You are correct. The NEC, in general, does not require an additional, external bonding conductor from the ground grid loop to the motor frame when the motor frame is already bonded via the Equipment Grounding Conductor (EGC) in the motor termination box.Here's a breakdown of why: NEC 430.244 (Grounding All Fixed Metal Parts)

This section requires that all exposed non-current-carrying metal parts of motor-driven equipment be grounded. This includes the motor frame.

NEC 250.134 (Equipment Fastened in Place or Connected by Permanent Wiring Methods)

This section details the methods for grounding equipment that is fastened in place or connected by permanent wiring methods (like most motors). It allows for grounding via: (A) Grounding through the metal raceway (if properly installed). (B)

An equipment grounding conductor (EGC) run with the circuit conductors. This is the most common method, and the one you described (the EGC in the motor termination box). The purpose of grounding is to create a low-impedance path for fault current. This allows overcurrent protective devices (circuit breakers or fuses) to quickly trip and clear a fault, minimizing the risk of electric shock and fire. A properly sized EGC, installed according to the NEC, provides this low-impedance path.

Why would someone think an additional external conductor is needed?

There are several potential reasons why someone might believe an additional external conductor is necessary, even though the NEC doesn't typically require it: Misunderstanding Equipotential Bonding

Equipotential bonding aims to minimize voltage differences between metallic objects within a specific area. While equipotential bonding can improve safety, it's often confused with grounding. Equipotential bonding focuses on reducing voltage differences, while grounding focuses on providing a path for fault current back to the source. In some situations, especially in areas with sensitive electronics or hazardous environments, supplementary bonding might be implemented in addition to the code-required grounding, but it's usually not mandated by the NEC for general motor installations.

Industry Practices

Certain industries (e.g., petrochemical, mining) might have internal standards or best practices that exceed the minimum requirements of the NEC. These standards might call for additional bonding for enhanced safety or reliability.

Perceived High Fault Current

In systems with very high available fault current, some engineers might specify supplementary bonding to further reduce impedance and ensure the overcurrent device trips reliably and quickly. This is usually done as part of a comprehensive engineering analysis.

Noise Reduction

In some cases, supplementary bonding is used to reduce electrical noise, particularly in sensitive electronic systems. However, this is more of a signal integrity consideration than a safety issue.

Misinterpreting NEC 250.92 (Bonding Other Enclosures)

This section requires bonding of metal enclosures containing service conductors and the service equipment enclosure. This section is not related to motors, but there may be a confusion.

Important Considerations



Proper Sizing

The EGC must be properly sized according to NEC Table 250.122, based on the rating of the overcurrent protective device protecting the motor circuit.

Continuous Path

The EGC must provide a continuous, low-impedance path back to the source. This means that all connections must be tight and corrosion-free.

Other Equipment

The discussion above only applies to the motor frame. Other equipment associated with the motor (e.g., control panels, starters) will have their own grounding requirements based on their individual listings and the NEC.

Local Codes and AHJ

Always check with the local electrical inspector (AHJ) to ensure compliance with local codes and interpretations. The AHJ has the final say on code interpretation.*

Equipotential grids in substations: This type of grid is often installed in substations for personnel protection in the event of a fault. As mentioned above, the equipotential grid reduces voltage differences and supplemental bonding may be used.

In conclusion, unless there are specific industry standards, engineering calculations, or local code requirements to the contrary, the NEC does not require an additional external bonding conductor from the ground grid loop to the motor frame when the motor frame is already bonded via the Equipment Grounding Conductor (EGC) in the motor termination box, provided the EGC is properly sized and installed. It's always a good idea to document the basis for your design decisions, including references to the NEC and any other relevant standards. Flag for review