Okay, this is a great question that digs into the heart of grounding and bonding philosophy in the NEC. You're right to be curious about the specific wording and restrictions regarding the connection point of the grounding electrode conductor (GEC) and the reasoning behind allowing a wire or bus main bonding jumper (MBJ) but disallowing a screw.

The Core Reason: Reliability and Low Impedance

The fundamental reason comes down to ensuring a reliable, low-impedance path for fault current to return to the source. This low-impedance path is critical for:1. Rapid Operation of Overcurrent Protective Devices (OCPDs)

When a ground fault occurs, a sufficient amount of current needs to flow quickly so the breaker or fuse can trip rapidly, minimizing damage and preventing dangerous voltage on conductive surfaces.2.

Limiting Touch Potential

By providing a low-impedance path, the voltage rise on exposed metal parts during a ground fault is minimized. This reduces the shock hazard to personnel.3.

Effective Bonding

The grounding system's primary function is to bond all conductive parts together so all those parts rise in voltage together during a fault. If properly bonded the potential difference between 2 pieces of equipment is close to zero.

Why a Wire/Bus MBJ is Preferred over a Screw



Surface Area & Contact Pressure

A wire or bus MBJ, properly sized and installed, provides a much larger contact surface area compared to a screw connection. A screw connection, even when tightened, can have limited contact points and is more susceptible to corrosion or loosening over time.

Lower Impedance Path

The larger surface area of the wire or bus creates a much lower impedance path. A lower impedance path helps to allow the flow of current to be closer to the calculated maximum fault current.

Mechanical Robustness

A properly installed wire or bus MBJ is mechanically more robust than a simple screw connection. It is less likely to become loose or compromised due to vibration, thermal cycling, or physical stress.

Long-Term Stability

The larger contact area and more secure connection of a wire or bus MBJ contribute to a more stable and reliable connection over the long term, even in harsh environments.

The Screw Limitation: A Point of Degradation and High Resistance Corrosion

Connection hardware is a weak point in any electrical connection. This is due to disimilar metals and contaminats can corrode the contact surface and create resistance.

Surface Area

As mentioned above a screw will have limited contact surface area and may limit the current flow.*

Oxidation

Connections can oxidize and create unwanted resistance.

Substantiation Insights (If You Can Find Them)

While I don't have access to the specific substantiation for that exact phrasing (NEC substantiations are published records, but tracking down the exact reasoning for every wording change can be challenging), the general principle of low-impedance fault current paths is the underlying justification for these kinds of restrictions. Looking at similar proposals related to grounding and bonding connections will likely reveal the same core concerns.

Your Theories Are Likely on the Right Track

I'm sure your theories align with these principles. The NEC emphasizes ensuring a durable, reliable, and low-impedance grounding path to facilitate rapid operation of overcurrent devices and minimize shock hazards. The specific wording restrictions are designed to enforce those principles in practical installations.

In Summary:

The reason the NEC prohibits using a screw (or a screw-only connection) as the main bonding jumper is because it doesn't provide the same level of reliability and low-impedance connection as a wire or bus. The larger contact area and more robust mechanical connection of a wire or bus are essential for safely and effectively clearing ground faults. The substantiation reflects the core principle of low-impedance fault current paths as the foundation of a safe and effective grounding system. Flag for review