Okay, let's break down these questions about Myers hubs, grounding, bonding, and hazardous locations in the context of an MCC.

Understanding Myers Hubs and Bonding The Issue

A standard Myers hub's primary function is to provide a watertight seal and secure mechanical connection for conduit entering an enclosure. While it may incidentally provide some electrical continuity, it's not designed or tested to be a reliable, low-impedance fault current path. Therefore, it's not listed and labeled as a bonding fitting.

Continuity vs. Bonding

This is a critical distinction. Continuity simply means there's an electrical path. Bonding means that path is specifically designed and maintained to have a low impedance to safely carry fault current back to the source for a circuit breaker to trip.

Why Continuous Pipe Doesn't Guarantee Bonding

You mentioned continuous pipe. While the pipe itself might be electrically continuous, the connections at each end (using the Myers hub) are the weak points. Factors like paint, corrosion, loose tightening, and the hub's inherent design can introduce impedance. During a fault, this impedance can cause: Voltage drop, which can delay or prevent overcurrent device operation. Heat generation, which is a fire hazard. Elevated touch potential on the conduit system.

Answers to Your QuestionsQuestion 1: How is the path to ground considered continuous with a Myers hub even though it's not considered bonded? Based on 250.97 would I have to have a ground bushing for anything a Myers hub goes into above 250volts even if it is not a concentric or eccentric knockout? (I knock out 95% of my holes with a KO punch or step bit) Answer

The path to ground isn't considered reliably continuous for fault current purposes with a Myers hub alone. That's the whole point of the bonding requirement. Just because you could measure continuity with an ohmmeter doesn't mean the connection is suitable for handling fault current.

NEC 250.97 and Bonding Jumpers

NEC 250.97 requires bonding jumpers in several situations:

Service Equipment

Bonding jumpers are always required for service equipment to ensure a low-impedance path back to the neutral.

Concentric/Eccentric Knockouts

This is the common one you hear about. If using these types of knockouts, the raised rings or displaced metal create a potential impedance issue, so bonding is explicitly required regardless of voltage.

Over 250 Volts to Ground

This is the key part for your question.

If you have equipment operating at over 250 volts to ground (phase to ground voltage), and your raceway system (the conduit and fittings) isn't designed to make a reliable electrical connection, you MUST use bonding jumpers around the raceway connection. Therefore, in your situation with the MCC, since Myers hubs are not listed for bonding, YES, according to 250.97, any conduit entering the MCC that carries conductors over 250 volts to ground needs to be bonded using a listed fitting, bushing, or jumper, regardless of whether you used a concentric/eccentric knockout. The fact that you used a KO punch or step bit doesn't change the requirement. The connection made by the hub is the issue.

Question 2: When I leave the MCC to go to a hazardous area, do all my pipes entering that MCC going to the hazardous location (not including the service conductors) need to be mechanically bonded in the head section of the MCC since Myers hubs are not considered a bond? Would I need a bonding bushing to make this code compliant since nearly all of them are 250 volt or higher? Answer



YES. This is absolutely crucial for safety in a hazardous location. Here's why: Hazardous Location Requirements

Hazardous locations have stricter grounding and bonding requirements because sparks or hot surfaces from a ground fault could ignite flammable or explosive atmospheres. Bonding minimizes the potential for voltage differences (and therefore sparks) between equipment enclosures and conduit systems.

Intrinsic Safety (IS)

Even if you're using intrinsically safe circuits in the hazardous area, proper bonding is still important. IS relies on limiting energy in normal and fault conditions. Poor grounding/bonding can compromise the IS system by allowing voltage potentials to develop.

Article 500+

In addition to 250.97, refer to NEC Article 500 and the applicable sections for your hazardous location classification (Class I, II, or III, and Divisions/Zones). These sections often explicitly require bonding, regardless of voltage, in hazardous locations.

MCC as a Transition Point

The head section of the MCC where the conduits enter is a critical transition point. You must ensure a reliable, low-impedance bonding path for any conduit running to the hazardous area. Since the Myers hubs aren't sufficient, you MUST use listed bonding bushings (with the correct number of lugs) and bonding jumpers.

Summary and Recommendations1. Replace or Augment the Myers Hubs

The most straightforward approach is to replace the Myers hubs with listed bonding hubs (if available for your application). These are specifically designed and tested for bonding.2.

Bonding Bushings and Jumpers

If you can't replace the hubs, install grounding/bonding bushings on the inside of the enclosure with bonding jumpers connected to a suitable grounding busbar inside the MCC. Ensure the busbar is properly sized and connected to the main grounding electrode conductor (GEC) system.3.

Proper Installation

Regardless of the method, ensure all bonding connections are clean, tight, and properly torqued according to the manufacturer's instructions. Remove any paint or corrosion from contact surfaces.4.

Documentation and Labeling

Keep detailed records of your grounding and bonding system, including the types of fittings used, the sizes of bonding jumpers, and the torque values applied. Consider labeling the bonded connections to show they are part of the grounding system.5.

Consult with an Engineer

If you're unsure about the proper grounding and bonding methods for your specific MCC and hazardous location application, consult with a qualified electrical engineer experienced in hazardous location design. They can perform a site-specific assessment and provide detailed recommendations.6.

Coordinate with the AHJ

Discuss your grounding and bonding plans with the local Authority Having Jurisdiction (AHJ) before starting the work. They may have specific requirements or interpretations of the code.

Important Notes



Listed Fittings

Always use fittings that are listed and labeled for their intended purpose (bonding, watertightness, etc.).

Grounding Electrode Conductor (GEC)

Ensure the GEC is properly sized and connected to the MCC and the building's grounding electrode system.

Equipment Grounding Conductor (EGC)

Don't rely solely on the conduit system for the equipment grounding conductor. Pull a dedicated EGC within the conduit, especially to the hazardous location, for redundancy and improved performance.*

Fault Current Study: For large installations, a fault current study can help determine the necessary ampacity and impedance requirements for your grounding and bonding system.By following these guidelines and consulting with qualified professionals, you can ensure a safe and code-compliant installation. Failing to properly bond your conduit system in a hazardous location is a serious safety hazard. Flag for review