Okay, let's break down the requirements for bonding in this scenario. Your plan involves transitioning from MC cable to EMT conduit for the final run to the EV charger, without making any splices within the junction box.

Understanding the Electrical Code (NEC) Principles

The key principles at play here are: Effective Ground-Fault Current Path

All metal enclosures and raceways must be bonded to create a low-impedance path for fault current to return to the source (the panel). This ensures that overcurrent protection devices (breakers) will trip quickly in the event of a ground fault, minimizing the risk of shock or fire.

Continuity of Grounding

The grounding conductor (in your case, the bare ground wire within the MC cable) is responsible for providing this low-impedance path. The metal raceways (EMT) must be connected to this grounding system to maintain continuity.

Bonding vs. Grounding

While the terms are often used interchangeably, bonding specifically refers to connecting non-current-carrying metal parts together to create an electrically conductive path. Grounding connects this system to the earth.

Analysis of Your Scenario

Here's how the NEC applies to your specific setup:1. MC Cable and Ground

You're using 6/2 MC cable with a grounding conductor. This is a good start. The grounding conductor within the MC cable will be connected to the ground bar in your panel.2.

Transition Box (4" Square)

This is the critical point. Even though you're not making splices inside the box, it still needs to be considered part of the overall electrical system.3.

MC Connector

You're using a listed fitting to connect the MC cable to the box. This is essential. Ensure the fitting is rated for grounding and is properly installed according to the manufacturer's instructions.4.

EMT Connector

You're using a set-screw EMT connector. This is also important, but the type of connector is less important than how it's used.

The Verdict: Bonding the Transition BoxYes, you DO need to bond the 4" square box. Here's why: NEC 250.96(A) and 250.4(A)(3)

These sections require metal enclosures and raceways that are likely to become energized to be connected to an effective ground-fault current path. The EMT conduit in your case falls under this requirement.

Conduit as a Grounding Path

When you transition to EMT, the EMT now becomes part of the grounding path to the EV charger. The box needs to be part of that continuous path.

Mechanical Connection Alone is Not Enough

While a tight mechanical connection between the EMT connector and the box might provide some continuity, it's not considered a reliable and permanent grounding connection by code. Vibration, corrosion, or loosening of the set screws can compromise this connection.

How to Properly Bond the Box

Here's how you can properly bond the box to the grounding system: Grounding Bushing or Bonding Locknut

The preferred method is to use a grounding bushing or a bonding locknut on the EMT connector where it enters the box. This provides a dedicated connection point for a grounding conductor.

Grounding Pigtail

Attach a grounding pigtail (a short length of insulated wire) to the grounding bushing or bonding locknut using a screw or compression connector.

Bonding to MC Ground

Connect the other end of the grounding pigtail to the grounding conductor in your MC cable inside the box. You can use a wire connector (like a wire nut or crimp connector) to make this connection. Ensure that the connections are rated for the size and number of conductors being spliced.

Grounding Screw (if available)

If the box has a pre-tapped hole for a grounding screw (often marked with a green grounding symbol), you can use a grounding screw and pigtail to bond the box. Connect the grounding screw to the box, and then connect the pigtail to the MC ground inside the box.

Important Considerations



Local Codes

Always check with your local electrical inspector to see if there are any local amendments or specific requirements in your area.

Proper Installation

Make sure all connections are tight and secure.

EV Charger Instructions

Always follow the EV charger manufacturer's installation instructions.

In Summary

Even though you're not making splices in the transition box, it is still essential to bond it to ensure a safe and reliable ground-fault current path for the EMT conduit run to your EV charger. Using a grounding bushing or bonding locknut with a grounding pigtail connected to the grounding conductor of the MC cable within the box is the best practice. When in doubt, consult with a qualified electrician.Disclaimer: I am an AI chatbot and cannot provide electrical advice. Always consult with a licensed electrician for any electrical work.* Flag for review