Does transformer kVA determine surge arrester voltage?

No. Surge arrester voltage selection is primarily determined by system voltage, grounding, maximum continuous operating voltage, temporary overvoltage conditions and insulation coordination.

Is an 8.4 kV MCOV arrester suitable for a 12.47 kV system?

An 8.4 kV MCOV arrester with a 10 kV duty-cycle rating is commonly used on effectively grounded systems in the 12.47 kV voltage range. Suitability must be confirmed using the actual maximum system voltage, grounding configuration, TOV conditions and manufacturer data.

Do ungrounded systems require different surge arresters?

They can. Ungrounded and impedance-grounded systems may develop substantially higher phase-to-ground voltage during a ground fault, requiring evaluation of a higher arrester MCOV and its temporary overvoltage capability.

Does transformer BIL affect surge arrester selection?

Yes. Arrester discharge-voltage characteristics should be coordinated with transformer BIL to provide an appropriate protective margin between the arrester protective level and the transformer's impulse withstand capability.

What Size Surge Arrester Is Needed for a 150 kVA, 12.47 kV Transformer? (8/24/2026)

Q: What size surge arrester is needed for a 150 kVA 12.47 kV transformer?

Transformer kVA does not directly determine surge arrester voltage. On an effectively grounded 12.47 kV system, a 10 kV rated arrester with an 8.4 kV MCOV is a common candidate, but maximum system voltage, grounding, temporary overvoltage capability, transformer BIL and manufacturer data must be verified.

Q: What is the phase-to-ground voltage on a 12.47 kV system?

For a three-phase wye system, 12.47 kV divided by the square root of three is approximately 7.2 kV phase-to-ground.

For a 150 kVA transformer on a 12.47 kV system, surge arrester selection depends on system voltage, grounding, MCOV and overvoltage conditions. An effectively grounded system commonly uses a 10 kV arrester with an 8.4 kV MCOV, while other grounding configurations may require higher ratings.

By LarsonElectronics.com, August 24, 2026

For a 150 kVA transformer connected to a 12.47 kV medium-voltage system, surge arrester selection is based primarily on the system voltage, grounding method, maximum continuous operating voltage (MCOV), temporary overvoltage capability and the insulation level of the protected equipment. The transformer's 150 kVA rating does not directly determine the arrester voltage rating.

On a 12.47 kV effectively grounded wye system, the nominal phase-to-ground voltage is approximately 7.2 kV. A 10 kV rated metal-oxide surge arrester with an 8.4 kV MCOV is a commonly encountered selection for systems in this voltage range, provided the arrester's MCOV and temporary overvoltage capability are suitable for the actual system. Ungrounded, impedance-grounded and some delta systems can subject an arrester to substantially higher phase-to-ground voltage during a ground fault and require separate evaluation.

Engineers specifying industrial transformers should coordinate the surge arrester with the transformer insulation level, system grounding, expected overvoltages and applicable IEEE or utility requirements rather than selecting an arrester solely from nominal system voltage.

A 12.47 kV System Has Approximately 7.2 kV Nominal Phase-to-Ground Voltage

For a three-phase, four-wire wye system, nominal phase-to-ground voltage can be calculated by dividing the phase-to-phase voltage by the square root of three:

V_LG = V_LL / v3

For a 12.47 kV system:

12.47 kV / 1.732 ˜ 7.20 kV

This 7.2 kV value is an important starting point for an effectively grounded system because a phase-to-ground arrester is continuously exposed to approximately the system's phase-to-ground voltage during normal operation.

It is important, however, not to simply choose an arrester whose MCOV equals the calculated nominal voltage. The engineer must account for maximum system operating voltage, grounding conditions and temporary overvoltages.

A 10 kV Arrester With 8.4 kV MCOV Is a Common Starting Point

For an effectively grounded 12.47 kV distribution system, a metal-oxide surge arrester with approximately the following ratings is commonly encountered:

Parameter	Typical Value or Consideration
Transformer rating	150 kVA
Nominal system voltage	12.47 kV line-to-line
Nominal phase-to-ground voltage	Approximately 7.2 kV
Common arrester duty-cycle rating	10 kV
Common arrester MCOV	8.4 kV
Primary selection factors	Grounding, maximum system voltage, TOV, transformer insulation level and protective margin

This should be treated as a common application example, not a universal specification. The actual arrester must be checked against the electrical system and the manufacturer's published characteristics.

Transformer kVA Does Not Determine Surge Arrester Voltage Rating

A common misconception is that a larger transformer requires a higher-kV surge arrester. The kVA rating and arrester voltage rating describe different characteristics.

The transformer's kVA rating indicates its apparent-power capacity. The arrester voltage rating and MCOV are selected according to the electrical system on which the arrester operates.

For example, a 75 kVA transformer and a 150 kVA transformer connected to the same 12.47 kV effectively grounded primary system could use the same voltage class of arrester if their system grounding, insulation coordination and other application requirements are equivalent.

Increasing transformer capacity from 150 kVA to 500 kVA therefore does not, by itself, require increasing the arrester from 10 kV to 15 kV.

MCOV Is a Critical Surge Arrester Selection Parameter

Maximum continuous operating voltage is the maximum specified power-frequency RMS voltage that may be continuously applied across a metal-oxide surge arrester under its defined operating conditions.

The MCOV must be high enough that the arrester can remain connected continuously without being overstressed by normal system voltage. At the same time, unnecessarily increasing MCOV can increase the arrester's protective level, potentially reducing the protective margin available for the transformer.

Surge arrester selection is therefore an insulation-coordination exercise. The goal is not simply to select the highest voltage rating that will survive the system. The goal is to select an arrester that can withstand expected system conditions while providing an appropriately low protective level for the equipment being protected.

System Grounding Can Change the Required Arrester Rating

Grounding configuration is one of the most important factors in medium-voltage arrester selection.

On an effectively grounded wye system, the voltage from an unfaulted phase to ground remains comparatively controlled during a single line-to-ground fault. This generally permits lower-MCOV arresters than would be appropriate on systems where the neutral is not effectively grounded.

On an ungrounded or impedance-grounded system, a line-to-ground fault can cause the voltage of the unfaulted phases relative to ground to rise significantly. Depending on the system, an arrester connected phase-to-ground may therefore experience substantially more voltage than it does during normal operation.

For that reason, the statement that every 12.47 kV system should use a 10 kV arrester with an 8.4 kV MCOV is not technically correct.

Ungrounded and Impedance-Grounded Systems Require Separate Evaluation

For ungrounded and impedance-grounded systems, engineers must evaluate the maximum voltage that can appear across the arrester during ground-fault and temporary-overvoltage conditions.

Manufacturer application guidance for metal-oxide arresters commonly distinguishes effectively grounded systems from ungrounded or impedance-grounded systems. Depending on the grounding arrangement and expected fault duration, a higher-MCOV arrester may be necessary.

A 15 kV arrester should not simply be selected because the system is ungrounded, however. The appropriate duty-cycle rating and MCOV should be determined from the actual system maximum voltage, grounding configuration, temporary overvoltage magnitude and duration, and the arrester manufacturer's TOV capability.

Temporary Overvoltage Capability Must Be Checked

Temporary overvoltage, commonly abbreviated TOV, is a power-frequency overvoltage that lasts longer than a lightning or switching impulse. Ground faults, resonance, load rejection and other system conditions can produce temporary overvoltages.

Metal-oxide arresters continuously conduct a very small current at normal operating voltage, but their conduction increases as applied voltage rises. Excessive voltage for too long can thermally stress the arrester.

Engineers should therefore compare the expected temporary-overvoltage magnitude and duration with the manufacturer's published TOV curve. This is particularly important for ungrounded, resistance-grounded and other systems where abnormal phase-to-ground voltage can persist.

IEEE C62.11 Establishes Requirements for Metal-Oxide Surge Arresters

IEEE C62.11 is the principal U.S. standard covering metal-oxide surge arresters for AC power circuits above 1 kV. The standard addresses arresters designed to limit transient overvoltages by conducting surge current and subsequently limiting power-frequency follow current.

IEEE C62.11 includes performance and testing requirements associated with metal-oxide surge arresters. For engineering applications, arrester selection should also account for insulation coordination, temporary overvoltage capability, discharge voltage and the characteristics of the protected equipment.

For Canadian industrial applications, engineers should also verify applicable CSA requirements, utility standards, provincial electrical requirements and equipment certification requirements. Projects involving equipment built to IEC practices may also encounter IEC 60099-4 for gapless metal-oxide surge arresters.

Transformer BIL Must Be Coordinated With the Arrester Protective Level

The arrester must do more than survive the system voltage. It must limit the surge voltage reaching the transformer to an acceptable level below the transformer's impulse withstand capability.

Transformer basic impulse insulation level, or BIL, indicates the transformer's ability to withstand specified impulse voltage conditions. The arrester's discharge-voltage characteristics establish the voltage that can appear across the protected equipment while the arrester is conducting surge current.

Proper insulation coordination provides sufficient protective margin between the arrester's protective level and the transformer's insulation withstand capability.

Engineers should verify the transformer's actual BIL from its nameplate, drawings or manufacturer data rather than assuming a BIL solely from nominal system voltage.

Arrester Location and Lead Length Affect Transformer Protection

Even a correctly rated arrester can provide inadequate protection if it is installed too far from the transformer or connected with unnecessarily long leads.

Surge currents have very fast rates of change. Lead inductance can create additional voltage during a surge, increasing the voltage that appears at the transformer terminals.

For transformer protection, arresters are generally installed as close as practical to the equipment being protected, with short and direct phase and ground connections. Excessive bends, loops and unnecessary conductor length should be avoided where permitted by the equipment design and installation requirements.

This is especially important on outdoor pad-mounted, pole-mounted and substation transformer installations exposed to lightning-generated surges.

NEC Article 280 Applies to Surge Arresters Over 1,000 Volts

For U.S. installations, NEC Article 280 addresses surge arresters on circuits operating over 1,000 volts. Applicable requirements include installation, connection and grounding provisions for medium-voltage surge arresters.

The NEC establishes installation requirements, but it should not be treated as the sole engineering method for selecting arrester MCOV, duty-cycle rating or protective level. Those decisions require coordination with the electrical system, equipment ratings, IEEE guidance, utility requirements and manufacturer application data.

The edition of the NEC adopted by the authority having jurisdiction should always be confirmed for the specific project.

A Practical 12.47 kV Transformer Example Starts With System Grounding

Consider a three-phase 150 kVA transformer with a 12,470-volt primary connected to an effectively grounded utility distribution system.

The nominal phase-to-ground voltage is:

12.47 kV / v3 ˜ 7.2 kV

If the system is confirmed to be effectively grounded and its maximum continuous phase-to-ground voltage remains within the arrester's limits, a 10 kV duty-cycle arrester with an 8.4 kV MCOV is a common candidate.

The engineer should then verify:

Maximum system operating voltage
System grounding configuration
Expected temporary overvoltage magnitude and duration
Arrester MCOV
Arrester duty-cycle rating
Transformer BIL
Arrester discharge-voltage characteristics
Required protective margin
Arrester class and energy capability
Installation location and lead length
Utility and authority having jurisdiction requirements

If those conditions are satisfied, the 8.4 kV MCOV arrester can be an appropriate selection. If the system is ungrounded, impedance-grounded or subject to elevated temporary overvoltages, the arrester rating must be reevaluated.

Arrester Selection Should Begin With Five System Values

For an existing transformer or a new medium-voltage installation, an engineer or industrial buyer should gather at least five pieces of information before specifying the arrester:

Nominal system voltage — such as 12.47 kV.
Maximum system voltage — not merely the nominal nameplate voltage.
Grounding configuration — effectively grounded, resistance grounded, impedance grounded or ungrounded.
Transformer insulation level — including the applicable BIL.
Expected temporary overvoltages — including magnitude and duration during abnormal system conditions.

These values provide a much stronger basis for arrester selection than transformer kVA alone.

Frequently Asked Questions

A 150 kVA transformer does not require a specific arrester voltage because of its kVA rating

Surge arrester voltage selection is primarily determined by system voltage, grounding, maximum continuous voltage, temporary overvoltages and insulation coordination. Transformer kVA does not directly establish the arrester voltage rating.

A 12.47 kV system has approximately 7.2 kV phase-to-ground voltage

On a three-phase wye system, 12.47 kV divided by the square root of three is approximately 7.2 kV. This is the nominal phase-to-ground voltage and is an important starting point for selecting phase-to-ground arresters on effectively grounded systems.

An 8.4 kV MCOV arrester is commonly used in this voltage range

On an effectively grounded 12.47 kV system, a 10 kV rated arrester with an 8.4 kV MCOV is a commonly encountered selection. The final choice must be verified against maximum system voltage, grounding, TOV capability, transformer BIL and manufacturer data.

Ungrounded 12.47 kV systems can require higher-MCOV arresters

Yes. During a ground fault, the unfaulted phases of an ungrounded or certain impedance-grounded systems can experience substantially increased voltage to ground. Arrester MCOV and TOV capability must be selected for those conditions rather than using the effectively grounded-system selection automatically.

Transformer BIL affects surge arrester coordination

Yes. The arrester's protective characteristics must be coordinated with the transformer's impulse insulation capability. The objective is to limit the surge reaching the transformer while maintaining an appropriate protective margin below the transformer's withstand level.

A Surge Arrester and Transformer Protection Content Cluster Builds Deeper Technical Coverage

This topic connects transformer selection, insulation coordination, grounding and medium-voltage protection. Supporting technical articles can build a complete transformer surge-protection resource around the subject:

Surge Arrester MCOV Explained
10 kV vs. 15 kV Surge Arresters
How System Grounding Affects Surge Arrester Selection
Surge Arrester Duty-Cycle Rating vs. MCOV
Transformer BIL and Surge Protection Explained
How to Coordinate Surge Arresters With Transformer BIL
Temporary Overvoltage and Surge Arrester Selection
Surge Arresters for 12.47 kV Distribution Systems
Surge Arresters for 13.2 kV and 13.8 kV Systems
Surge Arresters for Pad-Mounted Transformers
Surge Arresters for Pole-Mounted Transformers
Surge Arrester Placement and Lead Length
Effectively Grounded vs. Ungrounded Medium-Voltage Systems
Lightning Protection for Medium-Voltage Transformers
IEEE C62.11 Surge Arrester Requirements Explained
Distribution-Class vs. Station-Class Surge Arresters
How to Read a Surge Arrester Nameplate
How to Specify Surge Arresters on a Transformer RFQ

For assistance specifying industrial transformers and coordinating transformer voltage, insulation and application requirements, contact Larson Electronics.

Larson Electronics Building Trust Since 1973.