Okay, this is a common issue and requires a careful approach to ensure code compliance and safety. Here's a breakdown of the problem and possible solutions, considering NEC requirements

Understanding the Problem

You're trying to effectively bond multiple parallel raceways (RMC chimney nipples) carrying a large feeder (7 sets of 500 kcmil conductors) back to your cable tray, which is ultimately bonded to the main switchboard. The goal is to create a low-impedance ground-fault current path to trip the overcurrent protection devices (OCPDs) quickly.Your concerns are: Bonding Jumper Size

You're likely finding the bonding jumpers required by NEC 250.122 for the raceways are large, potentially larger than the tap connections on standard bonding bushings can accommodate.

Cable Tray Bonding

You're bonding the cable tray back to the main switchgear, but the conductor size may be problematic.

Practicality

Fitting large conductors into relatively small chimney raceways is challenging.

NEC Considerations 250.4(A)(5) Bonding of Electrically Conductive Materials and Other Equipment

Metal raceways must be made electrically continuous and bonded to provide a path that safely carries any fault current likely to be imposed on them.

250.92(A) Services

This covers the bonding of service equipment, which doesn't directly apply since you have a transformer, but it highlights the importance of grounding and bonding.

250.96 Bonding of Equipment Other Than Service Equipment

Requires bonding of metal parts that are likely to become energized. Allows use of listed fittings (bonding bushings) or methods described in 250.92(B) (jumper with proper connections).

250.102(C) Main and Equipment Bonding Jumpers

Dictates sizing for bonding jumpers within services and separately derived systems. This section typically doesn't apply to feeders within a building sourced from a transformer within the building.

250.122 Equipment Grounding Conductor Sizing

This is the section you're using to size the bonding jumpers, but its applicability here is potentially misapplied. This table is specifically for equipment grounding conductors, which run with the circuit conductors. You are trying to bond raceways, not create a separate EGC pathway.

300.10 Electrical Continuity of Raceways and Enclosures

Requires metal raceways to be electrically continuous.

314.4 Bonding

Addresses bonding of metal enclosures to a grounding electrode conductor or an equipment grounding conductor.

392.60(A) Grounding and Bonding: Metal Cable Trays

Provides specifics for bonding cable trays and says the EGC can be installed inside or securely fastened to the tray.

Proposed Solutions and Analysis1. Rethink 250.122 Application for Raceway Bonding



Important

The NEC generally doesn't require raceway bonding jumpers to be sized based on 250.122 for feeders. That table is intended for sizing the equipment grounding conductor (EGC) that runs with the circuit conductors. You already have the 350 kcmil EGC in the cable tray and the 500kcmil in the nipple/conduit.

Focus on Electrical Continuity

The primary goal for your raceway bonding is to ensure electrical continuity of the raceway system. This means ensuring that the raceways are properly bonded to each other and to a suitable grounding point.

Code Minimum is Key

Using a listed bonding bushing with a conductor attached is a good way to do that. The minimum size is not dictated necessarily by 250.122 but by 250.4(A)(5), ensuring a path that safely carries fault current.

Consider Local Amendments

Check your local jurisdiction. Some may have amendments to the NEC that might mandate specific sizing for these jumpers.2.

Bonding Bushings and Jumpers



Use Listed Bonding Bushings

Continue using listed bonding bushings on each RMC nipple. This is a critical step. Ensure they are UL-listed specifically for grounding/bonding purposes.

Jumper Size Practicality

The real question is jumper size. If the 350kcmil EGC is already in the tray, and 500kcmil EGC in the nipples, then the jumper is just to make the path continous.

Reduce Jumper Size (with Justification)

Given the 350kcmil EGC, You may be able to argue with the AHJ that smaller bonding jumpers, sized appropriately for the available tap on the bonding bushings are sufficient. For example, if you have a bonding bushing that is rated for max #6 AWG, and you use a properly sized compression connector. You are also creating the EGC from the 7 run of 500kcmil.

Document Your Design

Carefully document your reasoning for using smaller bonding jumpers. Provide calculations (fault current calculations) to show that the smaller jumpers are adequate to safely carry fault current and ensure proper tripping of the OCPD.3.

Cable Tray Bonding



Code Minimum is Key

Using a listed bonding bushing with a conductor attached is a good way to do that. The minimum size is not dictated necessarily by 250.122 but by 250.4(A)(5), ensuring a path that safely carries fault current.

Document Your Design

Carefully document your reasoning for using smaller bonding jumpers. Provide calculations (fault current calculations) to show that the smaller jumpers are adequate to safely carry fault current and ensure proper tripping of the OCPD.4.

Raceway Alternatives (If Feasible)



Parallel Grounding Conductors

If space and cost allow, consider running a separate, dedicated EGC alongside each set of 500 kcmil conductors within the RMC. This allows you to size the raceway bonding jumpers based on the individual circuit, rather than the entire feeder.

Larger Raceways

If feasible (though likely expensive), consider using larger RMC conduits or other raceway types that can accommodate larger tap connectors for the bonding jumpers.

Non-Metallic Raceways

Not a great option because of the risk.5.

Consult with a Qualified Electrical Engineer

This situation is complex and requires careful engineering judgment. A qualified electrical engineer can perform fault current calculations, analyze your system, and provide specific recommendations that meet code requirements and ensure safety.

Key Takeaways and Recommendations1. Don't blindly apply 250.122 to raceway bonding. It's more relevant for the EGC that runs with the circuit conductors. The primary goal is electrical continuity of the raceway system.2. Use listed bonding bushings! They provide a reliable and code-compliant connection.3. Consider smaller bonding jumpers with justification. Document your design and reasoning, including fault current calculations.4. Consult with an electrical engineer. They can provide expert guidance and ensure code compliance.

Example Scenario for Smaller Jumpers (Hypothetical - Consult an Engineer)

Let's say your fault current calculations show that even a #6 AWG bonding jumper is more than capable of safely carrying the fault current for the short time it takes the OCPD to trip. You use a listed bonding bushing rated for #6 AWG. You could argue that using a #6 AWG bonding jumper is sufficient, even though 250.122 might suggest a larger conductor if applied strictly. You would need to demonstrate* with calculations and engineering judgment that this is a safe and code-compliant solution.

Important Note: This information is for general guidance only and should not be considered a substitute for professional engineering advice. You are responsible for ensuring that your electrical installation complies with all applicable codes and regulations. Always consult with a qualified electrician and/or electrical engineer before making any changes to your electrical system. Flag for review