You're asking about whether you need to run a grounding conductor through each of the parallel nipples connecting a transformer to a panel, considering they're long running threads with locknuts. Here's a breakdown of the code and best practices, leaning heavily towards your interpretation being the safer and likely correct one

The Key Issue: Are these Nipples Considered Raceways?
Yes, highly likely. The NEC definition of a "raceway" is broad: An enclosed channel designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. While short nipples are sometimes allowed to be used as just support and not raceway (less than 24”), your description of 4" long, threaded nipples running parallel conductors strongly suggests they are being used as raceways. The length and use for parallel conductors solidifies this.

NEC Requirements (Simplified)



250.4(A)(5): Effective Ground-Fault Current Path

Electrical equipment and wiring must be installed in a manner that creates a low-impedance ground-fault current path... This is foundational. A proper ground fault path is essential for safety.

250.96(A): Bonding Other Enclosures

Metal raceways, cable trays, cable armor, cable sheaths, enclosures, frames, fittings, and other metal noncurrent-carrying parts that are likely to become energized shall be bonded together into an effective ground-fault current path.

250.102(C): Grounding Electrode Conductor Connection to Rod, Pipe, or Plate Electrode(s). This section doesn't directly apply to running nipples. It applies to the grounding electrode system which is different. 300.3(B): Conductors of the Same Circuit

All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, cable, or cord. This is crucial. All conductors of the same circuit including the EGC must be in the same raceway.

Why One Ground per Nipple is Generally Not Acceptable


Parallel Paths and Impedance

If you only bond one nipple, the other nipples may not provide a low-impedance path back to the source in the event of a ground fault. This is the critical safety concern. Ground fault current will take all paths back to the source, but you want it to take the lowest impedance path so that a breaker trips quickly. Multiple, independent paths are safer than relying on a single connection.2.

Induced Currents

The conductors running through each nipple will induce currents in the metal of the nipple. Without a proper EGC running in each, this can lead to circulating currents and potentially overheating.3.

Code Compliance

Because the nipples are essentially acting as parallel raceways, each must comply with NEC 300.3(B), that requires all conductors including EGC to be ran together.

What You Should Do (and Why You're Likely Correct)



Run an Equipment Grounding Conductor (EGC) Through Each Nipple. This is the safest and most code-compliant approach. The EGC must be sized according to NEC 250.122, based on the overcurrent protective device protecting the circuit.

Bond Each Nipple Properly

Ensure each nipple is securely bonded to the transformer enclosure and the panel using proper grounding locknuts or bushings listed for grounding.

Verify the Grounding Path

After installation, use a low-resistance ohmmeter to verify a low-impedance ground fault path from the transformer enclosure to the panel ground. A high resistance reading indicates a problem with your grounding connections.

Consult the Inspector

Ultimately, the Authority Having Jurisdiction (AHJ), your electrical inspector, has the final say. Discuss your installation with them before you begin work to ensure it meets their requirements.

In summary, while there might be very specific circumstances where an exception could be argued, the safest, most reliable, and most likely code-compliant method is to run an appropriately sized EGC through each of the parallel nipples and properly bond each nipple at both ends.* Your instincts are correct in this case. Prioritize safety and code compliance. Flag for review