You're dealing with a scenario that requires careful consideration of the National Electrical Code (NEC). Here's a breakdown of the relevant sections and what you likely need to do

Understanding the Key NEC Sections
NEC 250.24(A)(1) Exception 2 (Grounding Electrode Conductor for a Multi-Building or Multi-Occupancy Building)

This exception is often invoked in situations like yours. It allows you to ground the meter bank with a Grounding Electrode Conductor (GEC) sized to the service entrance conductors feeding the meter bank, rather than the individual units.

NEC 250.24(A)(2)

If you use the Exception 2, the meter bank neutral must be connected to the grounding electrode.

NEC 250.64(F)

This section permits the grounding electrode conductor to be run bare without protection from physical damage.

NEC 250.122 (Equipment Grounding Conductor)

This is for sizing equipment grounding conductors within circuits, not the grounding electrode conductor. This section is not relevant to sizing the GEC to the ground rods.

NEC 250.66 (Sizing Grounding Electrode Conductor)

This is the critical section. It tells you how to size the GEC to your grounding electrode (ground rods). You

do not use 250.122.

The Correct Approach1. Determine the Total Service Entrance Conductor Size Feeding the Meter Bank

Add up the ampacity of all of the conductors coming from the transformer to the meter bank. For example, if you have three 3/0 copper conductors (each rated for 200 amps), your total ampacity would be 600 amps.2.

Apply NEC 250.66 Table



Copper

If the service-entrance conductors are copper, Table 250.66 tells you the minimum size of the grounding electrode conductor. Look at the size that matches the overall size of your conductors (as you determined in step 1).

Typically, even a modest service will require at least a #2 copper grounding electrode conductor to the ground rods.3. Supplemental Grounding Electrode



NEC 250.53 (A)(2)

A supplemental ground rod is required unless a single rod has a ground resistance of 25 ohms or less. Often, this means you need two ground rods, a minimum of 6 feet apart.4.

Ground Rod Considerations



NEC 250.53(G)

Ground rods must be at least 8 feet long and driven to a depth where the top of the rod is at least 8 feet below grade, unless rock bottom is encountered.

NEC 250.53(B)

Be certain to follow the requirements for bonding the ground rods together.5.

Bonding the Neutral at the Meter Bank

Since you are using Exception 2 to NEC 250.24(A)(1), you must bond the neutral conductor to the grounding electrode system (the ground rods) at the meter bank. This is a critical safety requirement.

Why Your #4 Estimate is Likely Incorrect

The #4 copper you were considering is likely based on assuming a smaller service. With multiple apartments and a house panel, the overall service size is likely significantly larger. You cannot base the GEC size on the individual 100A disconnects. You must size it based on the total service.

Important Considerations Local Codes and Inspector

Always check with your local electrical inspector or authority having jurisdiction (AHJ). Local codes may have additional requirements or interpretations of the NEC. Their approval is essential.

Professional Consultation

If you're not entirely comfortable with these calculations and NEC requirements, it's highly recommended to consult with a qualified, licensed electrician. A mistake here can have serious safety consequences.*

Service Calculations

A proper load calculation for all the apartments and the house panel is necessary to determine the overall service size accurately.

Example (Hypothetical)

Let's say after a load calculation, you determine the service entrance conductors to the meter bank are 3/0 AWG copper conductors in parallel. Using Table 250.66, you would need a #2 AWG copper grounding electrode conductor to the ground rods.

In summary, you need to:1. Accurately calculate the total service size.
2. Use NEC Table 250.66 to determine the correct grounding electrode conductor size.
3. Install at least two ground rods according to NEC 250.53, bonded together.
4. Bond the neutral to the grounding electrode system at the meter bank.5. Contact your local inspector and consult with a qualified electrician if needed. Flag for review