Which of the following is not allowed to act as a concrete encased electrode?

The designation of what qualifies as a concrete-encased electrode is outlined in the National Electrical Code (NEC), specifically focusing on the size and type of conductor to ensure adequate grounding capabilities. A concrete-encased electrode must generally be at least 20 feet in length and consist of a copper or aluminum conductor that meets specific size requirements.

In this case, the option presenting 20 feet of #6 AWG bare copper does not align with the NEC requirements. Although it meets the length requirement, it does not meet the minimum size specification for a concrete-encased electrode, which is #4 AWG for copper conductors. Consequently, while it is a sufficient length, its gauge is insufficient for this application.

The other options may also have length or gauge concerns, but they are not the primary reason indicated here regarding the specific requirements for a concrete-encased electrode. The key takeaway is that for an effective grounding system, both the gauge and length must meet regulatory standards, emphasizing appropriate conductor sizes to ensure safety and functionality in electrical applications.