In electrical power distribution, the terms “loop feed” and “radial feed” describe how power flows from a source to the transformers that step down voltage for end users. While these terms originate from broader distribution system design, they are most commonly used to specify the high-voltage (HV) bushing configuration on pad-mounted transformers.
Choosing between a radial or loop feed transformer impacts the reliability, flexibility, and cost of your electrical network. This guide explains how these systems work, their physical differences, and how to select the right configuration for your project.
Before examining the transformers themselves, it is essential to understand the two types of distribution systems they serve.
Radial Feed System
A radial feed system is the simplest and most common form of power distribution. In this layout, power flows from a single source (like a substation) outward along a single path to the loads, much like spokes on a wheel. Each transformer receives power from only one point.
- Advantages: It is simple to design, operate, and protect. The initial cost is lower because it requires less equipment.
- Disadvantage: It offers no redundancy. If the single source fails or a fault occurs on the line, all downstream transformers and customers lose power until repairs are made.
Loop Feed System
A loop feed system is designed for higher reliability. In this configuration, the distribution line forms a loop, and transformers are capable of receiving power from two or more directions or sources.
- Advantages: It provides redundancy. If a fault occurs on one side of the loop or one source fails, power can be rerouted from the other direction, keeping the system energized. This makes it ideal for critical facilities.
- Disadvantage: It requires more complex protection schemes and a higher initial investment due to additional cables and switches.
Bushing Configurations： The Physical Difference
Radial Feed Bushing Configuration
A radial feed transformer is designed for a single-point power supply. Its high-voltage compartment contains three primary bushings (typically labeled H1, H2, H3)-one for each incoming phase conductor. This simpler design is common in residential subdivisions, small commercial areas, or as the last unit in a string of transformers where the line terminates.
Loop Feed Bushing Configuration
A loop feed transformer is designed for flexibility and redundancy. It features six high-voltage bushings-two full sets of three (typically labeled H1A, H2A, H3A for the “A” side and H1B, H2B, H3B for the “B” side). This allows for two primary feeds:
- Looping Transformers Together: Cables from the source enter the first unit’s “A” bushings, then exit from its “B” bushings to feed the “A” bushings of the next transformer. This daisy-chains multiple units together.
- Dual Source Feed: A single unit can be connected to two separate power sources. Internal selector switches allow operators to choose which source feeds the load.
Please note: a “loop of transformers” is different from a “loop system.” It’s possible to connect a series of transformers in a loop, but if they all rely on a single power source, then a failure at that source will cause all units to lose power-meaning the configuration remains radial.
| Feature | Radial Feed Transformer | Loop Feed Transformer |
| High-Voltage Bushings | Three (H1, H2, H3) | Six (H1A, H2A, H3A, H1B, H2B, H3B) |
| Design Complexity | Simple | More Complex |
| Power Source | Single point only | Single or dual source |
| Redundancy | None-single point of failure | High-alternate feed available |
| Typical Cost | Lower | Higher |
Redundancy, Protection, and Switching
Beyond just the number of bushings, loop feed transformers offer advanced features that enhance grid reliability and maintainability.
Redundancy and Fault Isolation
In a loop system with multiple transformers, internal bayonet fuses provide critical protection. If a fault occurs on the secondary side of one transformer, its primary fuse blows. This isolates the faulty transformer from the loop, allowing power to continue flowing past it to the other transformers in the circuit. This minimizes downtime and confines the outage to a single load.
Selector Switch Configurations
Loop feed transformers can be equipped with oil-immersed selector switches to manage power flow. These switches provide the ability to redirect current or isolate the transformer without de-energizing the entire loop. Common configurations include:
- Three Two-Position Load Break Switches: One switch controls the transformer itself, one controls the incoming “A” feed, and one controls the “B” feed. This offers maximum versatility, allowing an operator to isolate the transformer while keeping the loop energized.
- Four-Position Load Break Switch: This single switch (often called a “loop feed switch”) combines the functions, allowing operators to select Feed A, Feed B, or isolate the unit, though with slightly less granular control than the three-switch setup.
How to Choose the Right Transformer
Selecting between a radial and loop feed transformer depends on the project’s specific requirements for reliability, budget, and future growth.
When to Choose Radial Feed
Radial feed transformers are best suited for applications where cost is a primary concern and brief outages are tolerable.
Residential subdivisions and rural areas.
Simple commercial sites with predictable loads.
Serving as the termination point at the end of a loop-fed line.
Projects with tight budget constraints.
When to Choose Loop Feed
Loop feed transformers are the preferred choice for mission-critical applications where uptime is essential.
Hospitals, airports, and data centers where continuous power is vital.
University campuses and large industrial complexes that require high reliability.
Systems that require flexibility for maintenance, allowing a transformer to be repaired without shutting down the whole network.
Installations where future load growth might require adding more transformers to the loop.
A Note on Versatility
It is possible to use a loop feed transformer in a radial system. In such cases, the second set of bushings (B-side) is simply capped off or fitted with surge arresters. However, it is rare to use a radial feed transformer in a loop system, as its three-bushing design cannot accommodate a second feed without complex external modifications.
Conclusion
The choice between radial and loop feed transformers ultimately defines the reliability and complexity of your power distribution network. Radial feed transformers offer a simple, cost-effective solution for standard applications with three bushings and a single power source. Loop feed transformers, with their six-bushing design and optional internal switching, provide the redundancy and flexibility required for critical infrastructure. By assessing your need for uptime against your budget, you can select the configuration that ensures safe, efficient, and reliable power delivery.
