What are the primary bushing configurations on a pad-mounted transformer?

The most common primary bushing arrangements are radial feed and loop feed. A three-phase radial-feed transformer commonly provides one primary interface per phase, while a three-phase loop-feed transformer commonly provides two interfaces per phase so the primary circuit can enter and continue from the transformer location.

What is a radial-feed pad-mounted transformer?

A radial-feed pad-mounted transformer is normally supplied from one primary direction and is located at a terminating point on that portion of the distribution circuit. A three-phase radial unit commonly has three primary bushing interfaces.

What is a loop-feed pad-mounted transformer?

A loop-feed pad-mounted transformer normally provides two primary connection points per phase, allowing the primary circuit to enter and continue from the transformer location. A three-phase loop-feed transformer commonly has six primary interfaces.

What is the difference between loadbreak and deadbreak transformer bushings?

Loadbreak separable connector systems are designed for specified load-making and load-breaking operations when properly rated and operated. Deadbreak connector systems are intended to be connected or disconnected with the circuit de-energized.

Does a loop-feed transformer operate in a closed electrical loop?

Not necessarily. Many underground distribution loops are operated with a normally open point, creating radial electrical operation while preserving an alternate path for sectionalizing and service restoration.

Which IEEE standards apply to pad-mounted transformer primary connections?

IEEE C57.12.34 addresses connector, bushing, and terminal arrangements for three-phase pad-mounted distribution transformers within its scope. IEEE C57.12.38 addresses single-phase pad-mounted transformers within its scope, while IEEE 386 establishes requirements for shielded separable insulated connector systems.

How is the correct pad-mounted transformer bushing configuration selected?

Selection should consider radial or loop distribution topology, system voltage, continuous current, loadbreak or deadbreak requirements, cable and connector characteristics, switching arrangements, grounding, fault-current requirements, physical terminal arrangement, utility standards, and applicable IEEE requirements.

Pad-Mounted Transformer Primary Bushing Configurations Explained (9/15/2026)

Pad-mounted transformer primary bushing configurations determine how medium-voltage cables connect and whether the system uses radial or loop feed. Common designs use IEEE-standard separable connectors with loadbreak or higher-current deadbreak interfaces. Selection depends on system voltage, current, grounding, cable arrangement, switching requirements, and distribution design.

By LarsonElectronics.com, September 15, 2026

Pad-mounted transformer primary bushing configurations define how medium-voltage primary cables interface with the transformer and how power can be routed through an underground distribution system. The most common arrangements are radial-feed and loop-feed configurations using separable insulated connector systems. The correct arrangement depends on system topology, voltage class, continuous current, switching requirements, cable configuration, grounding, utility standards, and the transformer's application.

For industrial buyers and engineers specifying pad-mounted and industrial transformers, the primary bushing arrangement should be established during transformer specification. A transformer with the correct voltage and kVA rating can still be unsuitable for an installation if its primary terminals do not match the site's cables, connectors, current requirements, or distribution scheme.

Primary bushings provide the medium-voltage connection to a pad-mounted transformer

The primary, or high-voltage, bushings provide the insulated electrical interface between the incoming medium-voltage distribution cable and the transformer winding. On a typical compartmental-type pad-mounted transformer, the primary terminations are located in the high-voltage compartment and are physically separated from the low-voltage terminations.

Modern pad-mounted distribution transformers commonly use shielded separable insulated connector systems instead of exposed high-voltage terminals. These connector systems allow appropriately rated elbows, bushings, inserts, junctions, protective devices, and related components to form a shielded medium-voltage connection system.

IEEE 386 establishes requirements for shielded separable insulated connector systems, including loadbreak and deadbreak systems. The current IEEE 386 standard addresses connector systems for voltage classes from 15 kV through 35 kV and establishes ratings, construction features, interfaces, and testing requirements for these systems.

Radial-feed transformers have one primary cable connection per phase

A radial-feed pad-mounted transformer is supplied from one direction and represents a terminating point on that portion of the primary distribution circuit. For a three-phase transformer, a basic radial arrangement normally provides one primary connection for each phase.

Conceptually, the circuit is:

Source ? Primary Cable ? Transformer

A three-phase radial-feed transformer therefore commonly has three primary bushing interfaces, one for Phase A, one for Phase B, and one for Phase C.

Radial arrangements are relatively straightforward and are common where the transformer is located at the end of a dedicated underground feeder or where primary power does not need to continue through the transformer location to additional equipment.

Radial feed is appropriate for many dedicated industrial loads

Consider an industrial facility with a dedicated underground medium-voltage feeder supplying one pad-mounted transformer. If the feeder terminates at that transformer and no downstream primary circuit continues from the location, a radial configuration may provide the required connection without additional loop-feed bushings.

The tradeoff is system flexibility. If the single upstream primary path is unavailable, the transformer cannot normally be energized from another direction unless the distribution system has an alternate supply arrangement elsewhere.

Loop-feed transformers provide two primary connection points per phase

A loop-feed pad-mounted transformer normally provides two primary bushing interfaces for each phase. On a three-phase transformer, this commonly results in six high-voltage bushing interfaces.

Conceptually, the circuit can be arranged as:

Source ? Transformer Location ? Continuing Primary Feeder

The two interfaces per phase allow the underground primary circuit to enter and leave the transformer compartment. This arrangement is widely used in looped distribution systems serving multiple transformers or load points.

Configuration	Typical Three-Phase Primary Interfaces	Typical Application
Radial feed	3	Dedicated feeder or transformer at the end of a primary circuit
Loop feed	6	Primary circuit enters and continues from the transformer location

The number of interfaces alone does not completely define the transformer's switching capabilities. Internal switches, external switching equipment, connector type, circuit design, operating procedures, and utility requirements must also be considered.

A loop-feed configuration does not automatically provide closed-loop operation

One important engineering distinction is that a loop-feed transformer connection does not necessarily mean the distribution circuit is normally operated as a closed electrical loop.

Many underground distribution systems are physically constructed as loops but operated with a normally open point. This creates radial electrical operation while providing an alternate path that can be used during maintenance or after isolation of a faulted cable section.

For example, several pad-mounted transformers in an industrial campus may be connected by an underground loop. Under normal conditions, an open point divides the loop so loads are supplied radially. If a feeder section must be isolated, qualified personnel may reconfigure the system so unaffected transformers can be supplied from the opposite direction, subject to the system's switching design and operating procedures.

This arrangement can improve service restoration flexibility, but the actual reliability benefit depends on the complete distribution system rather than the transformer bushings alone.

Loadbreak bushing systems allow specific switching operations when properly rated

Many pad-mounted transformers use loadbreak separable connector systems. A common arrangement consists of a bushing well installed in the transformer, a compatible loadbreak bushing insert, and a shielded loadbreak elbow attached to the medium-voltage cable.

The bushing well itself should not be confused with the complete operating interface. The removable insert provides the standardized interface for the mating elbow in designs that use this construction.

Properly rated loadbreak connector systems are designed to perform specified load-making and load-breaking operations when used with compatible equipment and approved operating procedures. Their ratings and capabilities must be verified for the particular connector system.

The term loadbreak does not mean that any energized connector can be disconnected under any condition. Fault current, available system current, connector ratings, operating tools, equipment condition, work practices, and manufacturer instructions all matter. Medium-voltage switching should be performed only by qualified personnel under appropriate electrical safety procedures.

Deadbreak bushings are used where connections are not intended for load switching

Deadbreak separable connector systems are designed to be connected or disconnected when the circuit is de-energized. They are commonly used where higher continuous-current capability is required or where switching under load will be accomplished by another device.

Deadbreak systems are frequently encountered on larger transformers, higher-current distribution circuits, and installations using external or internal medium-voltage switching equipment.

Common connector-system current classes vary by equipment design. Rather than selecting a bushing simply by a familiar nominal current such as 200 A or 600 A, engineers should verify the continuous-current, short-time, fault-close, voltage-class, interface, and switching requirements against the current IEEE 386 standard and the connector manufacturer's published ratings.

Loadbreak and deadbreak describe connector capabilities rather than transformer capacity

A transformer's kVA or MVA rating does not by itself determine whether the primary interface must be loadbreak or deadbreak. Primary full-load current is determined by transformer capacity and primary voltage, but the connector system also must satisfy distribution-system and operating requirements.

For a three-phase transformer, approximate primary full-load current can be calculated as:

I = kVA ÷ (v3 × kV)

For example, a 2,500 kVA transformer with a 12.47 kV primary has an approximate full-load primary current of:

2,500 ÷ (1.732 × 12.47) ˜ 116 A

The calculated transformer load current is only one part of connector selection. The primary cable may be part of a loop carrying current for additional downstream transformers. A loop-feed bushing or connector can therefore be required to carry substantially more current than the transformer itself draws.

This distinction is particularly important when specifying larger industrial pad-mounted transformers.

Bushing voltage class must match the insulation system and connector system

Primary bushing selection is not based only on nominal system voltage. The entire separable connector system must have the appropriate voltage class and insulation characteristics for the distribution system.

Medium-voltage pad-mounted installations commonly use equipment in 15 kV, 25 kV, and 35 kV classes. The exact connector interface, insulation level, cable insulation system, grounding arrangement, and equipment ratings must be coordinated.

A system operating at 12.47 kV, for example, commonly uses equipment from the 15 kV class. A higher nominal distribution voltage may require a 25 kV or 35 kV class interface. Engineers should use the system's maximum operating voltage and applicable equipment standards rather than selecting components solely from the nominal voltage printed on a one-line diagram.

IEEE C57.12.34 establishes pad-mounted transformer arrangement requirements

IEEE C57.12.34 is a key North American standard for three-phase pad-mounted, compartmental-type, liquid-immersed distribution transformers. The standard covers units rated 10 MVA and smaller, with high-voltage systems of 34.5 kV nominal and below and low-voltage systems of 15 kV nominal and below.

Importantly for transformer specification, IEEE C57.12.34 addresses connector, bushing, and terminal arrangements for both radial-feed and loop-feed systems. It also establishes dimensional, mechanical, electrical, and selected safety requirements for this class of pad-mounted transformer.

For single-phase pad-mounted distribution transformers, IEEE C57.12.38 addresses applicable electrical, dimensional, mechanical, safety, connector, bushing, and terminal arrangements for radial- and loop-feed designs within its scope.

IEEE 386 defines separable insulated connector interfaces

IEEE 386 is another central standard when specifying primary connections. The standard establishes requirements for shielded separable insulated connector systems, including loadbreak and deadbreak interfaces used on medium-voltage distribution systems.

Using standardized interfaces helps provide compatibility among appropriately rated components, but an IEEE interface designation should not be treated as permission to mix arbitrary components. Voltage class, current rating, interface designation, conductor size, cable insulation diameter, connector construction, manufacturer requirements, and application must all be verified.

Bushing arrangement and primary switching should be specified together

The external bushing configuration is only one part of the transformer's high-voltage arrangement. Pad-mounted transformers can also incorporate primary switches and protective devices depending on the application.

For example, a loop-feed transformer may use internal loadbreak switches that allow sections of the primary circuit or the transformer itself to be isolated. Other installations may rely on external pad-mounted switchgear for feeder sectionalizing and transformer isolation.

Possible transformer configurations can include combinations of:

Radial-feed primary connections
Loop-feed primary connections
Loadbreak separable connectors
Deadbreak separable connectors
Internal primary switching
Primary fusing
Bayonet-type expulsion fuses where applicable
Current-limiting fuses where applicable
Surge arresters

The required combination depends on the transformer's rating, utility or facility protection philosophy, system fault current, switching requirements, coordination study, and maintenance strategy.

Primary bushing position and phasing must match the installation

Correctly specifying the number and type of bushings is not enough. Their physical arrangement and phase identification must also match the installation requirements.

Pad-mounted transformer drawings identify the locations of the high-voltage interfaces, low-voltage terminals, compartment barriers, operating handles, and other accessories. Cable routing and termination space should be reviewed before procurement, particularly when replacing an existing transformer.

A replacement transformer can have the correct electrical ratings but still create a field problem if bushing positions, compartment dimensions, cable entrance locations, phase arrangement, or connector interfaces differ from the existing equipment.

Replacement transformers require field verification

Consider a facility replacing an aging 15 kV-class loop-feed transformer. The nameplate may provide the transformer's kVA, voltage, impedance, and winding information, but those values alone do not fully define the replacement.

Before ordering, the engineering team should verify whether the existing transformer has radial or loop feed, the number and location of primary interfaces, loadbreak or deadbreak construction, connector current class, primary switch arrangement, cable sizes, cable insulation characteristics, phasing, compartment dimensions, pad dimensions, secondary terminal arrangement, grounding provisions, and required accessories.

Photographs and dimensional measurements can be useful, but they should supplement rather than replace nameplate information, drawings, cable data, and engineering verification.

System grounding affects primary transformer specification

System grounding must be considered when selecting a pad-mounted transformer and its primary equipment. Distribution systems may use grounded-wye, resistance-grounded, ungrounded, or other arrangements depending on facility and utility design.

The transformer's primary winding connection, system grounding, insulation requirements, surge arrester selection, switching equipment, and protective devices must be coordinated as a system.

For example, a transformer intended for operation on a grounded-wye distribution system should not be assumed interchangeable with a transformer specified for a different grounding arrangement merely because both systems have the same nominal line-to-line voltage.

NEC requirements apply to pad-mounted transformer installations

In U.S. industrial installations, pad-mounted transformer installations must comply with the adopted edition of the National Electrical Code and requirements of the authority having jurisdiction.

NEC Article 450 addresses transformers and transformer vaults, including applicable transformer protection and installation requirements. NEC Article 250 contains grounding and bonding requirements. Medium-voltage installations may also involve requirements in NEC Article 300 and other applicable provisions covering conductors, wiring methods, underground installations, and equipment operating above 1,000 V.

Where medium-voltage equipment is installed in accessible locations, guarding, enclosure, working-space, access, and qualified-person requirements must also be evaluated under the applicable NEC provisions.

NEC compliance does not replace IEEE equipment standards. The NEC governs installation and electrical safety requirements, while standards such as IEEE C57.12.34 and IEEE 386 address important transformer and connector characteristics. Project specifications, utility requirements, manufacturer instructions, and the authority having jurisdiction must also be considered.

Canadian pad-mounted transformer applications require applicable CSA coordination

For Canadian industrial installations, transformer and medium-voltage equipment should be coordinated with the applicable edition of the Canadian Electrical Code, Part I, CSA C22.1, provincial or territorial requirements, serving-utility standards, and applicable equipment certification requirements.

IEEE-based transformer and separable-connector specifications are widely relevant to North American equipment, but the governing installation requirements should always be confirmed for the specific Canadian jurisdiction and project.

The correct primary bushing configuration starts with the distribution one-line

The most reliable way to specify a pad-mounted transformer's primary bushings is to begin with the facility or utility one-line diagram and trace the primary circuit through the transformer location.

If the medium-voltage feeder terminates at the transformer, a radial-feed configuration may be appropriate. If the feeder must enter the transformer location and continue to another transformer or distribution point, a loop-feed arrangement may be required.

Engineers should then determine the required voltage class, continuous current, loadbreak or deadbreak interface, cable construction, connector interface, switching arrangement, fault-current capability, grounding, protection, and physical bushing arrangement.

Specification Item	Engineering Question to Resolve
Feed configuration	Does the primary feeder terminate at the transformer or continue through the location?
Voltage class	What system voltage and insulation class are required?
Current rating	What continuous current can the connector and loop circuit be required to carry?
Connector type	Is a loadbreak or deadbreak separable connector system required?
Cable interface	Do conductor size, insulation diameter, shielding, and connector components match?
Switching	Will switching occur at the connector, inside the transformer, or in separate switchgear?
Physical arrangement	Do bushing positions, cable entrances, compartment dimensions, and phasing match the site?

Frequently referenced pad-mounted transformer bushing configurations have distinct purposes

Radial-feed transformers normally provide one primary connection per phase

A radial-feed transformer is typically located at the terminating point of a primary feeder. A three-phase radial transformer commonly has three primary bushing interfaces.

Loop-feed transformers normally provide two primary connections per phase

A loop-feed configuration allows the primary circuit to enter and continue from the transformer location. A three-phase loop-feed transformer commonly provides six primary bushing interfaces.

Loop feed does not necessarily mean the system operates as a closed loop

Many underground loops contain a normally open point and operate electrically as radial systems. The loop provides an alternate path that can support sectionalizing and service restoration when the system is designed for those operations.

Loadbreak connectors provide defined switching capability

Loadbreak separable connectors are designed for specified load-making and load-breaking operations within their ratings and manufacturer requirements. Their use does not eliminate the need for qualified personnel, proper tools, system operating procedures, and electrical safety practices.

Deadbreak connectors require de-energization for disconnection

Deadbreak separable connectors are not intended to be disconnected under load. They are commonly applied where higher continuous-current capability is required or switching is performed elsewhere in the distribution system.

Primary bushing current can exceed transformer primary load current

On a loop-feed system, the primary connector may carry current supplying downstream equipment in addition to the transformer at that location. Connector current rating should therefore be determined from the distribution circuit requirements, not transformer kVA alone.

Transformer specification should address the complete primary interface

A complete pad-mounted transformer specification should identify more than voltage, phase, frequency, and kVA. Engineers and buyers should document the radial- or loop-feed arrangement, primary bushing interface, connector current and voltage class, cable requirements, switching configuration, protection, grounding, phasing, physical terminal arrangement, and applicable IEEE or utility requirements.

This system-level approach is especially important for replacement transformers because apparently small differences in bushing configuration can prevent existing medium-voltage cables from connecting correctly or alter the intended operation of the distribution circuit.

Larson Electronics 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 additional information about industrial transformers and transformer configurations, review the transformer equipment available for industrial power-distribution applications.

For assistance specifying a pad-mounted transformer, replacement transformer, primary bushing arrangement, or associated industrial power-distribution equipment, contact Larson Electronics.

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