Okay, I can help you find the relevant section in NEC 2017 regarding bonding locknuts at service entrances (although, strictly speaking, this might not be a "service entrance" depending on the exact configuration, but the principle is the same).The primary section you're looking for deals with effective grounding paths and ensuring proper bonding of metallic enclosures. Here's how to approach finding the specific code section:1. Key Concepts



Effective Grounding Path

This is crucial. The entire system needs a low-impedance path back to the source (utility transformer) so that overcurrent devices (breakers/fuses) will trip quickly in the event of a fault.

Bonding

Connecting metallic parts together to create that effective grounding path and equalize voltage potential.

Service Entrance vs. Separately Derived System

A service entrance is where the electrical service enters your building from the utility. Since you are dealing with a generator and an automatic transfer switch, this could be considered a separately derived system. However, the bonding principles remain the same.2.

NEC 2017 Sections to Consider



250.92 Equipment Bonding Jumpers

This section covers the required use of equipment bonding jumpers to ensure the electrical continuity and the capacity to conduct safely any fault current likely to be imposed. Pay particular attention to sections involving "rigid metal conduit" or "intermediate metal conduit". Since the supply to your ATS appears to be in conduit, this is highly relevant. Look for requirements that specifically state that where conduit enters a box or enclosure, it must be bonded. A bonding locknut (or other listed bonding fitting) is a common way to achieve this.

250.94 Bonding for Other Systems

This section addresses bonding of separately derived systems, which your generator and ATS would be. This section requires grounding and bonding be done in accordance with part V of article 250. When dealing with generators, the generator frame must be connected to the grounding electrode system, and the grounded conductor (neutral) must be bonded to the generator frame.

314.4 Electrical Continuity of Metal Raceways and Enclosures. 314.4(A) says, "Metal raceways and metal armor of cables shall be mechanically joined to boxes, fittings, cabinets, and equipment so as to provide effective electrical continuity.
3. Why a Bonding Locknut Matters

A standard locknut relies on tight contact and clean surfaces. Over time, corrosion, vibration, or loose tightening can compromise that contact. A bonding locknut has a sharp tooth or a bonding jumper provision that bites through any surface oxidation or paint, creating a more reliable, low-impedance connection.

Example of How to Explain it to Your Contractor

"The electrical inspection failed because the supply entrance to the automatic transfer switch (ATS) requires proper bonding according to NEC 2017. Specifically, sections 250.92 and 314.4(A) require the use of a bonding locknut or other listed bonding fitting to ensure a continuous, low-impedance ground path for fault current. A standard locknut is insufficient because it relies on surface contact that can degrade over time. Please install a bonding locknut at the supply entrance to the ATS to comply with the NEC and pass inspection."

Important Considerations



Local Amendments

Your local jurisdiction may have amendments to the NEC 2017. Check with your local building department to see if there are any modifications to these sections.

Inspector's Interpretation

Ultimately, the inspector's interpretation of the code is what matters for passing the inspection.

Qualified Electrician

If your contractor remains unresponsive, consider hiring a different, qualified electrician to correct the issue and provide documentation of the correction. This could be necessary to get the system approved.

Grounding Electrode Conductor: Make sure that the GEC connection to the generator and ATS is properly sized and installed, as this is also crucial for proper bonding.By providing the contractor with these specific code sections, you are giving them a clear understanding of the requirements and why the bonding locknut is necessary. Good luck! Flag for review