Grounding separately derived systems to water pipes

The new paragraph in Sec. 250-80(a) creates questions about the relationship between water piping and separately derived system grounding.One of the most common field problems is where to make a system grounding electrode connection for a separately derived system. Water piping systems are universally associated with grounding, although there are, of course, many other grounding electrodes. Therefore,

The new paragraph in Sec. 250-80(a) creates questions about the relationship between water piping and separately derived system grounding.

One of the most common field problems is where to make a system grounding electrode connection for a separately derived system. Water piping systems are universally associated with grounding, although there are, of course, many other grounding electrodes. Therefore, installers frequently look to water piping systems for this purpose, particularly if they are in the same area.

The EC&M panel's analysis

The first question is when is a water pipe a water pipe. This may seem obvious, but the prospective use of sprinkler piping caused a major controversy some years back, and the issue is commonly debated today. In the 1987 Code cycle, CMP 5 rejected a proposal to add sprinkler piping along with water piping (with conditions to assure effective grounding) into Sec. 250-26(c). The proposal would have expressly recognized a common industrial practice of using sprinkler piping for grounding separately derived systems, particularly in areas where effectively grounded building steel wasn't available. The panel said in its statement:

Many sprinkler piping systems do not

guarantee electrical continuity between

joints. On thee basis there is no

practical way to determine that the

piping is effectively grounded.

This panel action provoked a comment by this editor to the effect that words have meaning, and the existing wording allowed effectively grounded water piping. A sprinkler system (at least, one that uses water) is a piping system that conveys water, even if dry. Therefore it is a water piping system and is valid provided it is effectively grounded. The comment noted that grooved-joint piping systems have questionable continuity due to the rubber gaskets between joints, but that other systems such as those with sweated joints had unquestionable continuity. Even the grooved-joint systems can be made continuous with bonding jumpers. The comment concluded that the local AHJ should still be allowed to judge the local conditions. Although the panel rejected the comment, its statement was considerably more acceptable than on the original proposal:

A metal water pipe of a sprinkler

system might be used to meet the

requirements of Section 250-26(c)

provided the water pipe meets Section

250-81(a). The panel objects to the

original proposal because of the

emphasis it places on sprinkler


Based on both the literal text and this panel statement, we conclude that sprinkler piping is water piping and, where approved by the AHJ, it may be used as a grounding electrode.

But is it a Part H electrode (and does it matter)?

Part H of Art. 250, comprised of Sections 250-81 through 250-86, covers the "Grounding Electrode System." The third paragraph of Sec. 250-81 expressly disallows "interior metal water piping more than 5 ft from the point of entrance to the building" from being "used as part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system." There is an exception for industrial and commercial buildings with qualified maintenance and supervision, but only where "the entire length of the interior metal water pipe that is being used for the conductor is exposed." Although apparently unintended, this literally voids the exception if pipe passes through a partition, and an attempt to allow for this type of installation was rejected.

The reason for the rule is the prevalence of other trades disrupting continuity of interior water piping without appropriate supervision. Clearly that substantiation would apply to separately derived systems. Whether the Code actually requires a Part H electrode is another story, however. Sec. 250-54, which requires the same or commonly bonded electrodes to be used for all premises AC systems "specified in Sections 250-23 and 250-24," conspicuously fails to include separately derived ac systems under Sec.250-26. Many other sections of the Code, including Sec. 800-40(b)(1)(2) for telephone systems, recognize water pipe electrode connections that aren't in the first 5 ft beyond the water entrance.

With respect to the separately derived system rules themselves, Sec. 250-26(c) mandates that "the grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system." Then, Sec. 250-26(c)(2) for separately derived systems simply recognizes "the nearest available effectively grounded metal water pipe" for this purpose. We conclude that the local metal water pipe may or may not qualify as a Part H electrode depending on Sec.250-81 and Ex.2 thereto, however, with respect to separately derived systems (and only separately derived systems) it need not so qualify as long as it is effectively grounded.

Should it be bonded?

Bonding is another matter. Suppose, for example, the separately derived system is grounded to effectively grounded local building steel. The water piping system will be bonded to the building steel back in the service area. This is because the steel is a qualified Part H electrode and as such must be used as a service electrode per Sec. 250-81. The interior water piping system must be bonded at the same electrical location per Sec. 250-80(a) (first paragraph).

The question is whether the separately derived system grounding terminal, perhaps on the tenth floor, must be bonded to the local water piping. This is what the new second paragraph of Sec. 250-80(a) was attempting to reach. We know this because the substantiation described the potential hazard in having such an equipment grounding system unassociated with metal piping at that level during transient fault conditions.

The substantiation was partially based on an assumption that a separately derived system grounding connection to a water pipe would need to be within 5 ft of the water entrance, with which we clearly disagree. Nevertheless, we agree with the argument that if water pipe bonding connections are at great distances from a local system, there could be a severe hazard from transient voltages. It is clear that the intent was to mandate the local connection, even where the local piping wasn't the principal electrode.

Must it be bonded?

Unfortunately, the literal text of the new paragraph doesn't actually require much of anything. The principal reason is that, as written, the paragraph only applies where the separately derived system is using a made (or other) electrode under Sec. 250-26(c)(3):

(3) other electrodes as specified in

Sections 250-81 and 250-83 where

electrodes specified by (1) or (2) above are not


In other words, the only time the new paragraph (requiring local water pipe bonding) takes effect is when you are making a grounding connection that can only be made when an effectively grounded water pipe is unavailable. Remember, Sec. 250-80(a) (first paragraph) mandates a bonding connection to interior metal water piping by a conductor that meets the full minimum size rules in Table 250-94. The reason for this is exactly because other rules in the NEC recognize water piping as an electrode, and this requirement is what validates the assumptions behind those rules.

The result is "Catch-22." If an effectively grounded metal water pipe is available, you must use it as an electrode in preference to Sec. 250-26(c)(3) electrodes. Only if it isn't there must you bond to what isn't there. Finally, if you bond to effectively grounded local building steel, there also isn't any requirement to bond to the water piping system. However, the failure to make this connection opens the door to the hazard identified in the proposal substantiation. Until this is sorted out, we suggest that you make the connection, although we acknowledge no basis in the literal text to force compliance.

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