Okay, let's break down the NGR impedance bonding jumper requirement and how it applies to your parallel conductor scenario. You're right to be careful, as proper grounding is critical for safety and system protection.

Understanding the Impedance Bonding Jumper

The purpose of the impedance bonding jumper in an NGR system is to provide a low-impedance path for fault current from the equipment grounding system back to the neutral point of the transformer through the NGR. It's essential for the NGR to effectively limit the fault current and prevent overvoltages during a ground fault.The stipulation about not splicing and connecting directly from the first point of disconnect to the grounded side of the NGR is there to ensure this low-impedance path is maintained and not compromised by poor connections or unnecessary lengths of conductor. It also aims to minimize circulating currents.

Your Specific Scenario and Options

You have two parallel sets of 500 kcmil conductors from the transformer secondary to the switchgear (first point of disconnect), with each set having its own Equipment Grounding Conductor (EGC). You also have the NGR in a separate enclosure near the transformer.Let's analyze the options: Option 1: Combining EGCs at the Transformer and Running One Impedance Bonding Jumper (Your Proposed Solution) Pros

Simpler installation, potentially shorter total conductor length to the NGR.

Cons

This is where the "no splicing" rule comes into play and presents a challenge. Technically, if you are splicing these EGCs together, it might violate the intent of the code since the impedance bonding jumper technically doesn't start at the first point of disconnect. Additionally, it also might create circulating currents in the event of a ground fault.

Option 2: Running Each EGC Directly to the NGR Grounded Connection Pros

Complies more directly with the requirement that the impedance bonding jumper extends from the first point of disconnect to the grounded side of the NGR without splicing.

Cons

Requires more conductor and potentially a more complex installation.

Additionally

An additional equipment grounding conductor should be ran to the transformer frame, to provide a low impedance path for a fault that may originate at the transformer.

Recommendation and RationaleThe best and most compliant approach is Option 2: Run each Equipment Grounding Conductor from the switchgear's ground bus (first point of disconnect) directly to the NGR grounded connection separately without splicing. In addition, run an equipment grounding conductor from the transformer to the NGR.

Here's why


Compliance

This clearly adheres to the requirement of an uninterrupted impedance bonding jumper from the first point of disconnect to the NGR.2.

Low Impedance

Each EGC acts as its own low-impedance path. Combining them at the transformer, while electrically equivalent in some ways, can introduce potential impedance issues at the splice/connection point, especially during a high-fault current event.3.

Redundancy

Running separate EGCs provides a degree of redundancy in the grounding system.4.

Reduced Circulating Current

This provides a dedicated path, which reduces circulating currents.

Important Considerations



Size the EGCs Appropriately

The size of each EGC must be adequate to handle the available fault current and comply with the NEC tables (typically 250.122, although local codes may have amendments). Consider voltage drop across the run for larger faults.

Connection Quality

Ensure all connections are clean, tight, and properly made using listed connectors suitable for grounding applications. Use appropriate anti-oxidant compound.

Documentation

Thoroughly document the grounding system design and installation. This is critical for future maintenance and troubleshooting.

Local Codes and AHJ

Always check with your local authority having jurisdiction (AHJ) for any specific interpretations or requirements regarding NGR grounding. They may have stricter guidelines or interpretations that you need to follow. They have the final say.

In Summary:

While combining the EGCs at the transformer and running a single impedance bonding jumper might seem like a shortcut, the safest and most compliant approach is to run each EGC directly from the switchgear to the NGR grounded connection. And always provide a ground path for the transformer in addition to the bonding jumpers.By following these guidelines, you'll ensure a reliable and safe medium-voltage system with proper ground fault protection. Flag for review