What size surge arrester is typically used for a 13.2 kV transformer?

For some effectively grounded 13.2 kV systems, a 10 kV rated surge arrester with an 8.4 kV MCOV is a reasonable candidate. Final selection requires verification of maximum operating voltage, temporary overvoltage capability, and insulation coordination.

Does transformer kVA determine the surge arrester voltage rating?

No. Transformer kVA does not directly determine surge arrester voltage rating. System voltage, grounding, temporary overvoltage conditions, and transformer insulation requirements are primary selection considerations.

What is the difference between arrester rated voltage and MCOV?

Rated voltage is the manufacturer's designated arrester duty rating, while MCOV is the maximum specified RMS power-frequency voltage that can be continuously applied across the arrester.

Can a 10 kV surge arrester protect a 13.2 kV transformer?

Yes, under suitable system conditions. A phase-to-ground arrester on an effectively grounded 13.2 kV system experiences nominal phase-to-ground voltage rather than full line-to-line voltage. MCOV, TOV, and insulation coordination must be verified.

Do ungrounded 13.2 kV systems need higher rated arresters?

Often, yes. A ground fault can raise healthy-phase-to-ground voltage toward the full line-to-line system voltage. Arrester selection must account for the magnitude and duration of this temporary overvoltage.

Where should surge arresters be installed on a transformer?

Surge arresters should generally be installed as close as practical to the protected transformer terminals with appropriately designed phase and ground connections, while maintaining required electrical clearances and complying with manufacturer instructions.

LarsonElectronics.com

By LarsonElectronics.com, October 22, 2026

Selecting the correct surge arrester for a 1 MVA transformer operating on a 13.2 kV medium-voltage distribution system requires more than matching the arrester voltage rating to the transformer nameplate. Proper selection depends on the system grounding configuration, maximum continuous operating voltage (MCOV), temporary overvoltage (TOV) capability, transformer insulation withstand levels, and the expected surge exposure.

For a nominal 13.2 kV, three-phase, effectively grounded system, a 10 kV rated metal-oxide surge arrester with an MCOV of approximately 8.4 kV is a possible selection, provided the maximum continuous line-to-ground voltage does not exceed the arrester MCOV and its TOV capability satisfies the system requirements. Ungrounded, delta-connected, and impedance-grounded systems may require substantially higher arrester ratings.

Surge Arrester Selection for a 1 MVA, 13.2 kV Transformer

A surge arrester protects transformer insulation by limiting transient overvoltages caused by lightning, switching operations, and other electrical disturbances. Modern distribution-class arresters commonly use metal-oxide varistor (MOV) elements that conduct surge current while limiting the voltage appearing across protected equipment.

The transformer capacity of 1 MVA does not directly determine the arrester voltage rating. Instead, the arrester must be coordinated with the electrical system and transformer insulation characteristics.

Key Electrical Parameters for Arrester Selection

Nominal system voltage: 13.2 kV, three-phase, line-to-line.
Transformer capacity: 1,000 kVA (1 MVA).
Nominal line-to-ground voltage: Approximately 7.62 kV on a balanced, grounded-wye system.
System grounding: Effectively grounded, impedance-grounded, or ungrounded.
Arrester rated voltage: The manufacturer's designated duty rating, expressed in kV RMS.
Maximum continuous operating voltage: The maximum specified power-frequency RMS voltage that may be continuously applied across the arrester.
Temporary overvoltage capability: The arrester's ability to withstand elevated power-frequency voltage for a specified duration.
Transformer insulation level: The applicable lightning impulse withstand rating and other insulation coordination requirements.

Calculating Line-to-Ground Voltage on a 13.2 kV System

For a balanced three-phase, grounded-wye system, the nominal phase-to-ground voltage is calculated by dividing the line-to-line voltage by the square root of three.

V(L-G) = V(L-L) / v3

For a 13,200-volt system:

13,200 V / 1.732 = approximately 7,621 V

The nominal line-to-ground voltage is therefore approximately 7.62 kV RMS.

This calculation establishes the nominal operating voltage across a phase-to-ground arrester. It does not account for maximum system voltage, voltage regulation, abnormal operating conditions, or ground-fault temporary overvoltages.

For example, if the maximum continuous system voltage is 13.8 kV line-to-line, the corresponding balanced phase-to-ground voltage is approximately 7.97 kV. An arrester with an 8.4 kV MCOV may satisfy this continuous-voltage requirement, subject to manufacturer specifications and system conditions.

Recommended Surge Arrester Ratings for a 13.2 kV Transformer

The following examples illustrate how system grounding affects preliminary arrester selection. They are not universal specifications. Final ratings must be established through the applicable voltage, grounding, and insulation coordination analysis.

System Configuration	Preliminary Arrester Consideration	Engineering Requirement
Effectively grounded wye	10 kV rated arrester; approximately 8.4 kV MCOV	Verify maximum phase-to-ground voltage and TOV withstand.
Impedance-grounded system	Higher MCOV may be necessary	Evaluate ground-fault voltage rise, fault duration, and manufacturer TOV curves.
Ungrounded or delta system	Arrester MCOV may need to approach or exceed maximum line-to-line voltage	Account for sustained ground faults and elevated healthy-phase voltage.

Important: A nominal 10 kV arrester rating and an 8.4 kV MCOV are not interchangeable specifications. Arrester rated voltage, MCOV, and TOV withstand must be evaluated independently using the manufacturer's published data.

How Grounding Configuration Changes Surge Arrester Requirements

Effectively Grounded Wye Systems

Effectively grounded systems generally limit phase-to-ground voltage rise during single-line-to-ground faults compared with ungrounded systems. This can permit lower MCOV arresters and improved protective margins.

A 10 kV rated arrester with an 8.4 kV MCOV may be appropriate for some 13.2 kV effectively grounded systems. However, effective grounding must be verified through system characteristics, not assumed from the presence of a grounded neutral.

Ungrounded and Delta Systems

On an ungrounded system, a single-line-to-ground fault can cause the two unfaulted phases to approach full line-to-line voltage relative to ground.

For a 13.2 kV system, this can place approximately 13.2 kV RMS from an unfaulted phase to ground, before considering maximum operating voltage or other abnormal conditions.

Consequently, an arrester with an 8.4 kV MCOV may be unsuitable where this condition can persist. The selected arrester must have sufficient continuous-voltage and temporary-overvoltage capability for the expected fault duration.

The required arrester rating may be significantly higher than the selection used on an effectively grounded system. A 12 kV or 15 kV rated arrester should not automatically be considered sufficient without checking its actual MCOV and TOV performance.

Impedance-Grounded Systems

Resistance-grounded and reactance-grounded systems require evaluation of their specific ground-fault behavior. The degree of neutral displacement depends on grounding impedance, system capacitance, and fault conditions.

Low-resistance, high-resistance, and reactance-grounded systems should not be treated as electrically equivalent when selecting surge arresters.

Coordinating Surge Arrester Protection With Transformer BIL

Basic impulse insulation level (BIL), commonly expressed in kV crest, represents a standardized lightning impulse withstand level for electrical equipment.

For a transformer on a 13.2 kV distribution system, a 95 kV BIL is a common example for certain 15 kV-class transformer designs. However, the actual transformer BIL must be confirmed from its nameplate, drawings, and applicable equipment standard.

The arrester's protective level must be sufficiently below the transformer's applicable insulation withstand level to provide an appropriate coordination margin.

Engineers should evaluate:

The arrester's lightning impulse residual voltage at applicable discharge currents.
The steep-front protective characteristics where relevant.
The transformer's specified BIL and insulation coordination requirements.
Lead length, grounding conductor impedance, and physical separation between arrester and transformer terminals.
Expected lightning exposure, switching surges, and system operating conditions.

As an illustrative calculation, if a transformer has a 95 kV BIL and the effective protective voltage at the transformer terminals is 40 kV crest, the simple BIL-to-protective-voltage ratio is:

95 kV / 40 kV = 2.375

This ratio is an illustration, not a complete insulation coordination study or a universally required design margin. Actual coordination must account for the relevant surge waveforms, protective characteristics, installation effects, and applicable IEEE guidance.

Why Arrester Discharge Current and Energy Capability Matter

Voltage rating alone does not establish whether a surge arrester is suitable for a particular installation.

Surge arresters also have discharge-current classifications, energy-handling capabilities, and fault-current withstand characteristics.

Distribution-class arresters are commonly associated with nominal discharge-current ratings such as 5 kA or 10 kA, depending on the product and applicable standard. These values should not be interpreted as the maximum surge current an arrester can survive.

Selection must consider lightning exposure, system configuration, expected surge severity, repetitive duty, and the arrester manufacturer's published performance.

For exposed overhead distribution installations, the protective duty may differ substantially from that of a transformer supplied through underground cable within an industrial facility.

IEEE Standards Applicable to Surge Arrester Selection

IEEE C62.11 for Metal-Oxide Surge Arresters

IEEE C62.11 establishes performance and test requirements for metal-oxide surge arresters used on AC power circuits. It provides the technical framework for evaluating arrester ratings, operating characteristics, and qualification tests.

IEEE C62.22 for Surge Arrester Application

IEEE C62.22 provides application guidance for metal-oxide surge arresters on AC systems. It addresses considerations such as voltage selection, temporary overvoltages, protective characteristics, and insulation coordination.

IEEE C57 Transformer Standards

Applicable standards within the IEEE C57 family establish transformer design, testing, and insulation requirements. Relevant references may include IEEE C57.12.00 and IEEE C57.12.90 for liquid-immersed power and distribution transformers, as applicable to the equipment.

IEEE C57.12.01 and IEEE C57.12.91 provide corresponding requirements for certain dry-type transformer applications.

IEEE C62.82 Insulation Coordination Guidance

The IEEE C62.82 series provides guidance concerning insulation coordination, including the relationship between equipment withstand capability and expected overvoltage stresses.

Engineers should use the applicable editions and portions of these standards for the specific system and equipment.

NEC and Canadian Electrical Code Considerations

For installations governed by the U.S. National Electrical Code, NFPA 70 Article 242 addresses overvoltage protection, including surge arresters rated above 1,000 volts.

Applicable NEC requirements should be reviewed alongside Article 110, including equipment installation and manufacturer-instruction requirements, and Article 450 for transformer installations where relevant.

For Canadian installations, CSA C22.1, Canadian Electrical Code, Part I, and applicable provincial or territorial requirements govern installation practices. Relevant CSA and IEC arrester product standards may also apply depending on the equipment specification and procurement requirements.

Utilities and industrial facilities may additionally impose engineering standards, equipment specifications, and insulation coordination criteria that exceed minimum installation-code requirements.

Practical Example of a 1 MVA Transformer Installation

Consider a 1,000 kVA, three-phase, 13.2 kV primary transformer installed at an industrial facility.

Assume the following preliminary design conditions:

Nominal primary voltage: 13.2 kV line-to-line.
Maximum continuous operating voltage: 13.8 kV line-to-line.
System grounding: Verified effectively grounded.
Nominal primary line-to-ground voltage: 7.62 kV.
Maximum balanced operating line-to-ground voltage: Approximately 7.97 kV.
Transformer BIL: Assumed 95 kV, subject to nameplate confirmation.

A 10 kV rated metal-oxide arrester with an 8.4 kV MCOV could be evaluated as a candidate because its MCOV exceeds the calculated maximum balanced operating line-to-ground voltage.

Before approving the selection, the engineer must confirm that the arrester's TOV capability accommodates system ground faults and other credible temporary overvoltages, and that its protective characteristics provide acceptable insulation coordination.

If the same transformer were supplied from an ungrounded system, the arrester selection would need to be reconsidered because healthy-phase-to-ground voltage can rise substantially during a ground fault.

Common Surge Arrester Selection Errors

Using transformer kVA as the primary sizing criterion: Transformer capacity does not directly determine arrester voltage rating.
Confusing rated voltage with MCOV: These specifications serve different purposes and must be evaluated separately.
Ignoring system grounding: Ungrounded and impedance-grounded systems may produce higher temporary phase-to-ground voltages.
Using nominal voltage instead of maximum operating voltage: Voltage regulation and actual system limits can affect arrester suitability.
Ignoring temporary overvoltage duration: An arrester may withstand an elevated voltage briefly but not continuously.
Failing to coordinate with transformer insulation: An arrester with an unnecessarily high protective level may provide inadequate insulation protection.
Installing arresters too far from protected equipment: Connection impedance and lead length can increase the voltage experienced at transformer terminals.

Frequently Asked Questions About 13.2 kV Transformer Surge Arresters

Typical Surge Arrester Rating for a 13.2 kV Transformer

For some effectively grounded 13.2 kV distribution systems, a 10 kV rated surge arrester with an 8.4 kV MCOV is a reasonable candidate. The final selection depends on maximum operating voltage, temporary overvoltage conditions, and transformer insulation coordination.

Whether a 1 MVA Transformer Requires a Larger Surge Arrester Than a 500 kVA Transformer

Not necessarily. Transformers of different kVA ratings operating on the same system may use the same arrester voltage rating. Their insulation characteristics, installation configurations, surge exposure, and protection requirements must still be evaluated individually.

Difference Between Arrester Rated Voltage and MCOV

Arrester rated voltage is the manufacturer's designated duty rating. MCOV is the maximum specified RMS power-frequency voltage that may be continuously applied across the arrester. Both must be checked against system operating and abnormal-voltage conditions.

Whether a 10 kV Arrester Can Protect a 13.2 kV Transformer

Yes, under appropriate system conditions. A 10 kV rated arrester may be suitable on an effectively grounded 13.2 kV system because it is connected phase-to-ground rather than directly across the 13.2 kV line-to-line voltage. Suitability must be verified through MCOV, TOV, and insulation coordination calculations.

Whether Ungrounded 13.2 kV Systems Require Higher Arrester Ratings

Often, yes. During a single-line-to-ground fault, healthy-phase-to-ground voltage can approach the system's line-to-line voltage. The arrester must withstand the expected voltage magnitude and duration without thermal instability or unacceptable degradation.

Recommended Location for Transformer Surge Arresters

Surge arresters should generally be installed as close as practical to the transformer terminals being protected, using appropriately designed phase and ground connections. Actual placement must satisfy equipment clearances, manufacturer requirements, grounding practices, and the applicable electrical code.

Final Engineering Recommendations

For a 1 MVA transformer operating on a nominal 13.2 kV distribution system, the correct surge arrester cannot be selected from transformer capacity and primary voltage alone.

For an effectively grounded system, a 10 kV rated arrester with an 8.4 kV MCOV may be suitable after verifying maximum continuous operating voltage, temporary overvoltage capability, and transformer insulation coordination.

For ungrounded, delta, or impedance-grounded systems, higher arrester ratings may be required. The engineer should establish the applicable phase-to-ground voltage stresses and fault durations before selecting an arrester.

Final specifications should be supported by manufacturer performance data, the applicable IEEE C62 standards, transformer insulation requirements, and the facility's electrical protection design.

For additional information on industrial transformer configurations and applications, visit Larson Electronics Industrial Transformers.

Larson Electronics offers industrial transformer solutions and after-sales support for low-voltage, medium-voltage, and high-voltage transformers and switchgear, including assistance with assembly, installation, inspection, and commissioning across North America, subject to project scope and availability.

For transformer specifications, electrical equipment requirements, or application assistance, contact Larson Electronics.

Larson Electronics Building Trust Since 1973.

Articles

How to Select the Correct Surge Arrester for a 1 MVA Transformer on a 13.2 kV Distribution System (10/22/2026)