Pad-mounted transformers are critical for underground power distribution in the USA, with loop and radial configurations serving distinct reliability and cost needs.

1. Radial Feed Systems

• Design: Single power source with three primary bushings; no redundancy.
• Applications:
  • Residential subdivisions (e.g., standalone units serving a neighborhood).
  • Small businesses with one utility feed.
  • Solar farms feeding into the grid via a single point.
• Example:

A 2.5 MVA radial transformer (e.g., ZGS-Z-2500/44/4.16) used in industrial parks terminates cables at bushings with arresters for surge protection.

2. Loop Feed Systems

• Design: Six primary bushings (staggered A/B sides) for dual-source redundancy; supports daisy-chaining multiple units.
• Applications:
  • Hospitals/data centers requiring zero downtime.
  • Campus networks with chained transformers.
  • Crypto mining operations needing high reliability.
• Example:

A 2550 kVA loop unit (ZGSL-H-2550/34.5/0.415) with a 4-position load-break switch allows seamless source switching during outages.

Key Comparisons

USA Market Specifications

• Standards: IEEE C57.12.34 (pad-mount design), dead-front separable connectors (15-35 kV).
• Safety: Loop units use bayonet fuses to isolate faults, while radial units rely on grid-level protection.
• Adaptability: Loop transformers can operate in radial mode by capping unused bushings with arresters.

Real-World Scenarios

• Radial: A 315 kVA transformer powers a rural school - cost-effective but vulnerable to upstream outages.
• Loop: Six 2.5 MVA units daisy-chained for a Bitcoin mining farm ensure continuous power if one feed fails.

Summary: Radial systems prioritize simplicity and cost, while loop systems offer resilience for critical loads. Most USA utilities use loop feeds for urban networks and radial for remote sites.

Radial Feed Example

Application: Residential street lighting or small suburban homes

Transformer:

• Model: 167 kVA, 24.94 kV/7.2 kV (e.g., Cooper Power Systems PMU-167-R)
• Configuration:
  • H1 (connected to MV source cable)
  • H2/H3 (jumpered internally to tank ground)
• Protection: Surge arrester on H1, bayonet fuse on secondary

Utility source -> Pad-mount transformer (Radial) -> Customers

Why radial? Low cost, simple installation for non-critical loads with one power source.

Loop Feed Example

Application: Critical facilities (e.g., rural clinic, fire station)

• Model: 100 kVA, 34.5 kV/12.47 kV (e.g., Eaton Vault-Mount VL-100-L)
• Configuration:
  • A1/A2 (feed from Source A)
  • B1/B2 (feed from Source B)
  • 600A loadbreak interrupter switch (internal)

Source A <-> Pad-mount transformer (Loop) <-> Source B

Why loop? Redundancy for mission-critical loads; complies with NEC Article 700 for emergency systems.

Key USA-Specific Details

• Radial Feed Safety: Requires insulated elbow connector (e.g., Elastimold T-EDL); grounding lugs per IEEE C57.12.38.
• Loop Feed Switching: Must pass IEEE 386 tests for separable connectors (Open-Close-Open sequence).
• Visual Identification: Radial = 3-4 bushings; Loop = 4+ bushings (two isolated groups).

Single & Three-Phase Configurations

Single-Phase Radial Schematic:
Utility Source -> [H1]---->
                   TRANSFORMER -> Secondary (120/240V)
Ground ---------> [H0/H2]

Single-Phase Loop Schematic:
Source A -> [H1A][H2A]---->
                     TRANSFORMER -> Secondary
Source B -> [H1B][H2B]

Three-Phase Loop Daisy Chain:
Source -> [A-Side Bushings] -> Transformer 1 -> [B-Side Bushings] ->
         [A-Side Bushings] -> Transformer 2 -> [B-Side Bushings]
  

Comparison Tables

Grounding & Safety

• All systems use IEEE 386-compliant dead-front elbow connectors (15-35 kV).
• Wye systems: H0 grounded to tank & grid; delta systems use corner-grounded arresters.
• Protection: Bayonet fuses + secondary breakers; arresters on loop bushings in radial mode.

Deployment Examples

• Radial: Rural school (315 kVA) - single feed vulnerability.
• Loop: 6x 2.5 MVA units - ensures uptime for mining even with source failure.

Key Standard: IEEE C57.12.34 governs pad-mounted transformers. Loop systems cost 15-20 percent more but prevent downtime.