The short answer is no, you cannot reliably rely on the 100A EGC to provide a sufficient bonding path for faults on the 20A circuit in this scenario without also bonding the 20A EGC to the box.

Here's a breakdown of why, and what the NEC (National Electrical Code) says: Purpose of Bonding

The purpose of bonding a metallic enclosure is to create a low-impedance fault current path back to the source in the event of a ground fault. This low impedance ensures that enough current flows to quickly trip the overcurrent protective device (circuit breaker or fuse) and de-energize the circuit. This significantly reduces the risk of electric shock.

NEC Requirements

While the specific sections depend on the edition of the NEC, the following principles apply:

250.4(A)(3) Bonding of Electrical Equipment

Metal enclosures for electrical equipment must be bonded. This section emphasizes the importance of a low-impedance fault current path.

250.96 Bonding Other Enclosures

Similar to 250.4, this section requires the bonding of metal enclosures, raceways, and other non-current-carrying metal parts that are likely to become energized.

250.130 Equipment Grounding Conductor Connections

The NEC specifies how equipment grounding conductors (EGCs) must be connected.

250.146 Connecting Receptacle Grounding Terminal to Box

If you had receptacles in the box, this section is very relevant, but not in your current "no splice" situation.

Why Your Scenario is Problematic


Reliability of Connection

You're essentially relying on the connection between the box and the 100A EGC to be sufficient for all possible fault currents, including those on the 20A circuit. This is not a reliable or code-compliant approach. The 100A EGC provides grounding for its circuit. If a fault occurs on the 20A circuit, the fault current would have to travel: From the fault point inside the box Through the enclosure metal To the point where the 100A EGC is bonded to the box Through the 100A EGC back to the source.This extended path adds impedance and may not provide a sufficient fault current for the 20A breaker to trip quickly, particularly if the bond to the 100A EGC is loose or corroded.2.

Insufficient Fault Current Path

A fault on the 20A circuit might not generate enough fault current through the 100A EGC's connection to reliably trip the 20A breaker. A high impedance in the path will limit the fault current.3.

Potential for Difference in Potential

Without the 20A EGC directly bonded to the box, the box's potential can rise relative to the 20A circuit's ground point during a fault. This presents a shock hazard.

The Correct Approach


Bond Each EGC to the Box

The proper way to handle this is to bond both the 20A EGC and the 100A EGC to the metal box. Use a listed grounding connector (e.g., grounding lug, grounding clip) to ensure a secure and low-impedance connection to the box.2.

Pigtailing

If necessary, "pigtail" the EGCs. This means connecting both EGCs to a common grounding point inside the box (e.g., a grounding busbar or terminal block) that is then bonded to the box.

Safety First

Electrical safety is paramount. If you are not a qualified electrician, consult one. Incorrect wiring can be dangerous and violate the NEC, potentially leading to electric shock, fire, or equipment damage.

In summary, the NEC requires all metal enclosures that house electrical conductors to be bonded to ground to provide a low-impedance fault current path. This means the EGC of each* circuit passing through the box needs to be bonded to the box. Relying solely on the 100A EGC is not code-compliant and creates a potential safety hazard. Flag for review