Does loop feed provide redundant power?

Not by itself. Loop feed provides the connections needed for a loop distribution architecture. Redundancy and alternate-source capability depend on the complete system design, switching equipment, protection, source capacity and operating configuration.

Does a loop-feed transformer have a higher kVA capacity?

No. Radial versus loop feed describes the primary distribution connection arrangement. Transformer kVA capacity is determined separately by the transformer design and nameplate rating.

When is radial feed used for a pad-mounted transformer?

Radial feed is commonly used when a transformer is located at the end of a primary feeder and there is no need for the medium-voltage circuit to continue beyond the transformer location.

When is loop feed used for a pad-mounted transformer?

Loop feed is commonly used when a medium-voltage circuit must continue through multiple transformer locations or when the distribution system requires additional sectionalizing and service-restoration flexibility.

Radial Feed vs Loop Feed Pad-Mounted Transformers (8/24/2026)

Q: Which IEEE standards cover radial and loop-feed pad-mounted transformers?

IEEE C57.12.34 covers connector, bushing and terminal arrangements for radial and loop-feed configurations on applicable three-phase pad-mounted distribution transformers. IEEE C57.12.38 provides corresponding requirements for applicable single-phase pad-mounted transformers.

Radial-feed pad-mounted transformers normally receive primary power from one direction, while loop-feed transformers provide primary connections that allow the underground distribution circuit to continue through the transformer location. Radial feed is simpler, while loop feed can provide greater distribution flexibility, sectionalizing options and potential alternate-source capability when the overall system is designed for it.

By LarsonElectronics.com, August 24, 2026

Radial feed and loop feed describe the primary-side connection arrangement of a pad-mounted transformer. A radial-feed transformer is typically supplied from one incoming medium-voltage circuit, while a loop-feed transformer provides connections that allow the primary distribution circuit to enter and leave the transformer location.

The choice affects primary bushing configuration, cable routing, switching, sectionalizing, maintenance flexibility and distribution-system design. It does not, by itself, determine transformer kVA, secondary voltage or whether the transformer can carry more load.

Industrial buyers specifying pad-mounted and industrial transformers should determine the required feed arrangement from the facility one-line diagram, utility requirements and medium-voltage distribution architecture before ordering equipment.

Radial Feed Supplies the Transformer From One Primary Direction

A radial distribution system normally delivers power from the source toward the load through a single primary path. At the transformer location, the underground medium-voltage cable terminates at the transformer rather than continuing through that location as part of a loop.

For a typical three-phase dead-front radial-feed pad-mounted transformer, this often means one primary connection for each phase. The exact bushing arrangement depends on transformer design, voltage class, current rating and applicable specification.

Radial feed is commonly used where a simple primary distribution arrangement is appropriate and an alternate cable path through the transformer location is not required.

Loop Feed Allows the Primary Circuit to Continue Through the Transformer Location

A loop-feed pad-mounted transformer typically provides two primary connection points per phase. One set can receive the incoming underground feeder and the second set can continue the circuit toward another transformer, sectionalizing point or other medium-voltage equipment.

For many three-phase dead-front designs, this results in six primary bushings or bushing wells rather than the three commonly associated with radial-feed configurations.

IEEE C57.12.34 specifically addresses connector, bushing and terminal arrangements for both radial-feed and loop-feed three-phase pad-mounted distribution transformers. IEEE C57.12.38 provides corresponding requirements for applicable single-phase pad-mounted transformers.

Radial Feed and Loop Feed Differ Primarily in the Medium-Voltage Connection Arrangement

Characteristic	Radial Feed	Loop Feed
Primary circuit arrangement	Primary feeder normally terminates at the transformer location	Primary feeder can enter and continue through the transformer location
Typical three-phase dead-front primary connections	Three	Six
Cable routing	Single feeder path to the transformer	Incoming and outgoing feeder paths
System complexity	Generally simpler	Requires additional cable and system coordination
Sectionalizing flexibility	Generally more limited	Can support additional sectionalizing arrangements
Alternate-source capability	Normally limited by the single feeder path	Possible when the complete distribution system is designed and operated for alternate feeding

Loop Feed Does Not Automatically Mean the Transformer Has Two Energized Sources

A common misconception is that a loop-feed transformer is continuously powered from two independent sources. The term loop feed primarily describes the primary connection arrangement and the ability of the distribution cable to continue through the transformer location.

Many underground distribution loops are operated as open-loop systems. The circuit contains a normally open point so that it is not continuously energized from both directions.

If a feeder section is damaged or must be removed from service, appropriately rated switching and sectionalizing equipment may allow operators to isolate the affected section and restore portions of the circuit from another direction.

Whether this is possible depends on the design of the entire distribution system. A loop-feed transformer alone does not create redundancy.

Loop Feed Can Improve Distribution Flexibility

The principal advantage of loop feed is flexibility in the primary distribution system.

Depending on the system design, loop architecture can allow operators to:

Isolate a damaged underground cable section
Sectionalize portions of a medium-voltage feeder
Maintain selected equipment while preserving service elsewhere
Reconfigure the primary distribution path
Restore loads from an alternate direction when system capacity permits
Expand an underground distribution network more systematically

These capabilities can be valuable at industrial campuses, manufacturing facilities, data centers, hospitals, airports, military installations and other facilities where multiple transformers are distributed across a large site.

Radial Feed Provides a Simpler Distribution Arrangement

Radial feed remains appropriate for many industrial installations. If a transformer serves an isolated building or load and alternate primary routing is unnecessary, the additional connections and cable associated with a loop system may provide little practical benefit.

A radial system can reduce cable requirements and simplify the primary distribution layout.

For example, a transformer serving a remote warehouse from a dedicated medium-voltage feeder may not need a continuing primary circuit. A radial-feed transformer can provide the required service without adding a second set of primary feeder connections that the distribution system will not use.

Loop Feed Is Useful on Industrial Campuses With Multiple Transformers

Consider a manufacturing campus with four pad-mounted transformers serving separate buildings. A medium-voltage underground feeder can enter the first transformer location, continue to the second and third locations, and eventually reach another sectionalizing point or source path.

If the system is properly designed with suitable switches and protection, operators may be able to isolate a faulted cable segment while maintaining or restoring service to unaffected sections.

That flexibility can reduce the number of loads affected by a single cable failure.

The benefit comes from the complete loop distribution architecture, not simply from installing six bushings on each transformer.

Primary Switching Determines How a Loop System Can Be Operated

A loop-feed transformer may be equipped or coordinated with loadbreak switches, sectionalizing equipment, separable connectors, fuses and other primary components depending on the application.

Switching configuration should be specified intentionally. The transformer manufacturer needs to know how the incoming feeder, outgoing feeder and transformer primary are expected to be connected and isolated.

Common design considerations include:

System voltage and maximum system voltage
Available fault current
Continuous feeder current
Radial or loop-feed arrangement
Dead-front or live-front construction
Bushing and connector current ratings
Loadbreak switching requirements
Primary fuse arrangement
Surge arrester requirements
Grounding configuration
Utility or facility switching procedures

Dead-Front Construction Is Common in Underground Loop Systems

Many modern pad-mounted distribution systems use dead-front construction with separable insulated connectors. The medium-voltage cable terminates through shielded connectors rather than exposed live primary terminals.

Depending on voltage and current requirements, the transformer may use bushing wells and inserts or integral bushings designed for the applicable separable connector system.

For example, manufacturer specifications for three-phase pad-mounted transformers commonly identify three high-voltage bushings for radial-feed dead-front configurations and six for loop-feed dead-front configurations. Actual connector and bushing requirements must be matched to the system voltage, BIL, current rating and cable system.

Radial Feed and Loop Feed Do Not Determine Transformer kVA

Feed configuration should not be confused with transformer capacity.

A 500 kVA, 1000 kVA or 2500 kVA pad-mounted transformer could potentially be specified for a radial or loop-feed application when the transformer design and applicable standard support the required arrangement.

Transformer kVA is determined by load requirements, while radial versus loop feed is determined by the primary distribution topology.

Other specifications such as primary voltage, secondary voltage, impedance, BIL, taps, winding configuration, cooling, enclosure construction and protection must be evaluated separately.

Loop Feed Does Not Increase the Transformer kVA Rating

Providing two primary cable paths does not double transformer capacity. The transformer remains limited to its nameplate kVA and thermal rating.

The incoming and outgoing loop connections are part of the medium-voltage distribution circuit. They do not represent two transformer windings operating in parallel to increase output capacity.

This distinction is particularly important when replacing an existing pad-mounted transformer. A six-bushing loop-feed transformer should not be interpreted as having twice the primary capacity of a comparable three-bushing radial-feed transformer.

Existing Transformer Replacements Should Match the Distribution Topology

When replacing an existing pad-mounted transformer, radial versus loop feed should be verified before ordering the replacement.

Engineers should document:

Number and location of primary bushings
Bushing or bushing-well type
Connector current rating
Primary voltage class
BIL
Cable entrance locations
Phase orientation
Switching configuration
Fuse arrangement
Surge arresters
Grounding provisions
Physical cabinet dimensions and clearances

Matching voltage and kVA alone is not sufficient. A replacement transformer with the wrong feed configuration can be incompatible with the existing underground cable system even when its electrical transformation ratio is correct.

IEEE C57.12.34 Addresses Three-Phase Radial and Loop-Feed Pad-Mounted Transformers

IEEE C57.12.34 establishes requirements for applicable pad-mounted, compartmental-type, self-cooled, three-phase distribution transformers. The standard covers transformers up to 10 MVA, with high-voltage systems through 34.5 kV nominal and low-voltage systems through 15 kV nominal within its scope.

Importantly for radial versus loop feed, IEEE C57.12.34 addresses connector, bushing and terminal arrangements for both configurations.

IEEE C57.12.00 provides broader general requirements for liquid-immersed distribution, power and regulating transformers. Engineers should apply the standards appropriate to the specific transformer type and project requirements.

IEEE C57.12.38 Addresses Applicable Single-Phase Pad-Mounted Transformers

Single-phase pad-mounted transformer requirements are addressed separately. IEEE C57.12.38-2025 covers applicable liquid-filled, self-cooled, single-phase pad-mounted distribution transformers rated 250 kVA and smaller, with high voltages through 34.5 kV grounded-wye systems and low voltages through 480/240 V.

Like the three-phase standard, IEEE C57.12.38 includes connector, bushing and terminal arrangements for radial-feed and loop-feed configurations.

NEC Requirements Apply to the Transformer Installation

For U.S. installations, NEC Article 450 contains requirements for transformers and transformer vaults. Depending on the equipment and installation, other NEC articles can also apply to medium-voltage conductors, grounding, overcurrent protection and working space.

NEC requirements should be coordinated with the transformer listing, manufacturer instructions, IEEE standards, utility requirements and the authority having jurisdiction.

The NEC does not determine whether an industrial distribution system should use radial or loop feed. That decision is an engineering and utility-system design consideration based on reliability, switching, protection, cable routing, operating procedures and economics.

Canadian Installations Require Coordination With Provincial and Utility Requirements

For Canadian industrial projects, transformer and medium-voltage distribution design should be coordinated with the Canadian Electrical Code, applicable provincial or territorial requirements, CSA standards, utility specifications and equipment certification requirements.

Utility requirements can be particularly important for pad-mounted equipment because utilities may specify connector orientation, cable compartments, grounding, protection, metering, clearances and acceptable transformer configurations.

Radial Feed Is Appropriate When Simplicity Is the Primary Requirement

Radial feed is generally a strong candidate when the transformer is located at the end of a primary feeder and there is no requirement to continue the medium-voltage circuit beyond that location.

Typical applications can include isolated industrial buildings, remote pump stations, warehouses, individual process areas and facilities supplied by dedicated primary feeders.

The advantages are a straightforward cable arrangement, fewer primary termination points and potentially lower system complexity.

Loop Feed Is Appropriate When Distribution Flexibility Is Important

Loop feed is generally preferred when the medium-voltage distribution system must continue through multiple transformer locations or when the facility requires additional options for sectionalizing and service restoration.

Typical applications can include:

Large manufacturing campuses
Data centers
Hospitals and healthcare campuses
Airports
Military facilities
Universities
Large commercial developments
Utility underground distribution networks
Industrial sites with multiple substations or pad-mounted transformers

Loop feed should be selected because the overall distribution architecture requires it, not simply because it appears to provide greater redundancy.

A Pad-Mounted Transformer RFQ Should Clearly Identify the Feed Arrangement

An RFQ for a pad-mounted transformer should explicitly state whether radial or loop feed is required. The specification should also identify the primary cable and switching arrangement so the manufacturer can provide the correct high-voltage compartment configuration.

A practical transformer RFQ should include:

Required kVA
Number of phases
Frequency
Primary voltage and connection
Secondary voltage and connection
Radial or loop feed
Dead-front or live-front construction
Primary bushing type and current rating
BIL
Impedance requirements
Primary switching requirements
Fuse requirements
Surge arrester requirements
Tap requirements
Fluid requirements
Temperature-rise requirements
Grounding requirements
Enclosure and corrosion requirements
Applicable IEEE, CSA, utility or project standards

For replacement transformers, drawings and photographs of the existing primary compartment can also help confirm cable entrance locations, bushing orientation and switching configuration.

After-Sales Support Can Extend Through Transformer Commissioning

Larson Electronics offers after-sales support for low-voltage (LV), medium-voltage (MV) and high-voltage (HV) transformers and switchgear across North America. Support can include assembly, installation, inspection and commissioning to help verify that equipment is installed and placed into service in accordance with project requirements, manufacturer instructions and applicable procedures.

Frequently Asked Questions

Radial feed uses a primary path that normally terminates at the transformer

A radial-feed pad-mounted transformer is typically supplied by one incoming medium-voltage feeder, with the primary circuit terminating at that transformer location rather than continuing through it as part of a loop.

Loop feed allows the primary circuit to continue through the transformer location

A loop-feed transformer provides incoming and outgoing primary connections so the medium-voltage distribution circuit can continue to another transformer or distribution point.

A three-phase loop-feed transformer commonly has six primary connections

Many three-phase dead-front loop-feed pad-mounted transformers use two primary connections per phase, resulting in six high-voltage bushings or bushing wells. Radial-feed versions commonly use one per phase. The exact arrangement must be verified for the transformer design and applicable standard.

Loop feed does not automatically provide redundant power

A loop-feed transformer provides the connection arrangement needed for a loop distribution system, but redundancy depends on the complete system design, available alternate source, switching equipment, protection and operating configuration.

Loop feed does not increase transformer capacity

The transformer remains limited to its nameplate kVA and thermal ratings. Additional primary connections provide distribution flexibility and do not increase transformer output capacity.

Radial and loop-feed transformers are covered by IEEE standards

IEEE C57.12.34 addresses radial and loop-feed arrangements for applicable three-phase pad-mounted distribution transformers. IEEE C57.12.38 addresses corresponding arrangements for applicable single-phase pad-mounted transformers.

A Pad-Mounted Transformer Content Cluster Builds Deeper Technical Coverage

Radial versus loop feed is one part of a complete pad-mounted transformer specification. Supporting technical articles can build a broader engineering resource around pad-mounted transformer selection and application:

How to Specify a Pad-Mounted Transformer for an Industrial Project
Pad-Mounted Transformer Sizing Guide
Dead-Front vs Live-Front Pad-Mounted Transformers
Pad-Mounted Transformer Primary Bushing Configurations Explained
200 A vs 600 A Pad-Mounted Transformer Bushings
Pad-Mounted Transformer Bushing Wells and Inserts Explained
Loop-Feed Transformer Switching Configurations
Open-Loop Medium-Voltage Distribution Systems Explained
Pad-Mounted Transformer Loadbreak Switches Explained
Bayonet Fuses and Current-Limiting Fuses in Pad-Mounted Transformers
Pad-Mounted Transformer Surge Arresters Explained
Pad-Mounted Transformer BIL Selection
Pad-Mounted Transformer Tap Changers Explained
Pad-Mounted Transformer Nameplate Ratings Explained
Single-Phase vs Three-Phase Pad-Mounted Transformers
Pad-Mounted Transformer Grounding Requirements
How to Replace an Existing Pad-Mounted Transformer
What to Include in a Pad-Mounted Transformer RFQ

For assistance specifying industrial and pad-mounted transformers for radial-feed, loop-feed and other distribution applications, contact Larson Electronics.

Larson Electronics Building Trust Since 1973.