Practice question · NEC 250.104(B)

What Size Bonding Conductor for Metal Gas Piping?

By Mason Garry, Master Electrician · October 15, 2026 · 2 min read

What size bonding conductor for metal gas piping is a table-matching question. Gas piping does not size like a service, and it does not size off the water service. Under 250.104(B), it sizes like a circuit, using the rating of the circuit likely to energize the piping.

Answer: B) Table 250.122, based on the rating of the circuit likely to energize the piping. 250.104(B) sizes the gas piping bonding conductor per Table 250.122 using that circuit rating.

Where to Find It

Open your Code book to Article 250, section 250.104(B). That section covers metal gas piping. It sends you to Table 250.122 to size the bonding conductor, and it tells you what to base the size on: the rating of the circuit likely to energize the piping. Find 250.104(B) first. The table choice follows from the section, not from guessing which table sounds closest.

Why Table 250.122

The bank tab note says it in one line: "gas pipe bonds like a circuit, not a service." That is the whole distinction this question tests. Because 250.104(B) ties the bonding conductor to the circuit likely to energize the piping, you size it per Table 250.122 using that circuit rating.

Read the correct option closely, because it has two parts: the table, Table 250.122, and the basis, the rating of the circuit likely to energize the piping. Both parts have to be right. An option can name a real table and still be the wrong answer if it bases the size on the wrong thing.

Tab Article 250 at 250.104(B). When a bonding question names a specific piping system in the stem, go to the subsection for that system before you pick a table. Compare this with the metal water pipe bonding question, where the answer follows a different path.

The Trap

Option A, Table 250.66, pulls you toward service sizing, and option D pulls you toward the water service. Both miss the bank note: gas piping bonds like a circuit, not a service. Option C, Table 310.16, is a conductor ampacity table, not the table 250.104(B) sends you to for this bonding conductor. Match the piping system to its subsection first, then take the table that subsection names.

Exam Tip

In Article 250, the noun in the stem controls the table. Here the noun is gas piping, so start at 250.104(B) and let that section choose Table 250.122 for you. Do not pick a table from memory before you read the subsection. The exam plants familiar tables as options precisely to catch that shortcut.

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