What Is A Supplemental Electrode?
If you're using a made electrode to supplement a water pipe ground, it should meet the resistance or duplication provisions applying to such electrodes.
For many years, the NEC has not permitted a water pipe electrode to be the only electrode for any occupancy. You could use one of the made electrodes, such as a rod or pipe, as the additional electrode. Of course, if you have one of the other principal electrodes available (building steel, concrete encased, or ground ring) that means you don't need the made electrode. Remember, if you have more than one of those principal electrodes, then you need to include each available electrode in the grounding electrode system. That automatically provides the backup.
Many times, however, the metal water pipe is the only principal electrode. If you add a rod or pipe electrode, does it have to comply with the resistance limitations in Sec. 250-84? This section requires yet another made electrode when the first rod, pipe, or plate electrode has a resistance to ground above 25 ohms, although you don't have to go beyond one additional electrode.
The EC&M panel's response. We think the resistance requirement applies to made electrodes installed to backup water pipe electrodes. Assuming the first made electrode runs above 25 ohms, Sec. 250-84 requires an additional electrode of any type specified in "Sec. 250-81 or 250-83." The question is whether the water pipe can be that additional electrode. We think it can't, because making that allowance would, in effect, be saying the water pipe could back itself up. That is, if the water pipe needs a supplemental electrode, and the supplemental electrode needs an "augmented" electrode as described in Sec. 250-84, then the water pipe simply cannot be both the instigator of the necessity for supplementation and part of the means for supplementation at the same time.
In a way, use of the word "supplemented" in Sec. 250-81(a) is unfortunate because made electrodes, where installed for this purpose, don't actually supplement the water pipe. The usual resistance and dissipation capability of water pipe electrodes is so superior to made electrodes that a made electrode serves little initial purpose.
Supplemental electrodes have one principal purpose: they provide an electrode for the system in the event the metal water pipe is removed or made unavailable due to an insulating joint. If you install a single made electrode, and it doesn't meet the requirements in Sec. 250-84, then it won't serve its function when the water pipe disappears.
Many agencies that supply water service routinely remove metallic water laterals and change them to plastic pipe. They often do this without advising either the owner or the electrical inspector of the need to review the grounding arrangements. The result is a necessity for a standby component in the grounding electrode system. That is why this requirement went into the NEC.
It might be a good idea to eventually consider other names for these electrodes, rather than "supplemental," in order to clarify this concept in the minds of Code users. Perhaps we could use the term "reserve" electrode to more accurately convey the meaning. That would be some time off, however. Although the word "supplemental" will remain in the 1999 NEC, CMP 5 is including specific requirements to clarify that the same resistance requirement applies to these electrodes, as when they are the only electrodes. After all, the only reason for them is an assumption that they will be the only electrodes at some time in the future.