Okay, let's break down the grounding requirements for this multi-motor, multi-drive system. This is a common situation, and it's crucial to get it right for safety and code compliance.

Understanding the Grounding Principles Equipment Grounding Conductors (EGCs)

The purpose of the EGC is to provide a low-impedance path back to the source (typically the transformer) to facilitate the operation of the overcurrent protective device (OCPD) in the event of a ground fault. This low impedance ensures the OCPD trips quickly, minimizing the duration and severity of the fault.

Table 250.122

This table in the NEC (National Electrical Code) is your primary guide for sizing EGCs based on the rating of the overcurrent protective device protecting the circuit.

Series vs. Parallel Considerations

When multiple pieces of equipment are fed from a single OCPD via a wireway, the grounding must be carefully considered. The goal is to ensure each piece of equipment has a reliable and adequately sized EGC.

Your Specific Situation and Grounding Requirements

Here's how I recommend approaching the grounding in your setup, based on the information you provided:1. Feeder Ground to the Wireway (Important)



Yes, you need a #4 ground wire to the wireway. Table 250.122 dictates this. Since the 300A circuit breaker is protecting the feeders going to the wireway, the entire feeder requires a #4 AWG copper EGC (or equivalent). This #4 EGC must originate at the service equipment or separately derived system (transformer). This provides the fault current path for any faults occurring upstream of the drives. This #4 EGC should be landed securely in the wireway, providing a reliable grounding point for the drive branch circuits.2. Grounding Conductors to Each Drive



Taps from the Wireway

Each tap feeding a drive is now essentially its own branch circuit.

Here's the critical point

The EGC size for each drive is based on the rating of the overcurrent protection device protecting that drive (in your case, the internal 80A fuses in the drive).

Applying Table 250.122

For an 80A OCPD, Table 250.122 would normally require a #8 AWG copper EGC. However, the code allows you to use a smaller EGC if it is part of a listed flexible cord or cable assembly, so consult your cable specifications.

Therefore, you will need a #8 EGC from the wireway to each drive. This #8 EGC must be run with the power conductors feeding each drive.3. Motor Grounding



Individual Motor Overload Protection

You mentioned providing individual short-circuit and overload protection for each motor. This is essential. Typically, this is done within the drive itself, but you need to verify that this is the case. You must comply with NEC 430.52 and 430.
53.

Sizing the Motor EGC

The EGC for each motor is based on the rating of the motor branch-circuit overcurrent protection device protecting that motor. Since you are providing short circuit and overload protection to each individual motor, the drive output short circuit protection becomes the OCPD in question for the motor EGC. Since the drives are rated for 44 amps output each, and each drive controls two motors, we can estimate that the drives are each protecting a motor with a FLA of 18.2 amps with no more than 22 amps (44 amps / 2 motors = 22 amps per motor). Using Table 250.122, a #12 wire would likely be sufficient if using the standard 125% multiplier for motor protection. Consult the drive specifications and documentation to verify this information to be safe, and verify that the drive output current is considered the branch-circuit protection device for the motor, and not an additional level of overload protection.

Conduit/Cable Considerations

If the motor is connected using metallic conduit (e.g., EMT, rigid), the conduit itself can serve as the EGC, provided it's properly installed with listed fittings. If you're using cable (e.g., MC cable), the cable must contain an appropriately sized EGC. If you are using nonmetallic conduit, an EGC will be required. The EGC for the motor should be connected to the motor frame and back to the drive.

In Summary



Wireway Feed

#4 AWG EGC from the source (panel with the 300A breaker) to the wireway.

Wireway to Each Drive

#8 AWG EGC from the wireway to each drive, run with the power conductors.

Each Drive to Each Motor

A properly sized EGC from the drive to each motor, based on the motor branch-circuit overcurrent protection device (likely within the drive or external if the drive is only providing overload protection and not short circuit protection), but likely #12 AWG, run within the conduit or cable.

Important Considerations and Recommendations



Local Codes and AHJ

Always check with your local electrical inspector (AHJ - Authority Having Jurisdiction). They have the final say on code interpretation and may have specific requirements.

Manufacturer's Instructions

Always follow the manufacturer's installation instructions for the drives and motors. They may have specific grounding recommendations or requirements.

Clean Grounding

Ensure all grounding connections are clean, tight, and properly made. Use appropriate grounding lugs and connectors.

Documentation

Keep detailed records of your grounding system, including wire sizes, connection points, and any calculations you performed. This will be helpful for future maintenance and troubleshooting.

Professional Review

If you are not completely confident in your understanding of these requirements, I strongly recommend having a qualified electrical engineer or master electrician review your design and installation. Grounding is a critical safety issue, and it's worth getting it right.*

Grounding Electrodes: Ensure the grounding system is properly connected to a grounding electrode system (e.g., ground rods, building steel, concrete-encased electrode) as required by the NEC.By following these guidelines and consulting with qualified professionals, you can ensure a safe and code-compliant grounding system for your multi-motor, multi-drive air handler. Good luck! Flag for review