You're right to be concerned about the lack of bonding across the expansion joints in a large manufacturing building. While there isn't a single, explicit code section in the NEC that screams "BOND ACROSS EXPANSION JOINTS," the general principles of grounding and bonding, combined with practical considerations for a large manufacturing facility, strongly suggest that bonding these columns is necessary for safety and proper system function. Here's a breakdown of the applicable codes and the reasoning behind it: NEC 250.4(A)(1) Grounding of Electrical Systems

This section emphasizes the fundamental principle of grounding to limit voltage to ground and facilitate the operation of overcurrent protective devices. By bonding all metal parts of the structure together, you're ensuring a low-impedance path to ground, which helps in fault clearing.

NEC 250.4(A)(3) Bonding of Electrically Conductive Materials and Other Equipment

This section requires bonding of non-current-carrying metal parts of equipment to minimize the potential difference between them and other conductive objects in the vicinity. While the columns aren't "equipment" in the traditional sense, they can become energized under fault conditions if electrical equipment fails and grounds to the steel.

NEC 250.104 Bonding of Piping Systems and Exposed Structural Metal

This section specifically mentions bonding exposed structural metal that is likely to become energized. This is because large steel structures can be an efficient pathway for fault currents. NEC 250.104(C) would be specific to bonding other metal parts such as structural columns if it is likely to become energized.

NEC 250.64(E) Continuity of Grounding Electrode Conductor

Although more applicable to the grounding electrode conductor, it speaks to the intent of maintaining a continuous grounding path. Expansion joints disrupt that continuity if not addressed.

Practical Considerations and Engineering Judgment



Fault Current Path

During a ground fault, current will seek the lowest impedance path back to the source. Without bonding across the expansion joint, fault current might be forced to take a less direct and higher impedance path, increasing the risk of voltage potential differences and creating a shock hazard.

Large Manufacturing Facility

Manufacturing buildings typically have a high density of electrical equipment, including machinery, lighting, and distribution panels. The likelihood of a ground fault occurring somewhere within the facility is significantly higher than in a residential building.

Minimizing Potential Differences

The goal of bonding is to minimize voltage differences between different parts of the building's structure and equipment. These differences can be dangerous, especially when personnel are working with or near electrical equipment.

Touch Potential

If a fault occurs and one column is energized while another is not, a worker touching both columns could be exposed to a dangerous touch potential.

Step Potential

A large fault current could cause step potential difference.

Why Bonding is Necessary at Expansion Joints

Expansion joints are designed to allow for movement due to temperature changes, settling, and seismic activity. This movement breaks the electrical continuity of the structural steel. Without bonding jumpers, you essentially have two separate (or at least poorly connected) grounding systems, which can lead to: Increased impedance in the ground fault path. Potential differences between the two sections of the building. Increased risk of electrical shock. Improper operation of overcurrent protective devices.

How to Bond Across Expansion Joints



Bonding Jumpers

Use appropriately sized bonding jumpers made of copper or other approved materials to connect the structural steel columns on either side of the expansion joint. The size of the jumper should be based on the size of the service conductors and the fault current available at the site. Consult NEC Table 250.102(C)(1) as a starting point.

Flexible Connections

Ensure the bonding jumpers are flexible enough to accommodate the expected movement at the expansion joint without breaking or becoming damaged. Braided copper straps or flexible grounding braids are commonly used.

Secure Connections

Use approved connectors and methods to ensure a reliable and low-impedance connection between the bonding jumpers and the structural steel. Clean the steel to bare metal before making the connection.

Redundancy

Consider using multiple bonding jumpers at each expansion joint for redundancy and increased reliability.

Documentation and Engineering Review



Documentation

Keep thorough records of the bonding methods and materials used.*

Engineering Review

It is highly recommended to consult with a qualified electrical engineer to ensure that the bonding system is properly designed and installed, taking into account the specific characteristics of the building and the electrical system. The engineer can perform fault current calculations to determine the appropriate size of the bonding jumpers.

In summary, while there may not be a single line in the NEC that explicitly mandates bonding across expansion joints, the underlying principles of grounding and bonding, combined with the specific conditions of a large manufacturing facility, make it clear that bonding is essential for safety and proper system performance. The AHJ inspector is correct in their assessment. Document this issue in writing, cite the relevant NEC sections (250.4(A)(1), 250.4(A)(3), 250.104), and insist on proper bonding across the expansion joints. Flag for review