You need to size bonding conductors to gas piping the same as those for water pipes and structural steel; even though they aren't used as electrodes.
A new rule for bonding gas piping systems leads to the highly debated question: How big does the bonding conductor need to be? This intense debate increasingly concerns the gas industry as well as ours.
For years (until the 1962 NEC), you could use a gas pipe as an electrode by right, if a water pipe were unavailable. The 1962 NEC (and the next eight editions) still allowed the use, but only with consent of the gas supplier. Meanwhile, the gas industry resisted. NFPA 54, the National Fuel Gas Code, prohibited these connections for use as electrodes. In 1990, the NEC finally followed suit, disallowing gas piping as an electrode.
Bonding, however, is a much different issue. Even as NFPA 54 forbids using gas piping as a grounding electrode, or as a grounding conductor, it requires the aboveground portion be "bonded to any grounding electrode as defined by the NEC." This rule is the new Sec. 250-104(b):
(b) Metal Gas Piping.
Each aboveground portion of a gas piping system upstream from the equipment shutoff valve shall be electrically continuous and bonded to the grounding electrode system.
However, this rule doesn't say how big this conductor must be. Therein lies the major controversy. We think it should be sized essentially the same as for water piping. This means you use Table 250-66 in most cases.
Some national authorities (including the former panel chair) have pointed to actions by CMP 5 that demonstratepanel intent opposite to our position. We agree CMP 5 intended to allow whatever size equipment grounding conductor runs to electrical equipment connected to the gas piping system to serve as the required bonding conductor. We recognize this is a judgment call for the AHJ, but we offer five reasons in favor of using Table 250-66, just as you would for bonding water piping [Sec. 250-104(a)] and structural steel [Sec. 250-104(d)] elements.
The panel statement doesn't apply. This may seem like an odd thing to say, but in this unusual case, it's on the mark. This is because the panel didn't write this rule. The National Fuel Gas Code Committee wrote it in Sec. 3.14 of NFPA 54. Under NFPA extract policy, extracted material must not technically differ from the source document, although there is room for editorial modification to meet the needs of the extracting document.
CMP 5 reported a rule based on equipment grounding conductor sizing. That introduced a technical difference from NFPA 54, where the bonding requirement is independent from any electrical connection to the piping system. There are gas piping systems that supply equipment with no electrical supply. A rule based on equipment grounding conductors would implicitly omit this piping from coverage, in direct violation of NFPA 54. The NEC Correlating Committee reworded the rule to agree with NFPA 54 and avoid this problem. However, since NFPA 54 has no size specified, no size appears in Sec. 250-104(b).
The context favors Table 250-66. This issue concerns a metallic piping system that may run throughout a facility, whether or not it has electrical equipment connected to it. Sound familiar? It should. Sec. 250-104(a) imposes a similar rule on water piping. Whether or not we use them as electrodes, we've been bonding water-piping systems for generations. In addition, unlike other more arcane system piping, some may use gas pipes mistakenly for other grounding connections. In fact, in an exception, NFPA 54 expressly allows you to use gas pipes as electric conductors in ignition circuits, electronic flame detection circuits, and control circuits below 50V.
This is a new rule, different from Sec. 250-104(c). The 1996 NEC grouped gas piping with all other piping and simply allowed the equipment grounding conductor to any electric equipment supplied by the piping to furnish the required bonding connection. You'll find the rule today as Sec. 250-104(c). If there's no electric equipment connected, then you don't have to provide bonding. Sound familiar? It should. It's the same requirement as would apply to gas piping, if the panel intention were to become the rule.
If a new rule comes into the Code, we think it should be interpreted to make it just that: something new and different. To interpret this rule as only requiring an equipment grounding connection is to effectively remove it from the Code. If you doubt this, mentally remove this rule from the book, and then ask yourself what you would do to comply with Sec. 250-104(c), which would still remain. You'd make the same connections, at the same point, with the same sized conductors, as you would if you applied the requirement the way the panel intended.
The only way you'd do something different is to apply the rule irrespective of electrical equipment connections to the piping system. We think that's what NFPA 54 has in mind. They control the technical content of this requirement. The last time this came up (in their '96 revision cycle), NFPA 54 rejected an attempt to rewrite it to go to equipment grounding conductor sizing. In their current ('99 Annual Meeting) revision cycle, no one even made a proposal to change this rule.
It's practical. Once you admit the Code requires this bonding connection even without other electrical equipment connected to the gas piping (and that's open and shut), consider what size conductor you'd use, and how you'd justify your choice. No one would question Table 250-66. No rule points to a smaller size in this case. Ask yourself, if Table 250-66 applies to a gas pipe with no electrical equipment connected, why would a No. 14 (for example) be appropriate for the same pipe with a piece of electrical equipment on a 15A branch circuit? Remember, the comparable water pipe bonding rules reflect not just ampacity but also physical permanence.
Stand-alone bonding connections of this sort need to follow the rules for routing grounding electrode conductors [see Sec. 250-104(a) and 250-104(d) for references to Sec. 250-64(a), (b), and (e)]. This means bonding conductors smaller than No. 6 need to be in a raceway (or cable armor). You'll usually find an accessible point on the gas piping system near the water system. Why not just clip both with the same conductor?
In terms of likely current flow, the current over either an equipment grounding conductor or over a bonding conductor to such piping will end up about the same, since in either case the earth resistance is the limiting factor. Some gas suppliers require dielectric unions to prevent any current flow over their piping systems.