How to Size Surge Arresters for a 112.5 kVA Transformer on a 34.5 kV Delta System (11/5/2026)

Selecting surge arresters for a 112.5 kVA transformer on a 34.5 kV delta system requires evaluating system grounding, maximum continuous operating voltage (MCOV), temporary overvoltages, and transformer insulation ratings. This guide explains IEEE C62 surge arrester selection, correct voltage calculations, NEC and Canadian electrical considerations, and practical protection requirements for medium-voltage industrial transformers.

By LarsonElectronics.com, November 5, 2026

Selecting a surge arrester for a 112.5 kVA transformer connected to a 34,500-volt delta system requires more than matching the arrester voltage rating to the transformer nameplate. Engineers must evaluate the electrical system grounding configuration, maximum continuous operating voltage (MCOV), temporary overvoltage (TOV) conditions, transformer insulation withstand capability, and the arrester's protective characteristics.

For an ungrounded 34.5 kV delta system, a phase-to-ground surge arrester must be capable of withstanding elevated voltage during ground faults. A 48 kV rated arrester with approximately 39 kV MCOV is one conservative preliminary selection used in some utility specifications. Other ratings, including 39 kV rated arresters with approximately 31.5 kV MCOV, may be acceptable when supported by the manufacturer's temporary-overvoltage curves and the system's ground-fault clearing characteristics.

Key engineering principle: The transformer capacity of 112.5 kVA does not directly determine the required surge arrester voltage rating. System voltage, grounding, temporary overvoltage exposure, and insulation coordination are the primary selection factors.

What Size Surge Arrester Is Required for a 34.5 kV Delta Transformer

For a 112.5 kVA transformer supplied by an ungrounded 34.5 kV delta system, the initial surge arrester selection should account for the possibility that a healthy phase will approach full line-to-line voltage relative to ground during a single-line-to-ground fault.

Two illustrative phase-to-ground arrester selections are:

48 kV rated arrester with approximately 39 kV MCOV: A conservative preliminary selection for systems where ground faults may remain uncleared for extended periods or where the design requires substantial temporary-overvoltage withstand capability.
39 kV rated arrester with approximately 31.5 kV MCOV: A possible alternative where the anticipated voltage, ground-fault duration, and manufacturer-specific TOV capability support its use.

Neither rating should be treated as a universal recommendation. The selected arrester must withstand the actual continuous operating voltage and anticipated temporary overvoltages while providing adequate protection for the transformer's insulation system.

For comparison, an effectively grounded 34.5 kV wye system may use a lower phase-to-ground arrester rating because its normal phase-to-ground voltage is approximately 19.9 kV. However, effective grounding must be established from the source and system characteristics, not inferred from the transformer winding connection alone.

How Transformer Capacity Affects Surge Arrester Selection

A transformer rated at 112.5 kVA has a three-phase apparent power capacity of 112,500 volt-amperes. This rating determines the transformer's nominal current at a specified voltage but does not establish the voltage rating of the surge arrester.

For a three-phase transformer operating at 34.5 kV line-to-line, the nominal primary current is calculated as follows:

Primary current = 112,500 VA / (1.732 × 34,500 V)

Primary current ˜ 1.88 A

This relatively low primary current does not mean that a smaller voltage-rated arrester is appropriate. Surge arresters protect insulation against transient overvoltages, and their selection involves electrical characteristics different from normal transformer load current.

The transformer kVA rating remains relevant to equipment specifications, installation requirements, and system protection, but surge arrester voltage selection depends primarily on the voltage exposure across the arrester.

How Delta Grounding Affects Surge Arrester Voltage Requirements

A delta-connected transformer winding does not inherently establish whether the supplying electrical system is grounded or ungrounded.

A delta winding may be connected to an ungrounded system, a resistance-grounded system, or a system with an established grounding reference elsewhere in the network.

Consequently, engineers must distinguish between the transformer's winding configuration and the grounding characteristics of the upstream electrical supply.

Effectively Grounded Systems

On an effectively grounded 34.5 kV system, the nominal phase-to-ground voltage is approximately:

34.5 kV / 1.732 = 19.9 kV

Arresters are selected using the maximum continuous phase-to-ground voltage, grounding characteristics, and anticipated temporary overvoltages.

Ungrounded Delta Systems

On an ungrounded delta system, the phase-to-ground voltages are determined by system capacitances, insulation conditions, and other network characteristics.

During a single-line-to-ground fault, the two unfaulted phases can experience approximately full line-to-line voltage relative to ground.

For a 34.5 kV system, this means that a healthy phase may experience approximately 34.5 kV to ground at nominal system voltage. Actual maximum voltage may be higher because of normal operating voltage variations and system transients.

Ungrounded systems can also experience intermittent arcing-ground conditions and other overvoltages that require engineering evaluation.

Resistance-Grounded and Impedance-Grounded Systems

Resistance-grounded and impedance-grounded systems require evaluation of their grounding impedance, ground-fault current, fault detection, and clearing practices.

Some systems permit ground faults to remain present for a defined period, while others initiate rapid protective tripping.

These differences affect the magnitude and duration of voltage that surge arresters must withstand.

What Maximum Continuous Operating Voltage Means

Maximum continuous operating voltage, or MCOV, is the maximum designated power-frequency RMS voltage that may be continuously applied across a surge arrester.

For a phase-to-ground arrester, the MCOV must be suitable for the maximum continuous voltage between the connected phase and ground.

For example, if a 34.5 kV system operates at 105% of nominal voltage:

Maximum line-to-line voltage = 34.5 kV × 1.05 = 36.225 kV

On an ungrounded system experiencing a single-line-to-ground fault, an unfaulted phase may approach this voltage relative to ground.

An arrester with 39 kV MCOV exceeds this illustrative voltage exposure. An arrester with 31.5 kV MCOV does not continuously withstand 36.225 kV under its MCOV rating, but may tolerate the condition temporarily if its manufacturer-specified TOV capability and the expected fault duration permit.

Important: MCOV is not the voltage at which a surge arrester suddenly begins conducting. Metal-oxide arresters have nonlinear voltage-current characteristics and conduct varying current at different applied voltages.

Surge arrester selection must distinguish between continuous voltage withstand, temporary-overvoltage capability, and impulse protective performance.

How Temporary Overvoltage Affects Arrester Selection

Temporary overvoltages are power-frequency or near-power-frequency voltage increases that may persist from cycles to seconds or longer, depending on the system disturbance.

Potential causes include:

Single-line-to-ground faults
Grounding system characteristics
Load rejection
System resonance or ferroresonance
Switching operations
Abnormal neutral displacement

Arrester manufacturers publish TOV withstand characteristics that relate permissible applied voltage to exposure duration.

When an arrester's MCOV is below the maximum voltage that may occur during a ground fault, engineers must verify that the expected exposure falls within the manufacturer's TOV capability.

The analysis must consider initial arrester operating conditions, prior energy absorption, system voltage, and the time required to clear or correct the disturbance.

For an ungrounded delta system where ground faults may remain undetected or uncleared for extended periods, selecting an arrester with MCOV above the maximum expected line-to-line voltage can provide a more conservative starting point.

How Common Surge Arrester Ratings Compare on a 34.5 kV System

The following table illustrates commonly encountered arrester voltage ratings. Actual product ratings and application suitability must be verified from the manufacturer's data sheets.

Illustrative Surge Arrester Ratings for 34.5 kV Applications
Rated Voltage	Typical MCOV	Engineering Consideration
27 kV	22.0 kV	May be considered for effectively grounded applications; generally unsuitable for sustained full line-to-line ground-fault exposure
30 kV	24.4 kV	Commonly associated with effectively grounded applications; requires system-specific verification
36 kV	29.0 kV	May require substantial reliance on TOV capability during ungrounded-system faults
39 kV	31.5 kV	Possible ungrounded-system application when supported by TOV analysis and fault duration
48 kV	39.0 kV	Conservative preliminary selection for some ungrounded or impedance-grounded applications

Higher voltage ratings generally provide greater power-frequency voltage withstand capability but can also increase the arrester's residual or protective voltage.

Therefore, selecting the highest available arrester voltage rating is not automatically the best engineering solution. The arrester must provide an appropriate balance between temporary-overvoltage survival and transformer insulation protection.

How Transformer Insulation Ratings Affect Surge Arrester Selection

Surge arresters are intended to limit transient overvoltages to levels compatible with the insulation withstand capabilities of the protected equipment.

Basic lightning impulse insulation level, commonly called BIL, is an important transformer insulation characteristic.

For a 34.5 kV class transformer, the applicable BIL depends on the equipment design, winding insulation class, and specified standards. A 150 kV BIL is encountered in some 34.5 kV class transformer applications, but it must not be assumed for every transformer.

Engineers should obtain the actual transformer nameplate and design information before completing the insulation coordination study.

The study should compare:

Transformer lightning impulse withstand rating
Arrester lightning impulse protective level
Arrester front-of-wave protective characteristics
Relevant switching surge protective levels
Lead length and connection-induced voltage effects
Applicable insulation coordination margins

IEEE C62.22 and the IEEE C62.82 series provide relevant guidance for surge arrester application and insulation coordination.

A surge arrester with an acceptable MCOV may still provide inadequate insulation protection if its protective voltage is too high for the transformer or if installation lead effects are excessive.

Which IEEE Standards Apply to Medium-Voltage Surge Arresters

IEEE C62 standards provide the principal technical framework for metal-oxide surge arrester requirements, testing, and application on AC electrical systems.

IEEE Standards Relevant to Surge Arrester Selection
Standard	Application
IEEE C62.11	Requirements and testing for metal-oxide surge arresters on AC power circuits above 1 kV
IEEE C62.22	Application guidance for metal-oxide surge arresters on AC systems
IEEE C62.82.1	Insulation coordination principles and definitions
IEEE C62.82.2	Application guidance for insulation coordination
IEEE C57.12.00	General requirements for liquid-immersed distribution, power, and regulating transformers
IEEE C57.12.90	Testing of liquid-immersed transformers, including applicable dielectric tests

Dry-type transformers may be subject to IEEE C57.12.01 and IEEE C57.12.91 instead of the corresponding liquid-immersed transformer requirements.

Applicable standard editions, arrester classifications, and manufacturer-specific ratings must be confirmed for each installation.

How Surge Arrester Class Affects Transformer Protection

Surge arresters are available in different performance classifications and construction configurations.

For industrial and utility transformer applications, engineers may evaluate distribution-class, intermediate-class, or station-class arresters, depending on the applicable product standard and equipment design.

Selection considerations include:

Lightning impulse discharge capability
Switching surge energy requirements
Temporary-overvoltage withstand characteristics
Residual voltage performance
Short-circuit or pressure-relief capability
Housing insulation and environmental suitability
Pollution performance and creepage distance
Mechanical mounting and connection requirements

A 112.5 kVA transformer does not automatically require a station-class arrester solely because its primary voltage is 34.5 kV.

The appropriate arrester classification depends on system exposure, protective performance, reliability requirements, and the applicable engineering specifications.

Where Surge Arresters Should Be Installed on a Transformer

Surge arresters are generally installed as close as practicable to the equipment terminals they protect, with short, direct connections to the phase conductors and the grounding system.

Long arrester leads can increase the voltage experienced by transformer insulation during fast-rising surge events.

For a three-phase transformer, phase-to-ground arresters are commonly provided for each phase when required by the protection design.

Installation requirements include:

Suitable arrester voltage and insulation ratings
Appropriate electrical clearances
Short and properly routed connecting conductors
Effective grounding and bonding
Mechanical support suitable for the installation
Environmental and contamination considerations
Compliance with manufacturer installation instructions

For pole-mounted, pad-mounted, or substation transformers, the preferred physical arrangement depends on the equipment design and utility or facility engineering requirements.

How NEC Requirements Apply to Medium-Voltage Surge Arresters

The National Electrical Code, NFPA 70, includes requirements relevant to surge arrester installations within its scope.

Applicable provisions may include:

NEC Article 280: Surge arresters for circuits operating over 1,000 volts.
NEC Article 450: Transformer installation and protection requirements.
NEC Article 250: Grounding and bonding requirements.
NEC Article 110: General installation requirements, equipment suitability, and working clearances.
NEC Article 490: Equipment operating over 1,000 volts nominal, where applicable.

NEC Article 280 is particularly relevant because it addresses surge arrester installation, voltage ratings, and associated connections for higher-voltage electrical systems.

Utility-controlled installations may fall outside NEC jurisdiction under the applicable utility exclusion. In those cases, the National Electrical Safety Code, IEEE standards, and utility engineering specifications may be relevant.

The adopted NEC edition, local amendments, and authority having jurisdiction must be confirmed for each installation.

How Canadian Electrical Requirements Affect Surge Arrester Selection

Canadian industrial installations may be subject to CSA C22.1, the Canadian Electrical Code, Part I, along with provincial or territorial amendments and applicable utility requirements.

Relevant requirements include the Canadian Electrical Code provisions addressing high-voltage installations, grounding, bonding, equipment approval, and surge protection, as applicable to the installation.

Canadian installations may also require equipment certification or field evaluation acceptable to the local authority having jurisdiction.

IEEE and IEC arrester performance standards may inform equipment specifications, but compliance with a product standard does not automatically establish installation approval under Canadian electrical regulations.

For industrial transformers supplied from 34.5 kV systems in Canada, the same engineering fundamentals apply: confirm grounding configuration, maximum voltage, TOV exposure, arrester protective characteristics, and transformer insulation ratings.

How to Select a Surge Arrester for a 112.5 kVA Transformer

A practical engineering selection process includes the following steps:

Confirm the transformer configuration: Verify that 34.5 kV is the primary line-to-line voltage and identify the actual winding connections.
Identify the source grounding: Determine whether the supplying system is effectively grounded, ungrounded, resistance grounded, or otherwise impedance grounded.
Determine maximum system voltage: Obtain the highest anticipated continuous operating voltage from the utility or system design.
Evaluate temporary overvoltages: Determine expected ground-fault voltage magnitude, duration, and other relevant TOV conditions.
Select a suitable MCOV: Ensure the arrester withstands continuous voltage and applicable temporary overvoltage exposure.
Verify insulation coordination: Compare arrester protective levels with transformer BIL and other applicable insulation withstand characteristics.
Evaluate arrester performance: Confirm energy handling, short-circuit capability, environmental suitability, and appropriate arrester classification.
Review installation requirements: Verify clearances, mounting, grounding, bonding, and connecting lead arrangements.
Obtain engineering approval: Confirm the final selection against manufacturer data and utility or facility specifications.

For a 34.5 kV ungrounded delta system, a 48 kV rated arrester with approximately 39 kV MCOV may be used as a conservative preliminary reference. A lower-rated arrester may provide better protective levels when its TOV withstand capability is sufficient for the anticipated operating conditions.

The final selection should be documented through an insulation coordination and temporary-overvoltage evaluation.

What Information Should Be Provided When Requesting a Surge Arrester

Industrial buyers and engineers should provide the following information when requesting surge arresters for a medium-voltage transformer:

Transformer capacity: 112.5 kVA
Primary voltage: 34,500 V line-to-line
Transformer winding configuration: Delta, if confirmed
System grounding configuration
Maximum continuous system operating voltage
Ground-fault detection and clearing characteristics
Transformer BIL and insulation class
Required arrester mounting arrangement
Outdoor or indoor installation conditions
Applicable IEEE, NEC, CSA, or utility specifications
Available fault current and required arrester short-circuit capability
Environmental exposure and contamination conditions

Providing these details helps manufacturers and engineers identify appropriate arrester ratings without relying on assumptions about system grounding or fault duration.

For additional information about transformer configurations and electrical requirements, review industrial transformers.

Frequently Asked Questions About 34.5 kV Transformer Surge Arresters

What Surge Arrester Rating Is Suitable for a 34.5 kV Ungrounded Delta System

A 48 kV rated arrester with approximately 39 kV MCOV is one conservative preliminary option. A 39 kV rated arrester with approximately 31.5 kV MCOV may also be suitable when supported by manufacturer TOV data and the expected ground-fault duration. Final selection requires insulation coordination and system grounding verification.

Does a 112.5 kVA Transformer Require a Specific Arrester Voltage Rating

No. Transformer kVA determines apparent power capacity and nominal load current, not the required surge arrester voltage rating. System voltage, grounding, maximum continuous voltage, and temporary overvoltages govern voltage selection.

Can a 30 kV Surge Arrester Be Used on a 34.5 kV System

A 30 kV rated arrester with approximately 24.4 kV MCOV may be suitable for certain effectively grounded 34.5 kV applications. It should not be assumed suitable for an ungrounded delta system that can experience sustained line-to-line voltage relative to ground.

Why Do Ungrounded Delta Systems Require Different Arrester Considerations

During a single-line-to-ground fault, the healthy phases of an ungrounded delta system may rise to approximately full line-to-line voltage relative to ground. Surge arresters must withstand the resulting voltage for the anticipated fault duration.

What Is the Difference Between MCOV and Arrester Rated Voltage

MCOV is the maximum designated RMS power-frequency voltage that can be continuously applied across the arrester. Rated voltage is a separate standardized characteristic associated with the arrester's duty and temporary-overvoltage performance.

Does a Higher Arrester Voltage Rating Provide Better Transformer Protection

Not necessarily. Higher voltage ratings generally improve power-frequency withstand capability but may increase residual voltage during surges. Proper selection balances arrester survival with adequate insulation protection.

Which IEEE Standards Apply to Surge Arrester Selection

IEEE C62.11 addresses metal-oxide surge arrester requirements, IEEE C62.22 provides application guidance, and the IEEE C62.82 series addresses insulation coordination principles and applications.

Are the Same Surge Arrester Selection Principles Used in Canada

Yes. The electrical principles of system voltage, grounding, MCOV, TOV withstand, and insulation coordination apply in both countries. Canadian installations must also satisfy applicable CSA, provincial, utility, and equipment approval requirements.

Final Engineering Considerations for 34.5 kV Transformer Surge Protection

Surge arrester selection for a 112.5 kVA transformer on a 34.5 kV delta system begins with understanding the supplying electrical system rather than the transformer capacity alone.

An ungrounded delta system requires particular attention to ground-fault overvoltages, MCOV, and manufacturer temporary-overvoltage withstand characteristics. A conservative arrester selection may improve survival during sustained ground faults, but it must also provide sufficient protective margin for the transformer's insulation system.

Engineers should verify the system grounding arrangement, maximum operating voltage, transformer BIL, arrester residual voltage, fault duration, and applicable IEEE and electrical code requirements before specifying equipment.

For assistance evaluating industrial transformer specifications, medium-voltage applications, and equipment requirements, contact Larson Electronics.

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