Interior metal gas piping in a commercial kitchen is likely to become energized because it serves appliances supplied by a 240-volt, 30-ampere branch circuit. On what basis does the Code establish the minimum size of the bonding jumper for that piping system?
- From the equipment grounding conductor table, using the rating of the overcurrent device of the circuit likely to energize the piping
- BFrom the grounding electrode conductor table, using the size of the ungrounded service-entrance conductors, the same basis used for water piping
That table and that basis belong to the metal water piping system, which is bonded on the service conductor size because anything in the building can energize it. Gas piping is sized from the circuit likely to energize it.
- CFrom the ampacity table, using the ampacity of the largest feeder in the building that could contact the piping
Ampacity tables size conductors for continuous load current, not for fault duty. No bonding jumper anywhere in the Code is sized from the ampacity of a feeder, whichever feeder is picked.
- DIt is fixed at 6 AWG copper for every metal piping system other than water piping, regardless of the circuit involved
There is no fixed size, and the circuit is precisely what the rule turns on. The jumper scales with the overcurrent device of the circuit likely to energize the piping, so a 30-ampere circuit and a 400-ampere circuit give very different answers.
Why A is correct
Piping that becomes energized does so through a specific circuit, so the Code sizes its bonding jumper from that circuit's overcurrent device using the equipment grounding conductor table. The jumper only has to carry fault current from that circuit long enough to open its device, which is a very different job from bonding the water piping system.
What this question is testing
Tests whether you can distinguish the two piping bonding rules, one keyed to the service conductor size for water piping and one keyed to the branch-circuit overcurrent device for other piping, and whether you know that the circuit equipment grounding conductor may serve as the bonding means for the second case.
On the job
Gas piping bonding is one of the most argued items on a residential or restaurant final. The practical outcome is often that no separate jumper is needed at all, because the equipment grounding conductor running to the furnace, boiler or rooftop unit already satisfies the requirement. Corrugated stainless steel tubing systems are a different matter, since their manufacturers frequently require a direct bonding connection with its own minimum size stated in the installation instructions.
Memory technique
Water bonds to the whole house; gas bonds to the one circuit that can bite it.
Exam tip
Water piping looks at the service; every other piping system looks at the circuit that could energize it.
Where to look it up
Article 250, the bonding of piping systems and exposed structural metal section. The water piping subsection and the other piping subsection sit next to each other and point to different tables.