You've hit upon a very common point of confusion and a bit of an art within electrical engineering. You're correct that NEC 250, while comprehensive, doesn't directly address bonding conductor sizing for non-electrical equipment without a circuit feeding it and, therefore, no overcurrent protection (OCPD). Let's break this down and address your questions.

Understanding the Goal of Bonding Non-Electrical Equipment

First, it's crucial to understand why we bond non-electrical equipment to the grounding system. It's not primarily for clearing a fault from a circuit feeding the equipment (as that's not applicable here). The main reasons are: Equalizing Potential

To minimize potential differences between the equipment and other grounded structures/systems, reducing the risk of touch voltage hazards during a fault elsewhere in the system. This is especially important in industrial settings where there are many interconnected metallic structures.

Minimizing Stray Currents

Providing a low-impedance path to the ground for stray currents to flow safely.

Lightning Protection

In facilities with lightning protection systems, bonding is critical to providing a low-impedance path for lightning currents to ground, minimizing damage and personnel hazards.

Static Discharge

While less of a primary concern in many industrial settings (unless dealing with flammable materials), bonding helps dissipate static electricity.

Compliance with Industry Standards

Many industries (e.g., petrochemical, oil and gas, chemical) have internal standards or follow external standards that prescribe minimum bonding conductor sizes.

Why #2 AWG vs. #2/0 AWG – The Engineering Judgment Call

The senior designer's answer about it being up to the engineer to decide is both frustratingly vague and, unfortunately, somewhat accurate. Here's a more detailed explanation of the factors involved:1. Ground Fault Current Considerations (Indirectly)

While you're correct that there's no OCPD directly associated with the equipment, consider the potential ground fault currents that could flow through it in the event of a fault elsewhere in the system. Larger equipment is often exposed to a larger area and may have a higher probability of becoming involved in a fault from nearby electrical equipment. Larger equipment can also carry higher fault current if it becomes an indirect path for fault current due to poor installation practices. This isn't a precise calculation (since it depends on the entire facility's grounding system impedance and fault current levels), but it's a factor in the decision.2.

Physical Size and Contact Area

Larger equipment, like tanks and vessels, typically have a much larger physical size and contact area with the earth (or other grounded structures). This makes them more susceptible to induced voltages from nearby electrical faults or lightning strikes.3.

Corrosion

Larger conductors offer better resistance to corrosion, especially in harsh industrial environments. This is crucial for maintaining the integrity of the grounding system over time.4.

Mechanical Strength

Larger conductors are physically stronger and less likely to be damaged by mechanical stress, vibration, or accidental impact.5.

Industry Practice

In many industries, #2/0 AWG is often considered a minimum size for bonding conductors for significant pieces of non-electrical equipment, particularly in hazardous locations. This is often driven by internal safety standards and past experience.6.

Impedance

Larger conductors have lower impedance which provides better fault current carrying capabilities.

Guidance and Considerations for Sizing

While there's no hard-and-fast rule in the NEC for non-electrical equipment without circuits, here's how to approach the decision: Consult Existing Plant Standards

The most important thing to do is review your company's internal standards or specifications. These often have guidelines for bonding conductor sizes based on equipment size, location, and industry best practices.

Consider the Environment



Hazardous Locations

In classified hazardous locations (Class I, II, or III), more robust bonding is crucial to minimize the risk of ignition. #2/0 AWG is often a default minimum.

Corrosive Environments

Use larger conductors and/or more corrosion-resistant materials (e.g., copper or stainless steel) in corrosive environments.

Outdoor Locations

Outdoor equipment is more susceptible to lightning strikes and environmental degradation.

Assess the Size and Importance of the Equipment



Small Steel Columns

#2 AWG is often acceptable for small, structurally insignificant steel columns.

Large Steel Columns/Building Steel

Larger columns, especially load-bearing columns, benefit from larger bonding conductors (e.g., #2/0 AWG).

Tanks and Vessels

Typically, #2/0 AWG or larger is used for tanks and vessels, especially those containing flammable or hazardous materials. For very large tanks or vessels, you might even consider multiple bonding points with #2/0 AWG conductors.

Consider the Grounding System Impedance

A robust, low-impedance grounding grid reduces the potential difference between the equipment and ground during a fault. If the grounding grid impedance is relatively high, using larger bonding conductors can help compensate.

Documentation

Always document your rationale for the chosen bonding conductor size. This includes the factors you considered (equipment size, location, environment, industry standards) and any calculations or estimations you made. This is critical for future reference and safety audits.

NEC References (Indirectly Relevant)

While the NEC doesn't directly dictate the size in your case, these sections provide related guidance: NEC 250.102(E)

Sizing bonding jumpers on the supply side of service equipment. While not directly applicable, it provides a framework for sizing based on the size of the service conductors. You could loosely consider the size of the main service conductors feeding the facility when making your decision.

NEC 250.104(A)

Bonding of metal water piping systems. While a different application, it emphasizes the importance of bonding metal piping systems to the grounding electrode system.

NEC 250.104(B)

Bonding of other metal piping systems and exposed structural steel. This is closer to your situation. While it refers to the electrical system, it underscores the general principle of bonding to minimize potential differences.

NEC 250.66

Grounding Electrode Conductor sizing. This section outlines the size of the conductor connecting the grounding electrode system to the service equipment. This is not directly applicable, but it helps understand the scale of grounding conductors in general.

NEC 250.106

Sizing for Data Processing Systems. This section provides guidance for installations where noise and voltage fluctuation are of high concern.

NEC 250.130(C)

Sizing for Equipment Grounding Conductors. This section pertains to sizing EGCs based on the rating of the overcurrent protection device protecting the supply circuit.

In Summary

Sizing bonding conductors for non-electrical equipment is a judgment call based on a combination of factors, including: Your company's internal standards. The environment (hazardous, corrosive, outdoor). The size and importance of the equipment. The potential for fault currents. Mechanical strength and corrosion resistance. Industry practice.If no company specifications are in place, #2/0 AWG provides an adequate bonding conductor for large equipment. For small equipment such as small steel columns, #2 AWG is sufficient. When in doubt, err on the side of caution and use a larger conductor size. Always document your decision-making process. Flag for review