The NEC requires each service to have its own grounding electrode system. While it's possible to connect them together, you still need a grounding electrode system dedicated to each service.Here's a breakdown based on the NEC (specifically focusing on 2023, but generally applicable): NEC 250.50 (Connecting Grounding Electrode Systems)

"All grounding electrodes as described in 250.52 that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52 shall be installed and used."

NEC 250.52 (Grounding Electrodes)

Outlines acceptable grounding electrodes, including ground rods, ground plates, metal underground water pipes (with specific provisions), concrete-encased electrodes (Ufer grounds), and metal building frames (under certain conditions).

Why the Separation (Initially)?

The reason each service typically requires its own grounding electrode system is to:1. Minimize circulating currents

Connecting multiple services to a single grounding electrode system without proper engineering can, in some situations, lead to circulating currents in the grounding conductors. These currents can cause nuisance tripping of ground-fault protection devices, overheating, and even electromagnetic interference.2.

Ensure Effective Ground Fault Path

Each service must have a low impedance path back to the source. Sharing grounding systems could increase the impedance of the fault path, increasing the time to clear the fault and creating a hazard.3.

Safety and Redundancy

If one grounding electrode or connection fails, the other service will still have its own dedicated grounding system.

How to handle the scenario with two services


Install separate grounding electrode systems for each service. Each service disconnect must have its own set of ground rods.2. Bond the Grounding Electrode Systems Together

After establishing separate grounding electrode systems for each service, you can bond them together. This is actually required by NEC 250.50. You would run a bonding jumper between the grounding electrode systems.
3.

Sizing the Bonding Jumper

The bonding jumper must be sized according to 250.66 based on the largest service-entrance conductor of the larger service.

Important Considerations and Caveats



Local Codes

Always check with your local electrical inspector. Local codes can be stricter than the NEC.

Engineering Review

For complex installations (like multiple services in close proximity), it's highly recommended to have a qualified electrical engineer review the design. They can assess the specific conditions and determine the best grounding and bonding approach to minimize potential problems.

Ground Rod Quantity and Depth

The NEC generally requires at least two ground rods, unless a single rod has a resistance to ground of 25 ohms or less.

Spacing of Ground Rods

The ground rods must be at least 8 feet apart.

In summary, install a grounding electrode system for each service, then bond those systems together. This complies with the NEC and helps ensure safety and proper operation. Consult with your local inspector and consider an engineering review for a project like this. Flag for review