Yes, you can connect bus ducts to the bottom of the terminal box (also called a throat or connection box) of a distribution transformer (11kV/415V). While top connections are more common, bottom connections are feasible and sometimes advantageous. Here's a breakdown of the factors to consider

Advantages of Bottom Connection:
Space Constraints

In some situations, there might be limited headroom above the transformer, making a top connection impractical. A bottom connection can offer more flexibility in tight spaces.

Easier Accessibility

Depending on the overall layout, accessing the bus duct termination at the bottom might be easier for maintenance and inspections.

Shorter Run to Switchgear

If the switchgear or distribution panel is located directly below the transformer, a bottom connection could result in a shorter bus duct run, potentially reducing voltage drop and cost.

Factors to Consider (CRITICAL)


Transformer Terminal Box Design and Ratings



Manufacturer Approval



THIS IS PARAMOUNT. Contact the transformer manufacturer (and preferably show them the proposed layout) to confirm that bottom connection is permissible and will not compromise the integrity of the transformer or its warranty. Some transformers are not designed for bottom connections. Terminal Box Rating

Verify that the terminal box is adequately rated for the current, voltage, and fault current levels. Bottom entry may require a different terminal box design or reinforcement.

Clearance

Ensure sufficient clearance inside the terminal box for connecting the bus duct to the transformer bushings.

Support Structure

Confirm the terminal box is structurally capable of supporting the weight and forces imposed by the bus duct. May need to add support to the transformer tank.2.

Bus Duct Design and Installation



Vertical Run

A bottom connection will inherently involve a vertical bus duct run immediately after the terminal box. Consider the support requirements for this vertical section. Bus duct must be adequately supported close to the connection and along its length.

Moisture Protection

Bottom connections are more susceptible to moisture ingress. Implement proper sealing and weatherproofing techniques at the terminal box entry point. Consider condensation drains in the bus duct run.

Seismic Considerations

In seismically active areas, ensure the bus duct and its supports are designed to withstand seismic forces. The vertical run from the bottom connection can be more vulnerable.

Grounding

Properly ground the bus duct and connect it to the transformer's grounding system.

Expansion Joints

Include expansion joints to accommodate thermal expansion and contraction of the bus duct.3.

Coordination and Interference



Cable Trays/Raceways

Ensure the bottom connection and vertical run don't interfere with existing or planned cable trays, raceways, or other equipment.

Underground Utilities

Check for underground utilities (pipes, conduits) that might obstruct the bus duct route.

Access for Maintenance

Maintain adequate access for maintenance and inspection of the bus duct and transformer terminal box.4.

Cooling



Transformer Cooling System

Verify that the bottom connection does not impede the transformer's cooling system (radiators, fans, etc.). Proper airflow around the transformer must be maintained.5.

Safety



Working at Height

Consider the safety implications of working on a vertical bus duct run at a lower level, especially during installation and maintenance. Appropriate safety measures (scaffolding, fall protection) must be in place.

Arc Flash

Perform an arc flash study to determine the arc flash hazard levels and implement appropriate safety measures. The location of the bus duct termination might affect the arc flash boundary.6.

Code Compliance



Local Electrical Codes

Ensure that the design and installation comply with all applicable local electrical codes and regulations. NEC/CEC (National Electrical Code / Canadian Electrical Code) or other relevant codes may have specific requirements for bus duct installations.7.

Documentation



Drawings

Create detailed drawings showing the bus duct route, support locations, grounding connections, and other relevant information.*

Calculations

Document all design calculations, including voltage drop, fault current, and thermal expansion.

Key Steps to Take


Consult Transformer Manufacturer

This is the most important step. Get written confirmation from the transformer manufacturer that a bottom connection is acceptable and what specific requirements they have.2.

Design Review

Have a qualified electrical engineer review the design and installation plans to ensure compliance with codes and standards.3.

Arc Flash Study

Conduct an arc flash study to determine the safety requirements.4.

Detailed Drawings and Documentation

Prepare detailed drawings and documentation for the installation.5.

Qualified Installers

Use experienced and qualified electricians to install the bus duct and make the connection to the transformer.6.

Inspection and Testing: Thoroughly inspect and test the installation before energizing the system.In summary, while a bottom connection is possible, it requires careful planning, coordination, and adherence to safety and code requirements. Starting with consulting the transformer manufacturer is vital. Don't proceed without their approval. Flag for review