Technical Summary
The cable compartment of a loop-feed pad-mounted transformer contains two sets of MV bushings (source and load side), while a radial-feed unit contains only one. This is not simply a doubling of bushings — it changes the compartment depth, cable routing complexity, thermal profile, and access sequence for maintenance. This article compares both configurations from a cable compartment engineering perspective.
1. Configuration Comparison
| Radial feed | Loop feed |
|---|
| MV bushings (3-phase unit) | 3 (one per phase) | 6 (two per phase) |
| Compartment depth (typical planning range) | 300–400 mm | 400–600 mm |
| Cable quantity (MV) | 3 conductors | 6 conductors |
| Conduit count (MV) | Typically 3 | Typically 6, or one large duct bank |
| Working clearance requirement | Lower — one set of terminations | Higher — two sets to access; rear set may be behind front set |
| Barrier requirements | MV/LV separation | MV-MV phase grouping + MV/LV separation |
2. What the Extra Bushings Change
The loop-feed compartment is not simply a deeper radial compartment:
- Cable routing: Cables from the rear set of bushings route past — without touching — the cables from the front set. This S-curve routing imposes two bend radius constraints per cable (conduit exit bend + bushing entry bend) instead of one.
- Thermal profile: Six terminations inside the same compartment generate more heat than three. In outdoor installations this is typically negligible, but in enclosures with restricted ventilation it can raise the internal ambient temperature measurably.
- Access sequence: The source-side terminations (rear) are typically farther from the door, behind the load-side terminations. Maintenance on a rear bushing may require de-terminating a front bushing to gain access — a consideration for utilities that perform live-line work.
3. Specification Considerations
- State the required compartment depth explicitly — do not rely on the manufacturer selecting a depth that will work for the specific cable type and size on the project.
- Verify the concrete pad conduit layout. Loop-feed pads have conduits entering from opposite directions. The civil contractor must receive two separate conduit layout drawings — one for each feeder entry — and must not interchange them.
- Understand how the second feeder will be used. Some utilities use loop-feed only for switching flexibility, not for automatic service restoration. If the second feeder will remain de-energized most of the time, the added cost and complexity of the loop-feed compartment may not be justified.
4. Common Specification Errors
- Ordering loop-feed without specifying compartment depth: The manufacturer builds the standard depth, which may be adequate for radial-feed cable routing but too shallow for the rear bushing row in loop-feed.
- Assuming radial and loop-feed pads are interchangeable: They are not. A radial-feed pad cannot be field-upgraded to loop-feed without demolition.
- Not coordinating the bushing layout drawing with the pad design: The pad conduit layout must be based on the actual bushing positions — not a generic pad template.
Related Procurement Guides
For buyer-side RFQ and project-planning context, see these related TransformerGrid blog guides.