Okay, let's break down these scenarios regarding equipment grounding in your apartment complex panel upgrade

Scenario 1: Concentric Knockouts & Bonding Bushings
The Code

NEC 250.92 generally requires bonding for service equipment and 250.97 addresses bonding when concentric and eccentric knockouts are encountered in services.

Analysis

The key here is whether this is considered service equipment. While not the building service (main disconnect), a panel supplying multiple apartments might still be considered service equipment in some jurisdictions, especially if it's the first means of disconnect for those apartments.

If it's considered service equipment

If the new panels are being considered service equipment or are the first means of disconnect for any apartments (even if not the building service), NEC 250.92 & 250.97 likely applies. Bonding bushings and jumpers would be required to ensure a low-impedance ground fault path. The concentric knockouts create a potential discontinuity in the ground path.

If it's NOT considered service equipment

If the panel is definitively not the first means of disconnect for an apartment and isn't considered service equipment, the concentric knockouts are less of a strict code violation if the rigid conduit is properly tightened and makes good electrical contact to the enclosure. However, the best practice and often a local interpretation is to still use bonding bushings for added safety and to minimize impedance.

Recommendation

Err on the side of caution and use bonding bushings with jumpers. It's a small cost for increased safety and avoids potential issues during inspection. Even if not strictly required, it's a good practice. Also, ask the inspector!

Scenario 2: Rigid to Flex Transition and EGC The Code

NEC 250.118 lists acceptable types of equipment grounding conductors.

Analysis

The acceptability of using flex (flexible metal conduit - FMC) as an equipment grounding conductor depends on the following:

Size

A 1 1/4" FMC, if listed for grounding purposes, can be used as an equipment grounding conductor if it meets the size requirements of 250.122. You'll need to consult Table 250.122 based on the overcurrent device protecting the circuit. A 1 1/4" FMC probably has sufficient capacity, but you MUST verify.

Fittings

You must use fittings listed for grounding purposes. Standard connectors may not provide a reliable ground path.

Length

There are no explicit length limitations in the NEC for using FMC as an EGC for feeders but the longer the flex conduit the more impedance it will add.

A Safer and More Common Alternative

Install a separate equipment grounding conductor inside the flex conduit. This is generally the better practice. Pull a properly sized insulated green (or green with yellow stripe) grounding conductor (based on 250.122) through the flex. Terminate it properly at both ends (panel and whatever is being fed).

Why a Separate Grounding Conductor is Preferred



Reliability

Provides a dedicated, low-impedance ground path.

Flex Fatigue

Flexible metal conduit can be subject to movement and corrosion over time, which can compromise its grounding capability. A separate grounding conductor eliminates that risk.

Future Changes

If the flex is ever replaced (perhaps with a non-metallic conduit), the grounding conductor is already in place.

In Summary


Concentric Knockouts

Strongly consider bonding bushings with jumpers for the new panels, especially if they are considered service equipment. It's best practice and may be required. Consult with your local inspector.2.

Rigid to Flex

Verify 1 1/4" FMC can be used as an EGC, but strongly consider pulling a separate, appropriately sized insulated equipment grounding conductor through the flex conduit. This is generally the best and safest practice.

Important Considerations



Local Codes and Interpretations

Electrical codes can be interpreted differently by local jurisdictions. Always consult with your local electrical inspector before starting work. They can provide specific guidance for your area.

Qualified Electrician

This type of work should only be performed by a licensed and qualified electrician who is familiar with the NEC and local codes.

Documentation: Document everything, including the grounding methods you used. This will be helpful for future maintenance and inspections.This information is for guidance only and should not be considered a substitute for professional electrical advice. Safety first! Flag for review