Okay, let's break down the sizing of the Ufer bonding jumper in your scenario, considering the cascading nature of your grounding electrode system.

Understanding the Situation You have a common grounding electrode system. The grounding electrode conductor (GEC) to the main electrode (presumably the #1/0 AWG in your description) is a #1/0 AWG copper conductor. You have a Ufer ground (concrete-encased electrode). The bonding jumper from the #1/0 AWG common grounding electrode to the Ufer is #4 AWG copper. The bonding jumper extending from the #1/0 to other electrodes is larger than a #4 AWGRelevant NEC Sections



NEC 250.64(E): Grounding Electrode Conductor Taps

This section covers the conditions when you can "tap" the main grounding electrode conductor (GEC) to connect to additional electrodes.

NEC 250.66: Size of Alternating-Current Grounding Electrode Conductor

This table is used to determine the minimum size of the GEC based on the size of the service entrance conductors.

NEC 250.68(C): Connections to Concrete-Encased Electrodes

This section governs how you connect to a Ufer ground and any additional requirements.

Analysis


The Ufer Bonding Jumper Can Be Sized Independently

Because the Ufer is connected after the #1/0 AWG it can be sized independently.2.

Ufer Bonding Jumper Size (250.66)

NEC 250.66 directs you to Table 250.66 to determine the minimum size of the bonding jumper for the Ufer. However, there is an exception in 250.66(A). If the grounding electrode conductor is connected to a concrete-encased electrode as described in 250.52(A)(3), it shall not be required to be larger than 4 AWG copper.
3.

Is #4 AWG Sufficient? Yes, a #4 AWG copper conductor is the maximum sized wire for a concrete-encased electrode.

Conclusion

In your scenario, a #4 AWG copper bonding jumper from the #1/0 AWG common grounding electrode conductor to the Ufer ground is permitted by code even though the grounding electrode conductor that continues to other electrodes is larger than #4 AWG.

Important Considerations



Connections

Ensure all connections are made using listed and labeled connectors suitable for the materials and conductor sizes involved. The connection to the rebar in the concrete should be done per the manufacturer's instructions and any relevant local codes.

Inspection

This is a critical part of the electrical system, and it's highly recommended to have a qualified electrical inspector review the installation.*

Local Amendments

Always check with your local building department for any local amendments to the NEC that might affect the requirements.

Disclaimer: I am an AI chatbot and cannot provide professional electrical design advice. This information is for general guidance only. Consult with a qualified and licensed electrician for all electrical installations. Flag for review