You've hit on a very important point and a growing concern in modern electrical installations. Let's break it down to clarify the rules and address the scenario where the UFER is the only grounding electrode

Understanding the Code (NEC) - Key Sections
250.24(A)(5)

This section covers the connection of the grounding electrode conductor (GEC) to the neutral/grounded conductor at the service. It is about ensuring a proper connection at the service equipment.

250.50

This section states the need to connect all grounding electrodes together to form the grounding electrode system.

250.52

This section defines what constitutes a grounding electrode (including UFERs).

250.66

This section is crucial. It provides the sizing requirements for the GEC. It dictates how big the wire needs to be based on the size of the service entrance conductors.

250.66(A) through (C)

These subsections of 250.66 detail the specific sizing rules based on the electrode type and conductor material (copper or aluminum).

250.66(B)



This is where the #4 AWG copper minimum for a concrete-encased electrode (UFER) comes from. However, this section has a crucial limitation. 250.66(C)

This applies to ground rings.

Addressing Your Scenario (UFER as Sole Electrode)

You're right to question whether a #4 AWG copper is sufficient in all cases when a UFER is the only grounding electrode, particularly with large services. Here's the critical point: 250.66 still dictates the GEC size, even when only a UFER is present. However, Section 250.66(A) has to be followed where there are no exceptions. The #4 AWG copper limit in 250.66(B) is only a reduction when there are supplemental grounding electrodes. This is because you're relying on multiple grounding electrodes. If the UFER is the only grounding electrode, 250.66(B) cannot be used for reducing the wire size. You must size the GEC according to 250.66(A).

Example: 1000A Service with UFER Only

Let's say you have a 1000A service fed with (4) 500kcmil copper conductors per phase. Here's how you would determine the minimum GEC size when the UFER is the only grounding electrode:1. Sum the Circular Mils

The total circular mil area of one phase (all ungrounded conductors) is 4 500,000 = 2,000,000 kcmil2.

Use Table 250.66

Find the size of the GEC based on the circular mil area. Based on 2,000,000 kcmil you need a 3/0 Copper GEC per table 250.66.In this case, you cannot use a #4 AWG copper GEC because the UFER is the only electrode. If there were additional electrodes (like a ground rod in addition to the UFER), then 250.66(C) would allow you to use the #4 Copper.

Important Considerations Supplemental Electrodes are Highly Recommended

It's always best practice to install supplemental electrodes in addition to the UFER. This provides redundancy and improves the overall grounding system. In that case, the UFER can be #4 CU.

AHJ (Authority Having Jurisdiction)

Always consult with your local AHJ. They may have specific requirements or interpretations regarding grounding electrode systems.

Engineering Judgment

In complex installations, consider consulting with a qualified electrical engineer to ensure the grounding system is adequate and meets all applicable codes and standards.

In summary:

If the UFER is the only grounding electrode, you must size the GEC according to 250.66(A), which requires a larger conductor based on the size of the service entrance conductors. The #4 AWG copper limit for a UFER in 250.66(B) is only applicable when supplemental grounding electrodes are present. If no supplemental electrodes are present, the UFER wire is the GEC and needs to be properly sized for the service. Flag for review