https://www.nfpa.org/
• IEEE Std 142 — IEEE Green Book: Grounding of Industrial and Commercial Power Systems (conceptual and grounding design). https://standards.ieee.org/
• IEEE Std 80 — Guide for Safety in AC Substation Grounding (substation grid, step-and-touch limitations). https://standards.ieee.org/
• U.S. Department of Labor, OSHA — Electrical safety and grounding basics for buildings. https://www.osha.gov/
• International Association of Electrical Inspectors (IAEI) — practical guidance and articles on NEC interpretation. https://www.iaei.org/

Final technical recommendations

• Always identify whether the unit substation is service equipment or constitutes a separately derived system; this dictates the bonding location.
• Perform soil resistivity testing early in design to size electrodes and grid effectively; design for step-and-touch potential compliance per IEEE Std 80.
• Derive GEC and EGC sizes from NEC tables, but validate selections against calculated prospective fault currents and thermal I^2t exposures.
• Use robust mechanical connectors and list-approved clamps; prefer exothermic bonding where practicable for permanent, low-resistance connections.
• Document all bonding, conductor routings, tests, and one-line diagrams for AHJ review and long-term maintenance.

References cited above must be consulted for legal compliance; local electrical inspection authorities may impose supplemental or more restrictive requirements.