Does transformer kVA determine surge arrester voltage?

No. Surge arrester voltage selection is primarily determined by system voltage, grounding configuration, temporary overvoltage, and insulation coordination rather than transformer kVA.

What is the difference between arrester rated voltage and MCOV?

Arrester rated voltage and maximum continuous operating voltage are separate electrical characteristics. For example, a commonly available 10 kV rated metal-oxide arrester has an MCOV of 8.4 kV.

Should surge arresters be installed close to transformer bushings?

Yes. Short, direct connections near the protected transformer terminals generally improve surge protection by reducing additional voltage associated with lead inductance and surge propagation.

What Size kV Surge Arrester Is Required for a 45 kVA Transformer on a 12.47 kV System (10/16/2026)

Q: Can a 15 kV surge arrester be used on a 12.47 kV system?

A 15 kV rated arrester may be suitable for some 12.47 kV applications. Its MCOV, temporary overvoltage capability, and protective characteristics must be checked against the system and transformer insulation requirements.

Q: Do ungrounded 12.47 kV systems require higher-rated surge arresters?

Ungrounded systems frequently require higher arrester voltage ratings because unfaulted phases can approach the full line-to-line voltage relative to ground during a ground fault. The appropriate rating depends on the magnitude and duration of the overvoltage.

For a 45 kVA transformer on a 12.47 kV effectively grounded system, a 10 kV surge arrester with an 8.4 kV MCOV is a common starting point. Final selection depends on system grounding, overvoltage conditions, and insulation coordination in accordance with IEEE C62.11, IEEE C62.22, and utility requirements.

By LarsonElectronics.com, October 16, 2026

Selecting the correct surge arrester for a 45 kVA transformer connected to a 12,470-volt (12.47 kV) distribution system requires more than matching the arrester's voltage rating to the transformer's primary voltage. Engineers must evaluate the system grounding configuration, maximum operating voltage, temporary overvoltage conditions, and the transformer's insulation withstand capability.

For a 45 kVA transformer on a 12.47 kV effectively grounded wye distribution system, a 10 kV rated metal-oxide surge arrester with an 8.4 kV maximum continuous operating voltage (MCOV) is a common preliminary selection. However, this rating is not universally appropriate. Systems that are ungrounded, impedance-grounded, or subject to extended ground-fault overvoltages may require a higher-rated arrester.

The following engineering guidance explains how to evaluate surge arrester ratings for medium-voltage transformers used in U.S. and Canadian industrial and utility applications.

What Surge Arrester Rating Is Commonly Used on a 12.47 kV Transformer

For a nominal 12.47 kV, three-phase, four-wire, effectively grounded wye system, the normal line-to-ground voltage is approximately 7.2 kV.

Because distribution-class surge arresters are commonly connected between the energized conductor and ground, the arrester's MCOV must accommodate the maximum continuous voltage at its terminals.

Nominal system voltage: 12.47 kV line-to-line
Nominal line-to-ground voltage: 7.2 kV
Common preliminary arrester rated voltage: 10 kV
Common corresponding MCOV: 8.4 kV
Applicable arrester technology: Gapless metal-oxide distribution-class surge arrester

A 10 kV arrester with an 8.4 kV MCOV provides continuous operating voltage margin above the nominal 7.2 kV line-to-ground voltage. Its suitability must still be verified against the highest expected system voltage and temporary overvoltage conditions.

The 10 kV arrester rating should not be confused with its MCOV or protective voltage. These are different electrical characteristics with different functions.

How System Voltage Determines Surge Arrester Selection

The first step is identifying whether the specified 12,470 volts represents the three-phase line-to-line system voltage or the actual voltage applied to the transformer primary winding.

For a balanced, grounded-wye system:

Line-to-ground voltage = Line-to-line voltage / v3

Using a nominal system voltage of 12,470 volts:

12,470 / 1.732 = approximately 7,200 volts

Therefore, the nominal phase-to-ground voltage is approximately 7.2 kV.

This calculation is relevant to a conventional phase-to-ground arrester installation. It does not establish the appropriate arrester rating for every grounding arrangement or connection method.

How the Transformer Primary Connection Affects the Calculation

A 45 kVA transformer may have a single-phase primary connected between a phase conductor and neutral, a single-phase primary connected between two phase conductors, or a three-phase primary with a wye or delta winding configuration.

These arrangements must be distinguished from the grounding configuration of the supplying distribution system.

For example, a delta-connected transformer supplied from an effectively grounded 12.47 kV wye distribution system does not automatically require the same arrester rating as an ungrounded delta distribution system.

Engineers must establish the voltage between each arrester terminal and ground, including abnormal operating conditions, rather than selecting the arrester solely from the transformer winding connection.

How Maximum Continuous Operating Voltage Affects Arrester Selection

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

MCOV is particularly important because the arrester remains connected to the energized electrical system throughout normal operation.

For a 12.47 kV effectively grounded system, an 8.4 kV MCOV is a commonly applied value, provided the actual maximum continuous line-to-ground voltage remains within the arrester manufacturer's rating.

As an illustration, if the distribution system operates at 105% of nominal voltage:

7.2 kV × 1.05 = approximately 7.56 kV

This remains below an 8.4 kV MCOV. However, the 105% condition is only an example. Actual maximum system voltage must be established from utility operating limits, voltage regulation practices, and system design documentation.

An arrester should not be selected simply because its MCOV exceeds nominal voltage. The engineer must also evaluate temporary overvoltages, protective characteristics, and insulation coordination.

How Grounding Configuration Changes the Required Arrester Rating

Grounding configuration is one of the most important factors in selecting medium-voltage surge arresters.

During a single-line-to-ground fault, the voltage on unfaulted phases can rise relative to ground. The magnitude and duration of this voltage depend on the system grounding method and protective-device clearing time.

System Configuration	Voltage Consideration	Selection Guidance
Effectively grounded wye	Approximately 7.2 kV nominal line-to-ground	10 kV rated arrester with 8.4 kV MCOV is a common starting point
Impedance-grounded system	Ground-fault overvoltage depends on grounding impedance and fault duration	Evaluate temporary overvoltage before choosing MCOV and rated voltage
Ungrounded system	Unfaulted phase-to-ground voltage may approach line-to-line voltage during a ground fault	Higher MCOV may be necessary; verify fault duration and system operating conditions
Delta distribution system	Depends on whether the system is ungrounded, corner-grounded, or otherwise grounded	Determine actual arrester terminal voltage and grounding arrangement

For an ungrounded 12.47 kV system, a sustained ground fault can expose arresters on unfaulted phases to approximately 12.47 kV phase-to-ground at nominal system voltage. In such applications, an arrester with an MCOV above that sustained voltage may be required.

A nominal 15 kV arrester rating should not be assumed adequate without checking its actual MCOV. For example, a commonly available 15 kV rated arrester may have a 12.7 kV MCOV, leaving limited margin for operation above nominal system voltage.

Higher-rated devices may be appropriate, but the final selection must be established through manufacturer data and an engineering review.

Why Transformer kVA Does Not Directly Determine Surge Arrester Voltage

The 45 kVA transformer rating describes its apparent power capacity. It does not directly establish the surge arrester's required rated voltage or MCOV.

A 45 kVA transformer and a 500 kVA transformer supplied from the same distribution system may use surge arresters with identical voltage ratings when their grounding, maximum operating voltage, insulation requirements, and exposure conditions are equivalent.

Transformer capacity can influence other aspects of the installation, including conductor sizing, protective devices, and available fault current. However, surge arrester voltage selection is primarily a system-voltage and insulation-coordination decision.

The engineer must also distinguish between an arrester's nominal discharge current classification and the system's available short-circuit current. These are different characteristics and cannot be substituted for one another.

How Transformer Basic Insulation Level Affects Surge Protection

Basic lightning impulse insulation level, commonly called BIL, identifies the standardized lightning impulse withstand level associated with electrical equipment insulation.

Surge arresters protect transformer insulation by limiting transient overvoltages. The selected arrester's protective characteristics must provide an appropriate margin below the transformer's insulation withstand level after accounting for installation effects.

For equipment in the 15 kV insulation class, a 95 kV BIL is a common transformer insulation level, but it must be confirmed from the actual transformer nameplate and applicable design standard.

Important characteristics include:

Lightning impulse protective level
Steep-current impulse protective level
Switching surge protective characteristics where applicable
Transformer BIL and other applicable insulation withstand ratings
Lead length and conductor routing
Grounding and bonding impedance
Expected surge exposure and arrester energy capability

The arrester's rated voltage is not its discharge or clamping voltage. Engineers must consult the manufacturer's published protective characteristics to determine the actual voltage developed across the arrester during specified surge-current conditions.

How Arrester Installation Distance Affects Transformer Protection

A surge arrester installed near the transformer high-voltage terminals generally provides better protection than one installed farther away, all other conditions being equal.

Long arrester leads and grounding conductors can introduce additional inductive voltage during rapidly changing surge currents. Traveling-wave effects can also influence the voltage reaching the transformer terminals.

For these reasons, arrester connections should be kept as short and direct as practical, consistent with manufacturer installation instructions, required electrical clearances, and applicable utility standards.

Effective bonding to the transformer tank and grounding system is an important part of the overall protection arrangement.

How Temporary Overvoltage Capability Is Evaluated

Temporary overvoltage, or TOV, refers to a power-frequency overvoltage that persists for a duration associated with abnormal system operation.

Potential causes include:

Single-line-to-ground faults
Neutral displacement
Load rejection
System resonance and ferroresonance
Abnormal switching configurations
Voltage regulation or grounding system abnormalities

Arrester manufacturers publish temporary overvoltage withstand characteristics that relate permissible voltage magnitude to duration under specified conditions.

Engineers should compare the expected system TOV magnitude and duration against the selected arrester's published TOV capability, accounting for relevant operating conditions and manufacturer limitations.

For systems where ground faults can remain energized for extended periods, TOV performance may govern the arrester selection more strongly than nominal operating voltage.

Which IEEE Standards Apply to Medium-Voltage Surge Arresters

IEEE standards provide the principal technical framework for surge arrester selection and application in U.S. medium-voltage distribution systems.

IEEE C62.11 Covers Metal-Oxide Surge Arrester Requirements

IEEE C62.11 addresses metal-oxide surge arresters for alternating-current power circuits above 1 kV.

Its requirements and test methods cover characteristics such as voltage ratings, protective performance, and operating capability.

When specifying an arrester, engineers should verify that the product conforms to the applicable edition of IEEE C62.11 and is suitable for the intended installation environment.

IEEE C62.22 Provides Surge Arrester Application Guidance

IEEE C62.22 provides guidance for applying metal-oxide surge arresters on AC power systems.

Its application principles include consideration of continuous operating voltage, temporary overvoltage, surge exposure, and insulation coordination.

These considerations are especially relevant when selecting between different arrester voltage ratings for grounded and ungrounded medium-voltage systems.

IEEE C57 Standards Address Transformer Requirements

Relevant standards in the IEEE C57 family address transformer ratings, insulation requirements, testing, and application considerations.

For example, IEEE C57.12.00 covers general requirements for liquid-immersed distribution, power, and regulating transformers, while IEEE C57.12.90 addresses associated test methods.

Applicable transformer standards depend on equipment construction and type. Engineers should verify the specific transformer standard rather than assuming all 45 kVA transformers share identical insulation requirements.

How NEC Requirements Apply to Transformer Surge Protection

In the United States, the National Electrical Code (NFPA 70) establishes installation requirements for electrical systems within its scope.

Relevant provisions may include:

NEC Article 242: Overvoltage protection, including surge arrester installation requirements
NEC Article 250: Grounding and bonding requirements
NEC Article 450: Transformer installation and protection requirements
NEC Article 490: Equipment operating at more than 1,000 volts, nominal

Article 242 is particularly relevant to surge arrester applications, while grounding and bonding provisions affect installation safety and the effectiveness of the protection arrangement.

The applicable NEC edition, local amendments, equipment listing requirements, and authority having jurisdiction must be considered.

Utility-controlled distribution installations may fall outside portions of the NEC scope and instead be governed by utility standards and other applicable electrical safety requirements.

How Surge Arrester Selection Differs in Canadian Installations

Canadian medium-voltage transformer installations require consideration of the applicable Canadian Electrical Code, CSA C22.1, provincial or territorial requirements, and local utility standards.

CSA C22.2 No. 269.2 addresses surge arresters in the Canadian standards framework.

For projects spanning the United States and Canada, engineers should confirm the arrester's applicable certifications, ratings, and compliance documentation rather than assuming that compliance with one national standard automatically establishes compliance with every project requirement.

Voltage selection principles remain similar, but the installation rules, certification requirements, and utility specifications may differ.

Example of Selecting a Surge Arrester for a 45 kVA Transformer

Consider a 45 kVA transformer installed on a nominal 12.47 kV, three-phase, four-wire, effectively grounded wye distribution system.

The project specifications identify the following conditions:

Transformer capacity: 45 kVA
Distribution system voltage: 12,470 V line-to-line
Nominal phase-to-ground voltage: 7,200 V
Grounding configuration: Effectively grounded wye
Arrester connection: Phase-to-ground
Installation: Outdoor distribution transformer

Step 1: Establish the nominal voltage across the arrester.

For the specified grounded-wye system, the nominal phase-to-ground voltage is approximately 7.2 kV.

Step 2: Establish the maximum continuous operating voltage.

Confirm the highest expected sustained phase-to-ground voltage using utility operating data and applicable system voltage limits.

Step 3: Identify a preliminary arrester rating.

A 10 kV rated distribution-class metal-oxide arrester with an 8.4 kV MCOV is a common preliminary candidate.

Step 4: Evaluate temporary overvoltage conditions.

Confirm that the arrester can withstand expected abnormal power-frequency voltages for the applicable protective-device clearing times.

Step 5: Verify insulation coordination.

Compare the manufacturer's published protective characteristics with the transformer BIL and account for the effects of arrester lead length and installation arrangement.

Step 6: Confirm equipment and installation requirements.

Review applicable IEEE standards, utility specifications, environmental conditions, and electrical installation codes.

After these checks, the engineer can determine whether the preliminary 10 kV rated arrester is suitable or whether another rating or arrester design is required.

Common Surge Arrester Selection Errors

Choosing an Arrester Based Only on Transformer kVA

Transformer kVA is not the primary determinant of surge arrester rated voltage. The system voltage, grounding configuration, and insulation coordination requirements are more important.

Confusing Arrester Rated Voltage with MCOV

A 10 kV rated arrester does not necessarily have a 10 kV MCOV. The rated voltage and maximum continuous operating voltage must be evaluated separately.

Assuming Every 12.47 kV System Is Effectively Grounded

Industrial distribution systems may use ungrounded or impedance-grounded configurations. Applying a grounded-wye arrester selection without verifying the actual system can lead to inappropriate protection.

Ignoring Temporary Overvoltage Duration

An arrester may tolerate an elevated voltage briefly but not continuously. Fault-clearing times and TOV withstand characteristics must be evaluated together.

Installing Arresters with Excessive Lead Length

Long or poorly routed connections can increase the voltage experienced by transformer insulation during fast transient events. Short, direct connections and effective bonding improve protective performance.

Frequently Asked Questions About 12.47 kV Transformer Surge Arresters

What Size Surge Arrester Is Commonly Used for a 45 kVA Transformer on a 12.47 kV System

A 10 kV rated metal-oxide surge arrester with an 8.4 kV MCOV is a common starting point for a 45 kVA transformer supplied from a 12.47 kV effectively grounded wye system. Final selection requires verification of maximum operating voltage, temporary overvoltage, and transformer insulation coordination.

Does a 45 kVA Transformer Require a Different Arrester Than a 75 kVA Transformer

Not necessarily. Transformers of different kVA ratings may use the same arrester voltage rating when connected to systems with equivalent voltage, grounding, and insulation coordination requirements.

Can a 15 kV Surge Arrester Be Used on a 12.47 kV System

A 15 kV rated arrester may be appropriate for some 12.47 kV applications, but its suitability depends on MCOV, grounding, TOV, and protective performance. A higher arrester rating can also produce a higher protective voltage, potentially reducing the insulation protection margin.

What Is the Difference Between a 10 kV Arrester and an 8.4 kV MCOV

The 10 kV value identifies the arrester's rated voltage, while 8.4 kV identifies its maximum continuous operating voltage. Neither value is the arrester's lightning impulse protective level.

Do Ungrounded 12.47 kV Systems Need Higher-Rated Surge Arresters

They frequently do. During a ground fault, unfaulted phases can approach the full line-to-line voltage relative to ground. The selected arrester must withstand the resulting voltage magnitude and duration under the expected system conditions.

Should Surge Arresters Be Installed Near the Transformer Bushings

Yes. Installing surge arresters close to the protected transformer terminals, with short and direct electrical connections, generally improves protection by reducing additional voltage associated with connection inductance and surge propagation.

Engineering Recommendations for 12.47 kV Transformer Surge Protection

For a 45 kVA transformer connected to a nominal 12.47 kV distribution system, a 10 kV rated arrester with an 8.4 kV MCOV is a reasonable preliminary selection when the supplying system is effectively grounded and operating conditions support that rating.

However, final selection should always be based on the actual electrical system. Engineers should verify the system grounding arrangement, maximum continuous voltage, TOV exposure, transformer BIL, arrester protective characteristics, and installation configuration.

For ungrounded or impedance-grounded systems, higher arrester voltage ratings may be necessary to accommodate abnormal phase-to-ground voltages. Those ratings must be evaluated against the manufacturer's published performance data and the applicable IEEE guidance.

Properly coordinated surge protection helps reduce the risk of transformer insulation failure, equipment damage, and unplanned industrial downtime.

Industrial Transformer Selection and Technical Support

Industrial and utility transformer projects may require coordinated evaluation of primary voltage, grounding arrangements, insulation ratings, protective devices, and surge protection requirements.

Explore industrial transformers from Larson Electronics for distribution and industrial power applications.

Larson Electronics also provides after-sales support for low-voltage (LV), medium-voltage (MV), and high-voltage (HV) transformers and switchgear, including assembly, installation, inspection, and commissioning support across North America.

For assistance with transformer specifications, equipment selection, or project requirements, contact Larson Electronics.

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