You're likely correct. Let's break down the NEC requirements and best practices.Here's the reasoning why you likely need a local disconnect: NEC 430.102 Location

This section mandates a disconnecting means for the motor and controller (VFD). It specifies that the disconnecting means must be "within sight" of the motor controller (VFD) location.

"Within sight" is defined in Article 100 as "Visible and not more than 50 feet distant." Accessibility

Even if technically "within sight" (and 20' up might arguably be within sight if nothing is obstructing the view), general principles of safety and practicality dictate accessibility. An 800A disconnect 20 feet in the air is not accessible for routine maintenance, troubleshooting, or emergency shutdown.

Serviceability

The whole point of a disconnect is to de-energize the equipment for safe servicing. A 20' high disconnect renders the VFD practically unserviceable without specialized equipment (lifts, etc.) every time, increasing the likelihood of unsafe practices.

Lockout/Tagout (LOTO)

OSHA regulations require effective lockout/tagout procedures. It's much easier and safer to implement LOTO when the disconnect is readily accessible at the VFD.

Emergency Shutdown

In emergency situations, immediate access to a disconnect is crucial. Waiting to get a lift to reach a high-mounted disconnect could have dire consequences.

Here's why your coworker might be questioning it (and potential counterarguments)



The bus duct disconnect might be considered the motor disconnect

If the bus duct disconnect is both within sight of the VFD and lockable, some might argue that it satisfies the basic NEC requirement. However, this is a very weak argument considering the accessibility and serviceability issues.

Counterargument

Accessibility is paramount. The intent of the code is to ensure a safe and practical means of disconnecting power. A high-mounted disconnect fails on both counts.

Counterargument

Lockability may be questionable. The bus duct disconnects often require additional hardware to lock out, making it less convenient and prone to improper lockout practices.

Cost

Adding another 800A disconnect is a significant expense.

Counterargument

Safety and code compliance outweigh cost considerations. The cost of an accident or injury far exceeds the cost of the disconnect.

In Conclusion

While technically, you might find an interpretation where the high-mounted disconnect could* be argued as compliant with minimum code requirements,

a readily accessible, local disconnect next to the VFD is the safest and most practical solution and the best practice. You are right. It is also much easier to justify to an inspector.

To strengthen your argument


Cite NEC 430.102 focusing on "within sight" but also emphasizing the spirit of safety and practical operation that underlies the code.2. Mention OSHA LOTO requirements.3. Stress the safety implications of having to use lifts or ladders to de-energize the VFD for service.4. Consider local amendments or interpretations. Some jurisdictions have stricter interpretations than the national code.

Recommendation: Have a discussion with your supervisor or engineer, clearly outlining the safety and practical benefits of a local disconnect. It's better to err on the side of caution and install the additional disconnect. You might also ask the local electrical inspector for their opinion before proceeding. Flag for review