Bonding a Corrugated Stainless Steel Gas Line
An electrician is roughing in a new house and notices that the gas piping is corrugated stainless steel tubing (CSST) running through the attic and walls to a tankless water heater, range, and dryer. The plumber has bonded nothing. The electrical service is 200 A with a grounding electrode system consisting of two ground rods and a metal water pipe. The electrician is asked whether the CSST must be bonded, where, and with what size conductor. The local inspector has flagged the job, citing lightning-related arc damage to CSST in the area. The electrician also must bond the metal water piping and the gas piping at the water heater.
1. Must the CSST gas piping be bonded, and if so, how should it be done?
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Yes. CSST manufacturers' installation instructions require direct bonding of the CSST to the grounding electrode system, and NEC 250.104(B) requires metal gas piping to be bonded. The bonding conductor is sized per 250.104(B) using Table 250.122 based on the service overcurrent device - for a 200 A service, that is 6 AWG copper. However, many CSST manufacturers require a larger bonding jumper (often 6 AWG minimum, some require 4 AWG) and specify bonding at the manifold or at the point where the CSST enters the building.
The bonding connection must be made to a metallic fitting or a listed bonding clamp on the CSST, not to the plastic jacket. It must run to the grounding electrode system or the service equipment enclosure. Bonding the CSST is distinct from using the gas piping as a grounding electrode - it must not be used as a grounding electrode conductor. The judgement call is that manufacturer instructions govern and may exceed the code minimum; the electrician should check the specific CSST listing and follow the stricter requirement.
2. How should the metal water piping and the gas piping at the water heater be bonded, and what size conductors apply?
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Metal water piping that is part of the grounding electrode system must be bonded to the service with a grounding electrode conductor sized per Table 250.66 - for 2/0 copper service conductors, that is 4 AWG copper. If the water pipe is not a grounding electrode (e.g., plastic service), it still must be bonded per 250.104(A) with a conductor sized per Table 250.122 (6 AWG for 200 A) or larger if the piping is likely to become energized.
The gas piping at the water heater is bonded as part of the gas piping bonding requirement; the same 6 AWG (or manufacturer-required size) applies. The bond must be accessible and made with listed clamps. The judgement call is distinguishing the grounding electrode conductor (Table 250.66) from the equipment bonding jumper (Table 250.122) - using the wrong table is a common error. The electrician should also verify the water pipe bond is within 5 ft of the point of entry per 250.68(C).
3. The inspector cites lightning damage to CSST. What does bonding actually accomplish, and what are its limits?
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Bonding equalizes potential between the CSST and the building's grounding system so that a lightning surge or fault current does not arc across the thin CSST wall to nearby grounded metal, which can puncture the tubing and release gas. It gives fault and surge current a low-impedance path back to the grounding electrode system rather than through the gas tubing.
Limits: bonding does not make the CSST immune to a direct lightning strike; it reduces the risk of sideflash and arcing at connections. It also does not substitute for a lightning protection system. The electrician should not promise the homeowner that bonding eliminates all lightning risk. The judgement call is understanding the purpose of equipotential bonding versus surge protection - bonding is required and effective for potential equalization, but it is not a lightning arrestor.