Okay, Jim, this is a good catch and definitely warrants careful review. You're right to question this, as the #8 THHN on a 90A breaker feeding a VFD outputting 78A is a serious violation and a potential fire hazard. The fact that the original design was approved doesn't absolve you from making it right.Here's a breakdown of the NEC rules that apply and how to determine the correct conductor size

Key NEC Articles and Concepts:
NEC 430.122 (Conductor Size and Protection – Adjustable-Speed Drive Systems)

This is THE key article for VFDs. It provides specific requirements for conductors supplying adjustable-speed drive systems (VFDs). It states that conductors must have an ampacity not less than 125% of the rated input current of the drive.

Note: You mentioned the VFD had a rated output current of 78A. You need to find the input current rating on the VFD's nameplate. It's almost certainly higher than 78A due to losses in the VFD. NEC 430.52 (Motor Branch-Circuit Short-Circuit and Ground-Fault Protection)

This article deals with the protection of the motor and feeder conductors. While it allows for oversizing protection devices in some motor applications, it doesn't override the conductor ampacity requirements in 430.122 for VFDs.

NEC 310.16 (Ampacities of Insulated Conductors)

This table provides the ampacity ratings for conductors based on insulation type, temperature rating, and installation conditions.

NEC 310.15(B)(1)(c) [formerly 310.15(B)(2)(a)] Ampacity Adjustment for Ambient Temperature

Ambient temperatures above 30°C (86°F) require ampacity derating. Rooftop HVAC units are often in high ambient temperature environments.

NEC 310.15(B)(3)(a) [formerly 310.15(B)(3)(a)] Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

If the conductors are bundled or run in a conduit with other conductors, you need to derate the ampacity. Neutral conductors only carry current under unbalanced load conditions and should be counted when calculating the number of current carrying conductors.

NEC 250 (Grounding and Bonding)

Proper grounding and bonding are critical for safety and performance.

Voltage Drop

Consider voltage drop over the 300-foot run. Excessive voltage drop can cause the motor to overheat and/or not operate correctly.

NEC 110.3(B)

Equipment shall be installed and used in accordance with any instructions included in the listing or labeling. Many VFDs specify a minimum conductor size and/or maximum overcurrent protection based on their input current.

Step-by-Step Analysis and Correction


Determine the VFD's Rated Input Current

This is absolutely crucial. Find the nameplate on the VFD and locate the rated input current (in Amps). Do not assume it's less than the 90A breaker rating. It is very common for the input amperage to be close to, or even greater than the rated amperage of the OCPD supplying the VFD.2.

Calculate the Minimum Conductor Ampacity

Multiply the VFD's rated input current by 125% (NEC 430.122). Let's assume, for example, the VFD input current is 80A. 80A 1.25 = 100A3.

Adjust for Ambient Temperature and Conductor Bundling



Ambient Temperature

Check the typical ambient temperature near the rooftop HVAC unit, especially during the hottest months. Use NEC 310.15(B)(1)(c) to find the appropriate temperature correction factor for your conductor insulation type (e.g., THHN). If, for instance, you need to derate by 0.88, the required ampacity increases: 100A / 0.88 = 113.6A

Conductor Bundling

If the conductors are run in a conduit with other current-carrying conductors, use NEC 310.15(B)(3)(a) to apply the appropriate derating factor. If you have 4-6 current-carrying conductors, the derating factor is 0.
80. 113.6A / 0.8 = 142A4.

Select the Conductor Size

Refer to NEC 310.16 to find a conductor size with an ampacity after all derating that meets or exceeds the calculated value. For example, if you needed 142A, you'd need at least #1/0 THHN. Make sure to consider the termination temperature rating of the equipment (75°C or 90°C) and select the ampacity accordingly.5.

Verify Voltage Drop

Use a voltage drop calculator to ensure that the voltage drop from the panel to the VFD doesn't exceed 3% (a good general guideline). Long runs like 300 feet are very susceptible to voltage drop, especially if you try to use smaller conductors. Increase the conductor size if necessary to stay within acceptable voltage drop limits.6.

Re-evaluate Overcurrent Protection

While the VFD may require a 90A breaker, verify that the selected conductor size can handle that level of protection. The next standard size down from 90A is 80A. If you upsized the conductors to #1/0 THHN, the 90A breaker will be adequate overcurrent protection for your feeder. Always verify that the VFD manufacturer doesn't require overcurrent protection below 90A (refer to VFD manufacturer's specification). If the manufacturer requires overcurrent protection below 90A, you will be limited to the manufacturer's requirements.7.

Grounding and Bonding

Ensure proper grounding and bonding of the VFD and motors, following NEC 250. This is crucial for safety and proper VFD operation.
8.

Documentation

Keep meticulous records of all calculations, conductor sizes, and breaker ratings.

Addressing the Two Motors Shared by One VFD



The impact of the two motors on the VFD's conductor sizing is minimal. The primary concern is the input current required by the VFD to operate both motors. As long as you correctly size the conductors based on the VFD's input current (and apply the NEC 430.122 125% rule), the fact that it's driving two motors is already factored in. Motor Overloads are Critical

The fact that each motor has its own overload protection is good. Make sure the overloads are properly sized according to each motor's nameplate FLA.

Coordination with VFD Parameters

Ensure the VFD's motor parameters (voltage, current, frequency, etc.) are correctly programmed for BOTH motors. The VFD needs to be configured to protect the motors connected to it.

The #10 THHN on the 60A Breaker for the Other VFD

This is another likely violation. Repeat the same analysis as above, starting with the input current of that VFD, and correct the conductor size accordingly.

Important Considerations



Consult a Qualified Engineer

Given the complexity and potential liability, it's highly recommended to consult a qualified electrical engineer to review your calculations and ensure the installation is fully compliant with the NEC and all applicable codes.

Documentation is Key

Thorough documentation of all calculations, conductor sizes, breaker ratings, and grounding/bonding details is essential for future reference and potential inspections.

Inform the Casino/Resort Management

Explain the code violations and safety risks to the casino/resort management and emphasize the importance of correcting the issues.

NEC 90.4 Enforcement

Be aware that the electrical inspector has the final say and may interpret the code differently. Discuss your approach with the inspector before proceeding with the work.

In Summary:

The existing installations are clearly unsafe and not NEC-compliant. Focus on the VFD's input current, apply the 125% factor from NEC 430.122, and then account for ambient temperature, conductor bundling, and voltage drop. Consult with a qualified engineer and the electrical inspector to ensure a safe and compliant solution. Correcting these issues is paramount to protecting people and property. Good luck! Flag for review