Good morning! I understand you want to ensure you're correctly connecting, grounding, and bonding the MIGBs (Main Interconnecting Grounding Busbars) to a 225 kVA transformer. This is a critical safety issue, and I'll help you understand the basics. However, I am not a substitute for a qualified electrician or engineer. This information is for educational purposes only. ALWAYS consult with and hire a licensed electrician for this type of work. They are trained to ensure safety and code compliance.Key Concepts



Grounding

Provides a low-impedance path for fault current to return to the source (transformer), allowing overcurrent protection devices (breakers/fuses) to operate quickly and safely.

Bonding

Connects all non-current-carrying metallic parts (enclosures, raceways, etc.) to create an electrically continuous path, minimizing voltage differences and the risk of shock.

MIGB (Main Interconnecting Grounding Busbar)

A central point for grounding and bonding connections. It ensures all grounding and bonding conductors are connected to a common point.

General Steps (Consult an Electrician for Specifics)


Transformer Grounding



The Transformer's Neutral Point

The neutral point of the transformer's secondary (usually the wye point) must be grounded. This is typically done with a

Grounding Electrode Conductor (GEC). This GEC connects to a suitable grounding electrode system (ground rods, metal water pipe, building steel, etc.). The size of this GEC is determined by the size of the transformer and the applicable electrical code (NEC in the US). Transformer Enclosure

The transformer enclosure must be bonded to the grounding system. Usually, this involves a bonding jumper from the transformer enclosure to the MIGB.2.

MIGB Connections



Location

The MIGB should be located close to the transformer and readily accessible.

Incoming GEC

The GEC from the grounding electrode system connects to the MIGB.

Neutral-to-Ground Bond (if applicable)

If the transformer is serving as the service equipment, the neutral conductor is bonded to the grounding system at the transformer (or the first disconnecting means). This is where you'd typically bond the neutral conductor to the MIGB. Important: Only bond the neutral to ground once in a system. Multiple neutral-to-ground bonds can cause circulating currents and safety hazards.

Equipment Grounding Conductors (EGCs)

EGCs from all downstream panels, equipment, and raceways connect to the MIGB. These EGCs provide a path for fault current to return to the source.

Bonding Jumpers

Bonding jumpers connect the MIGB to any nearby metallic parts that could become energized during a fault, such as metal conduits, building steel, or other enclosures.3.

Sizing Conductors



GEC

Sized according to the NEC table 250.66 (in the US). It is based on the size of the ungrounded service conductors or equivalent area for parallel conductors.

EGCs

Sized according to NEC table 250.122 (in the US), based on the rating of the overcurrent protection device (breaker/fuse) protecting the circuit.

Bonding Jumpers

Typically sized according to NEC 250.102 (in the US).
4.

Materials

Use appropriately rated and listed connectors, lugs, and conductors for grounding and bonding. Copper conductors are commonly used, but aluminum conductors are permitted in some cases. Ensure all connections are clean, tight, and corrosion-resistant. Anti-oxidant compound is recommended for aluminum connections.

Simplified Diagram (Conceptual - Consult an Electrician for Specifics)

Grounding Electrode System(Ground Rods, etc.)|| GEC (Grounding Electrode Conductor)|+-------------------+|       MIGB        | <--- Main Interconnecting Grounding Busbar+-------------------+/  |        |  \/   |        |   \/    |        |    \/     |        |     \/      |        |      \/       |        |       \EGC to    EGCs to   Bonding    Neutral-to-GroundDownstream   Panels/    Jumpers      Bond (if applicable)Panels     Equipment  to Enclosures  (only at service)& Raceways