Okay, let's break down the potential code violations and the arguments for and against this practice. You're right to question this, as there are definitely potential issues.

Here's why pulling MC cable through EMT without proper bonding at a 4-square box likely violates the NEC



300.10 Electrical Continuity of Metal Raceways and Enclosures

"Metal raceways, cable armor, and other metal enclosures for conductors shall be metallically joined together into a continuous electrical path so as to provide electrical continuity and shall be connected to grounding electrodes as prescribed in Article 250." This is a fundamental principle. The EMT conduit system must be electrically continuous and grounded. Simply passing MC cable through it doesn't achieve this.

314.40(A) Metal Boxes

This section covers the requirement of equipment grounding. Unless you have something else grounding the EMT (like properly installed connectors to grounded boxes elsewhere in the run), it violates this section.

250.4(A)(3) Impedance

This is perhaps the most important and often overlooked aspect. The NEC wants to ensure a low-impedance fault current path back to the source. This facilitates the rapid operation of overcurrent devices (breakers/fuses) to clear a fault quickly. Running the MC through a length of unbonded EMT increases impedance. A high-impedance ground fault can prevent a breaker from tripping, creating a dangerous shock hazard and potential for fire.

300.12 Mechanical Continuity - Raceways and Cables

Raceways (like EMT) and cables must be mechanically continuous between boxes, fittings, or other enclosures. While the cable is continuous, the electrical raceway is interrupted. The unbonded EMT sections are essentially floating pieces of metal.

Why their argument ("NEC doesn't say you can't") is flawed



NEC as a Minimum Standard

The NEC is not a "permission" document; it's a minimum safety standard. Just because something isn't explicitly forbidden doesn't mean it's acceptable if it compromises safety or violates the intent of the code. The intent here is to ensure a safe, continuous, and grounded electrical system.

Code Interpretation

The NEC relies on reasonable interpretation. An inspector would almost certainly view unbonded EMT as a violation of the principles of continuity, grounding, and low impedance fault current path.

What the 4-Square Box and Bonding Does

A properly installed 4-square box, with listed EMT connectors and a grounding jumper to the box and then to the MC cable armor, provides the following:1.

Electrical Continuity

It bridges the gap between the EMT sections, maintaining electrical continuity of the raceway.2.

Grounding

The box, being grounded, provides a path for fault current to return to the source via the EMT system.3.

Bonding

The bonding jumper connects the MC cable armor to the grounded box, ensuring that the cable armor is also part of the grounding system.

In summary

Running MC cable through EMT without bonding most likely violates the NEC because it compromises the integrity of the EMT raceway as a continuous, grounded, and low-impedance fault current path. It's a shortcut that creates a potential safety hazard.

What to do


Politely but Firmly Explain

Explain the code sections and the safety concerns outlined above. Focus on the potential for increased impedance and the impact on fault current clearing.2.

Involve Supervision

If they're unwilling to listen, escalate the issue to your supervisor or the responsible electrical foreman. Safety is paramount.3.

Consult with an Electrical Inspector

If there's still disagreement, request a clarification from the local electrical inspector. Their interpretation is what matters. A call to the inspector may be necessary to come inspect the jobsite.

Important Considerations



Listing and Labeling

Make sure all materials (EMT connectors, boxes, bonding jumpers) are properly listed and labeled for their intended use.

Local Amendments

Be aware that some local jurisdictions may have amendments to the NEC that further restrict or clarify these types of installations.

AHJ

When in doubt, always consult with your local Authority Having Jurisdiction (AHJ), which is typically the electrical inspector.

Safety First: Never compromise on safety to save time or money. A properly installed electrical system protects lives and property.I hope this helps! It's a nuanced issue, but the core principles of grounding and bonding should always be followed. Flag for review