Okay, I understand the question. You're installing a VFD (Variable Frequency Drive) or a power panel on a metal strut/stand, and you're wrestling with the requirements of NEC Article 250 regarding bonding. You want to know if you need a separate grounding conductor (like a column ground) to the building grounding system.Here's a breakdown to help you navigate this situation, based on common NEC interpretations

Key Concepts & Considerations:
Bonding vs. Grounding

This is crucial. Bonding connects metallic parts together to create a low-impedance path for fault current. Grounding connects the system to the earth. You are dealing mostly with bonding here.

Purpose of Bonding

To ensure that metallic parts that could become energized due to a fault are at the same potential. This minimizes touch voltage and facilitates the operation of overcurrent protective devices (breakers or fuses) in the event of a fault.

Fault Current Path

You need to provide a reliable, low-impedance path for fault current to return to the source (the transformer or service equipment). This is how you get a breaker to trip.

General Rules & NEC References (Important: Always refer to the current edition of the NEC)


Metal Enclosures are Required to be Bonded

The VFD and power panel enclosures are metallic, and NEC 250.92 and 250.96 generally require them to be bonded.
2.

Strut/Stand as a Bonding Conductor (Possible, but Requires Careful Consideration)

You can potentially use the metal strut/stand itself as the equipment grounding (bonding) conductor, BUT this is where the details matter.

Reliable Connection

The strut/stand must have a reliable, low-impedance connection to the VFD enclosure, the power panel enclosure, and to the building's grounding electrode system (or other suitable grounding point – see below). This means: Properly tightened hardware (bolts, nuts, washers). Clean, paint-free contact surfaces. Removing paint or coatings at the connection points is essential. Consider using listed grounding/bonding bushings or washers designed for this purpose. Avoid relying solely on pressure contact between dissimilar metals (e.g., aluminum and steel) without proper surface preparation and anti-corrosion compounds.

Continuous Electrical Path

The strut/stand must be electrically continuous. If it's multiple sections, you need to ensure each section is bonded to the next using bonding jumpers or other approved methods.

Adequate Ampacity

The strut/stand must be of sufficient size and construction to safely carry the maximum available fault current. This is usually not a problem for typical strut, but it's something to consider. If you have a very high fault current potential, it might be necessary to supplement the strut with a bonding conductor.3.

Supplemental Bonding Conductor (Often Recommended, and Sometimes Required)

In many cases, it's best practice, and sometimes required, to run a separate bonding conductor (e.g., a green insulated wire) in addition to relying on the strut/stand itself. This provides a more reliable and dedicated low-impedance path.

When Required (Check NEC 250)

If the strut/stand connection is not deemed "reliable" enough. (This is a judgment call, but erring on the side of caution is always wise). In areas with high corrosion potential, a separate conductor offers better long-term reliability. If the strut/stand is not inherently electrically continuous.

Where to Connect the Supplemental Conductor



Building Grounding Electrode System

Ideally, this is the best connection point.

Other Suitable Grounding Point

This might include the metal frame of the building, a nearby metal water pipe (used as a grounding electrode), or the grounding bus in a nearby panelboard that is properly grounded. Ensure the grounding point you choose is, in fact, a properly established and maintained grounding electrode system point.



Sizing

The size of the bonding conductor is determined by NEC Table 250.102(C)(1) or 250.66, based on the size of the service entrance conductors or the size of the overcurrent device protecting the circuit feeding the VFD/power panel.
4.

Important Considerations for VFDs



VFDs and Noise

VFDs are notorious for generating electrical noise. Proper grounding and bonding are critical for minimizing noise and ensuring proper VFD operation. Consult the VFD manufacturer's recommendations for grounding and bonding. They often have specific requirements that go beyond the basic NEC minimums.

Dedicated Grounding Conductor

Some VFD manufacturers require a dedicated grounding conductor run back to the source panel, separate from any other grounding paths. This is to minimize noise and ensure proper operation of the VFD's internal filtering. Always follow the manufacturer's instructions.

In Summary


Bonding is Essential

You must bond the VFD and power panel enclosures to ensure a low-impedance fault current path.2.

Strut/Stand Can Potentially be Used

The strut/stand can potentially be used as part of the bonding path, if you ensure a reliable, low-impedance, and continuous connection.3.

Supplemental Bonding Conductor is Recommended (and Often Required)

It is highly recommended to run a separate bonding conductor from the VFD/power panel (or the strut/stand) to the building grounding electrode system or a suitable grounding point. This provides a more reliable and robust grounding path, especially for VFD applications.4.

Follow VFD Manufacturer's Instructions

Always consult and follow the VFD manufacturer's grounding and bonding instructions. These are often more stringent than the basic NEC requirements.5.

Consult with a Qualified Electrician

If you're unsure about any aspect of the grounding and bonding, consult with a qualified electrician. Improper grounding can be dangerous and can cause equipment malfunction.

Example Scenario

Let's say you're installing a VFD on a strut stand. Here's how you might approach it:1.

Mount the VFD

Securely mount the VFD to the strut stand.2.

Clean Contact Points

Clean the paint or coatings from the strut stand where the VFD mounting feet will make contact.3.

Install Bonding Jumpers (if needed)

If the strut stand is in sections, install bonding jumpers between the sections to ensure electrical continuity.4.

Run a Supplemental Bonding Conductor

Run a green insulated bonding conductor (sized according to NEC Table 250.102(C)(1)) from a grounding lug on the VFD enclosure to the building's grounding electrode system (e.g., to the main grounding bus in the service entrance).
5.

Tighten all connections

Ensure all connections are tight and secure.6.

Follow VFD Manufacturer's Instructions

Double-check the VFD manufacturer's grounding and bonding requirements and follow them meticulously.

Disclaimer: I am an AI chatbot and cannot provide electrical code advice. This information is for general guidance only. Always consult the current edition of the NEC and a qualified electrician for specific code interpretations and requirements in your jurisdiction. Electrical work can be dangerous, and improper grounding can lead to serious injury or death. Flag for review