Industrial Web Search Buyer's Guide Sector
Transformers, switchgear, and distribution equipment used to convert, control, and protect electrical power in industrial, commercial, and utility systems. This sector covers the equipment itself, as well as the ratings, enclosures, and installation requirements that determine whether it can be used where you intend to install it.
Overview
A working orientation to the sector before you compare specific equipment: how the equipment is classified, what actually constrains the choice, and the kinds of company you will end up talking to.
Transformers are categorized first by their cooling and insulation methods. There are liquid-immersed transformers, which are surrounded by an insulating liquid. These units tend to be more efficient and tolerate overloads better. In practice, the liquid decision is made by the building rather than the transformer: the fire code provisions that attach to each liquid type determine where a liquid-filled unit may be installed at all, which is why the installation location comes before the construction choice. On the other hand, dry-type transformers are cooled by air or a solid compound, completely avoiding liquid containment. They are commonly used indoors and can be constructed as either cast-resin or vacuum-pressure-impregnated units.
Switchgear serves the dual purpose of switching and protecting electrical systems, and the terminology used in this context is more precise than everyday language suggests. Metal-clad switchgear refers specifically to a defined medium-voltage construction that features drawout breakers and grounded metal barriers separating different compartments. Within this framework is the broader category of metal-enclosed switchgear. It's important to note that switchboards, panelboards, and motor control centers are unique products built to different standards, not merely smaller versions of switchgear.
As we look across both families of equipment, several ratings govern the selection process: voltage class, impulse withstand level, capacity, impedance, interrupting and short-circuit withstand ratings, enclosure type, and, when applicable, arc-resistant construction. Most of these ratings are determined by analyzing the system rather than simply selecting from a catalog.
Three types of companies operate in this sector, and understanding the differences is crucial because they determine who owns the interfaces. Original equipment manufacturers (OEMs) design and build transformers and switchgear assemblies, hold the type test evidence, the laboratory tests that prove a design meets its standard, confirming published ratings, and take responsibility for anything constructed to a project-specific rating. Distributors and manufacturers' representatives manage multiple lines, maintain stock of standard ratings, and provide a practical route for obtaining catalog units. Lastly, integrators, panel builders, and packagers assemble unit substations, control panels, and coordinated lineups, the multi-section assemblies of switchgear and related equipment engineered to work as one, assuming responsibility for the connections between components rather than the components themselves. This distinction is particularly important in this sector, as it is often at the interfaces among a transformer, its primary protection, and the downstream distribution where responsibility can become unclear.
Sourcing Considerations
The decisions below are the ones that most often cause regret later. The first three, the system study, the installation location, and who owns the interfaces, determine everything after them, including the answers to the other three.
Voltage class, impulse withstand level, impedance, interrupting rating, and short-circuit withstand all come from an analysis of the system in which this equipment will be installed. Ordering against a capacity figure alone means someone else assumes the rest, and an assumed fault current is the assumption that matters most.
Where the equipment goes determines what it can be. Indoor installation, proximity to occupied space, and the type of building construction all bear on whether a liquid-filled unit is permitted and what containment or separation it needs. Establish the location and governing code requirements before choosing between dry-type and liquid-immersed types.
Switchgear, switchboard, panelboard, and motor control center are four different products built to four different standards, and "metal-clad" is a narrower term than "metal-enclosed". Specifying loosely invites a supplier to quote something cheaper that meets the words you used. State the standard to which the assembly must be built.
A transformer experiences both no-load and load losses. The significance of each depends on the loading duration and intensity. Always request both efficiency figures and compare them against your actual usage. Generally, no-load loss is more critical for transformers that are continuously energized but lightly loaded, like building service transformers. In contrast, load loss predominates in heavily and continuously loaded transformers, such as those serving process equipment.
A consequence of how the equipment will be operated and where it sits: arc-resistant switchgear is tested to direct the effects of an internal fault away from personnel, and it is rated by the sides it protects. Whether you need it and which accessibility type follow from a safety assessment of how the equipment will be operated and maintained. It also affects footprint, so it cannot be added late.
Because the ratings come from a study and much of this equipment is built to order, lead time follows from the decisions above. Lead time can govern a construction schedule more than any other procurement item. Establish it at the concept stage, confirm what is in stock and what is made to order, and understand how a change to the specification affects the delivery date.
Glossary
The terms you will meet on an equipment schedule, a nameplate, or a supplier quote, in plain English.
Switchgear designed and tested so that the pressure and hot gases from an internal arcing fault are directed away from personnel. Tested under a published IEEE guide and rated by accessibility type, which defines which sides of the equipment are protected. It is specified from a safety assessment rather than added as an option.
The designation describing which sides of arc-resistant equipment are protected from the effects of an internal arcing fault. Type 1 protects the front only; Type 2 protects the front, back, and sides; suffixes extend protection to conditions such as open doors or inter-compartment faults. The type belongs on the specification, because equipment protected on fewer sides costs less and protects less.
The organization, office, or individual responsible for enforcing the applicable codes and approving equipment and installations in a given location, such as a state or municipal electrical inspector. Installation questions on this page, including whether a liquid-filled unit may go indoors, are ultimately answered by this authority, which is why confirming requirements locally appears throughout the standards section.
The insulation's rated ability to withstand a high voltage surge, such as a lightning strike or switching transient, expressed in kilovolts. It is specified alongside voltage class and is one of the electrical features that distinguishes otherwise similar transformer models.
The insulated assembly that carries a conductor through the transformer tank or enclosure wall. Bushing type, rating, and configuration affect connection method and physical arrangement, so they belong in the specification rather than being left to the manufacturer's default.
A dry-type transformer whose windings are encapsulated in cast epoxy resin. Tolerant of humidity, dust, and mechanical stress, and generally specified where the environment is harsh but a liquid-filled unit is unsuitable or not permitted.
The standardized letter code on a transformer nameplate describing how the unit is cooled, identifying the internal medium and how it circulates and whether cooling stages such as fans raise the rating. Two capacity figures on one nameplate usually correspond to two cooling stages, so the class is needed to read the rating at all.
The two ways transformer windings are connected. The configuration determines how the transformer handles unbalanced load, whether a neutral is available, and how it interacts with grounding and harmonics. Primary and secondary configurations are specified separately.
In United States federal efficiency regulation, a transformer within defined capacity ranges that steps voltage down for distribution to end users. The regulatory definition is narrower than common usage and excludes several transformer types, which matters because efficiency rules attach to the regulatory definition.
A design in which the circuit breaker can be racked out of its compartment for inspection, maintenance, or replacement without dismantling the assembly. Standard in metal-clad switchgear and a significant factor in what maintenance costs over the life of the equipment.
A transformer cooled by air or a solid insulating compound rather than liquid. Avoids the fire code and containment requirements associated with liquid-filled units, which is why it is common indoors, generally at lower efficiency and overload capability than a liquid-immersed equivalent.
The rating describing what an enclosure protects against, such as dust, water, or corrosion, defined in the United States by NEMA enclosure type numbers. The international IP system describes similar protection differently, and the two do not translate directly, so specify both the system and the number.
The temperature at which a liquid continues to burn after ignition. It is the defining property separating conventional mineral oil from liquids classified as less flammable, and it determines which installation rules in the electrical code apply to a liquid-filled transformer.
Expressed as a percentage, the voltage required to circulate rated current through the transformer with the secondary shorted. It governs how much fault current can flow downstream and how much the output voltage sags under load, so it is coordinated with the protective devices rather than accepted as supplied.
The insulation system's rated thermal capability and the temperature increase above ambient the transformer is designed to operate at. Together they determine loading capability and expected insulation life, and a unit specified at a lower rise generally runs cooler and lasts longer at the same load.
The maximum fault current a breaker or fuse can safely interrupt, stated in amperes or kiloamperes. It must exceed the available fault current at the point of installation, which makes a fault current study a prerequisite rather than an optional exercise.
A rating indicating a transformer's ability to serve nonlinear loads that draw harmonic currents, such as variable frequency drives and switching power supplies. A standard transformer feeding significant harmonic load will run hot, so K-factor is specified where such loads dominate.
The apparent power capacity of a transformer, in kilovolt-amperes. It is the headline size figure, but it means little without the voltage class, impedance, temperature rise, and cooling class alongside it.
An insulating liquid with a fire point at or above the threshold defined in the electrical code, including silicone, synthetic ester, and natural ester fluids. Its use permits installation options that conventional mineral oil does not, which is why it appears on indoor and building-adjacent projects.
A transformer whose core and coil assembly sits in an insulating liquid. Generally more efficient and more tolerant of overload than a dry-type equivalent, and subject to fire code requirements that depend on which liquid is used.
The two components of transformer loss. No-load loss occurs whenever the transformer is energized, regardless of load. Load loss varies with the current drawn. Because a transformer is typically energized continuously and loaded variably, the balance between them drives lifetime energy cost.
Medium-voltage switchgear built with drawout circuit breakers and grounded metal barriers isolating major components including the main bus and incoming and outgoing connections. Compartmentalization is what distinguishes it, and it is defined by an IEEE standard that covers a specific voltage range.
A broader category of switchgear housed in a grounded metal enclosure. All metal-clad switchgear is metal-enclosed, but not all metal-enclosed switchgear is metal-clad, because metal-clad requires specific compartmentalization and drawout construction. The two terms are frequently used interchangeably, but they should not be.
An assembly of motor starters, drives, and associated control in a common enclosure with a shared bus. Distinct from switchgear and switchboards in purpose and in the standard it is built to, and specified by unit types and structural arrangement.
A liquid-filled distribution transformer in a locked, tamper-resistant enclosure designed to sit on a concrete pad at grade without a fence or vault. Common for commercial and industrial services fed from underground distribution.
A distribution assembly of branch circuit breakers, mounted in a cabinet and accessible from the front. Smaller in capacity and interrupting rating than a switchboard, and a different product built to a different standard rather than simply a smaller version.
The fault current a transformer or assembly can survive without damage for a defined duration. Distinct from interrupting rating, which is about clearing a fault rather than surviving it, and confirmed by test rather than calculation.
A free-standing assembly of protective devices and instruments, typically front accessible and often with fixed rather than drawout devices. Built to a different standard than switchgear, generally at lower cost and with less maintenance flexibility.
An assembly combining switching and interrupting devices with associated control, metering, protective, and regulating devices in a coordinated structure. In North American usage, it implies a specific set of construction and testing requirements rather than describing any enclosure containing breakers.
A mechanism allowing the transformer turns ratio to be adjusted to compensate for supply voltage variation. De-energized tap changers are adjusted with the unit out of service. Load tap changers operate while energized, and cost considerably more. Which is required follows from how much the supply voltage varies.
A coordinated assembly combining an incoming section, a transformer, and a low-voltage distribution section as a single lineup. Purchasing it as a single unit places responsibility for the interfaces with a single supplier, which is the main argument for it over separately procured components.
A circuit breaker that interrupts current in a sealed vacuum interrupter. The interrupting technology most widely specified for new medium-voltage switchgear, valued for long contact life and low maintenance compared with older interrupting media.
A dry-type transformer construction in which windings are impregnated with resin under vacuum and pressure. An alternative to cast resin, generally at lower cost, with different tolerance for moisture and contamination.
Standards
What each standard governs and why a buyer should care. Which ones apply depends on the equipment type, the voltage class, where it is installed, and who has jurisdiction.
Transformer design, testing, and efficiency
Published by IEEE, this standard sets requirements for liquid-immersed distribution and power transformers with a highest voltage winding of 601 V or higher. It outlines performance and interchangeability criteria but excludes certain transformer types, such as instrument and arc furnace transformers. Ensure your unit is within scope.
Published by IEEE. The counterpart general requirements standard for dry-type distribution and power transformers, including units with solid cast or resin encapsulated windings. Where a specification cites the liquid-immersed standard for a dry-type unit, or the reverse, that is worth correcting before it reaches a supplier.
Published by IEEE. The test codes for liquid-immersed and dry-type transformers, respectively, defining how the tests behind published ratings are performed. They matter to a buyer because they make quoted performance from different manufacturers comparable, and they are referenced in federal efficiency test procedures.
Administered by the US Department of Energy under 10 CFR Part 431, Subpart K. Sets minimum efficiency for transformers meeting the regulatory definition of a distribution transformer, which is bounded by capacity ranges and excludes several transformer types. The regulatory definition is narrower than common usage, so confirm whether your unit is covered rather than assuming the rules do or do not apply.
Published by the National Electrical Manufacturers Association. NEMA publishes standards and test methods for transformers, including the test method for measuring energy consumption of distribution transformers that federal test procedures have drawn on. NEMA is the National Electrical Manufacturers Association whose standards sit alongside IEEE design standards in most equipment specifications.
Switchgear and assembly standards
Published by IEEE. The standard for metal-clad switchgear, covering medium-voltage assemblies containing drawout, electrically operated circuit breakers, compartmentalized with grounded metal barriers that isolate instrumentation, main bus, and incoming and outgoing connections. It covers rated maximum voltages from 4.76 kV to 48.3 kV, with defined continuous current ratings for the main bus. If a specification says "metal-clad," that's the standard it means.
Published by IEEE. The standard for metal-enclosed low-voltage power circuit breaker switchgear, covering assemblies at 1000 Vac and below and 3200 Vdc and below. The low-voltage counterpart to the metal-clad standard, and a different product from a switchboard despite superficial similarity.
Published by IEEE. The guide for testing metal-enclosed switchgear for internal arcing faults, applying to equipment that uses air as the primary insulating medium and is rated 38 kV ac or below. Its scope reaches beyond switchgear to metal-enclosed bus, medium-voltage controllers, motor control centers, and switchboards. Ratings are expressed by accessibility type, describing which sides of the equipment the protection covers, and the tests assume doors and covers are properly secured.
Published by UL Solutions. Separate standards govern switchboards, motor control centers, medium-voltage controllers, and metal-enclosed low-voltage power circuit breaker switchgear. They are referenced in the arc-fault testing guide and the electrical code, and they are the reason switchboards, motor control centers, and switchgear are distinct products rather than interchangeable terms.
Installation, enclosures, and safety
Published by the National Fire Protection Association. Governs transformer installation, including provisions for less-flammable liquid-insulated transformers, nonflammable fluid-insulated transformers, and vault construction. Less-flammable liquids are defined by a minimum fire point, and qualifying units may be installed indoors under stated conditions including building construction type and voltage limits. Code editions are revised on a cycle, and adoption varies by jurisdiction, so confirm the applicable edition with your authority.
Published by the National Electrical Manufacturers Association. Define what an enclosure protects against, covering dust, water, ice, and corrosive conditions, and whether the rating applies indoors, outdoors, or in hazardous locations. The international ingress protection system published by the International Electrotechnical Commission describes similar protection on a different basis, and the two do not translate exactly, so specify which system a number refers to.
Published by ASTM International and the International Electrotechnical Commission. Separate specifications cover mineral oil, silicone fluid, natural ester, and synthetic ester insulating liquids. Which liquid is used determines the fire point and therefore which installation provisions of the electrical code apply, so the liquid specification and the installation requirement are decided together.
Published by IEEE and the National Fire Protection Association. Cover calculation of incident energy from an arcing fault and safe work practices for personnel working on or near energized equipment. Relevant at purchase because equipment selection, protective device settings, and arc-resistant construction all affect the calculated hazard and therefore what personal protective equipment operating staff will need.
Frequently Asked Questions
Direct answers to the questions buyers ask most often when sourcing in this sector.
A liquid-immersed transformer has its core and coil assembly submerged in an insulating liquid that efficiently dissipates heat. A dry-type transformer is cooled by air or a solid insulating compound, built either with windings encapsulated in cast resin or impregnated with resin under vacuum and pressure. Liquid-immersed units are generally more efficient and more tolerant of overload, and are usual at higher capacities and voltages. Dry-type units avoid liquid containment entirely, which simplifies indoor installation and removes the fire code provisions that attach to liquid-filled equipment. The decision is usually based on the installation location and applicable code requirements rather than on electrical performance alone.
BIL stands for basic impulse insulation level, which describes an insulation system's rated ability to withstand a short, high-voltage surge, such as a lightning strike or a switching transient, expressed in kilovolts. It is specified alongside the voltage class because two transformers can share a voltage rating yet have different surge-withstand capability. It matters practically as well as technically: federal efficiency regulations treat differences in BIL rating as a differentiating electrical feature between otherwise similar transformer models, so BIL is part of what defines the unit rather than an accessory.
They are three different products built to three different standards, and the distinction is not one of size alone. Switchgear is an assembly of switching and interrupting devices, with associated control, metering, and protective devices, built and tested to specific construction requirements, typically with drawout devices and rear or full-compartment access. A switchboard is a free-standing assembly of protective devices, usually front-accessible and often with fixed rather than drawout devices, built to a different standard and generally at a lower cost. A panelboard is a cabinet of branch-circuit breakers accessible from the front, with smaller capacity and interrupting rating. Specifying loosely allows a supplier to quote the least expensive product that matches the word used, so state the standard the assembly must meet.
Metal-enclosed is the broad category: switchgear housed in a grounded metal enclosure. Metal-clad is a specific construction within it, defined by an IEEE standard covering medium-voltage assemblies that contain drawout, electrically operated circuit breakers and are compartmentalized so that grounded metal barriers isolate components including instrumentation, the main bus, and both incoming and outgoing connections. The standard addresses rated maximum voltages from 4.76 kV to 48.3 kV. All metal-clad switchgear is metal-enclosed; not all metal-enclosed switchgear is metal-clad. The terms are often used interchangeably in conversation, which is exactly why a specification should use them precisely.
That depends on a safety assessment of how the equipment will be operated and maintained, not on a general rule. Arc-resistant equipment is designed and tested to direct the pressure and hot gases produced by an internal arcing fault away from personnel. Testing follows a published IEEE guide that applies to air-insulated equipment rated 38 kV ac or below and covers switchgear, metal-enclosed bus, medium-voltage controllers, motor control centers, and switchboards. Ratings are expressed by accessibility type, which defines which sides of the equipment the protection covers, and the tested performance assumes doors and covers are properly secured. Because arc-resistant construction affects footprint and clearances, it should be decided early rather than added to an existing layout.
It is an insulating liquid with a fire point at or above the threshold set by the National Electrical Code, a requirement that conventional mineral oil does not meet. Silicone fluids, natural esters, and synthetic esters are the common types. The practical significance is installation: transformers filled with a qualifying liquid may be installed indoors under stated conditions, including limits on building construction type and voltage, and with requirements such as liquid confinement, while mineral-oil units indoors generally require a vault. If a project needs a liquid-filled transformer inside or adjacent to a building, this is usually why a less-flammable liquid appears in the specification. Confirm the applicable code edition with your authority having jurisdiction, since editions are revised and adoption varies.
In the United States, the Department of Energy sets minimum efficiency standards for transformers that meet the regulatory definition of a distribution transformer under 10 CFR Part 431. That definition is bounded by capacity ranges for liquid-immersed and dry-type units and explicitly excludes several transformer types, making it narrower than the term's everyday use. The test procedures behind it draw on IEEE test codes and a NEMA test method. The practical step for a buyer is to confirm whether the specific unit falls inside the regulatory definition, because equipment outside it is not covered by the standard even though it may look similar to equipment that is.
Impedance, expressed as a percentage, is the voltage needed to circulate rated current through the transformer with its secondary shorted. It has two practical consequences. A lower impedance allows more fault current to flow into the downstream system, thereby raising the interrupting rating required of downstream equipment. A higher impedance limits fault current but results in greater voltage drop under load. Impedance is therefore coordinated with the protective devices and the downstream equipment rather than accepted as whatever the manufacturer supplies, and it is one of the figures that should appear on the specification alongside capacity and voltage.
For a transformer: capacity, primary and secondary voltage, winding configuration, impedance, impulse withstand level, temperature rise, cooling class, insulation type, tap requirements, enclosure or tank arrangement, and the installation location, including whether it is indoors. For switchgear: system voltage, available fault current, required interrupting and short-circuit withstand ratings, bus continuous rating, the standard the assembly must be built to, enclosure type, whether arc-resistant construction is required and to what accessibility type, and the protective devices and metering to be included. Available fault current is the most often omitted item, and it determines whether the equipment is safe to install at all.
Buyer's Guides
In-depth guides covering the decisions above.
Buyer's Guide
Electrical requirements, impedance and losses, insulation and cooling, standards and testing, what actually drives lead time, and the package to send a supplier.
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