Which of the following is unacceptable as a ground according to NEC standards?

The use of a 10-foot gas pipe as a grounding electrode is unacceptable according to NEC (National Electrical Code) standards for several reasons. The primary purpose of grounding is to provide a low-resistance path to earth for electrical faults, ensuring safety by preventing electrical shock. Gas piping systems are typically not designed to handle electrical current or function as a grounding pathway.

For effective grounding, materials must be conductive and have a connection to the earth, while also being reliable and durable against corrosion and mechanical damage. Metal conduit, water pipes, and ground rods are all recognized and acceptable grounding methods under NEC guidelines. Metal conduit and water pipes can provide a good electrical connection to ground since they are regularly bonded and can handle some electrical current. Ground rods are specifically designed for grounding purposes, making them ideal.

Gas pipes do not meet these criteria, as they are primarily used for transporting gas and may not maintain the necessary continuity or reliability over time when it comes to electrical grounding. Thus, using gas pipes violates safety protocols and standards set forth by NEC, making them an unacceptable choice for grounding.