Sizing the Grounding Electrode Conductor
The first question in the Code Quandaries department, Stumped by the Code, August 2000 issue, implies that for any size service, the maximum size grounding electrode conductor (GEC) required when connected to a made electrode is a No. 6 copper conductor, when there is no other grounding electrode available. I don't believe this is the intent of Sec. 250-66(a).
The NEC also requires you to establish a fault current path under Sec. 250-2(d). The rule states, "The fault current path shall be permanent and electrically continuous, shall be capable of safely carrying the maximum fault likely to be imposed on it, and shall have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions."
The code does not address instances in which a single ground rod serves as the grounding electrode. However, Sec. 250-52, Made and Other Electrodes, states, "Where none of the electrodes specified in Sec. 250-50 is available, one or more of the electrodes specified in (b) through (d) shall be used." These references include rod and pipe electrodes.
Notwithstanding the discouraged usage of a single ground rod as the grounding electrode, there are plenty of these existing. Where they do exist, shouldn't the GEC be sized in accordance with Sec. 250-66? If not, the fault current path will likely not have sufficiently low impedance to facilitate the operation of overcurrent devices under fault conditions.
Robert W. Roush, P.E.
Chief Electrical Engineer, Frankfurt Short Bruza
As per Sec. 250-66(a), the largest size GEC connected to a ground rod is not required to be larger than No. 6 copper. There would never be any reason to have a GEC larger than No. 6 copper connected to a ground rod, because it's only sized to help lightning get to the earth, not to clear a line-to-case fault.
The fault current path requirements contained in Sec. 250-2(d) have nothing to do with sizing the GEC connected to a ground rod. The GEC connected to a ground rod will not carry sufficient fault current to remove dangerous voltage from line-to-case faults. This is because the earth is a poor conductor of electricity. Its resistivity is around one billion times that of copper [IEEE Std. 142, Sec. 2.2.8] and the earth does not permit sufficient fault current to flow back to the power supply to open the circuit overcurrent protection device.
A single ground rod is permitted (but probably two would be required to comply with Sec. 250-56) by the NEC, and the GEC is not required to be larger than No. 6. Remember this conductor is not sized to carry fault current. It's only sized to provide a path for lightning to get to the earth. And since the earth is one billion times more resistant than copper, a No. 6 copper conductor is more than large enough.
I think you're mixing the requirements of the fault current path [Sec. 250-2(d) and Sec. 250-122] with the requirement of the GEC [Sec. 250-66].
NEC Consultant, Mike Holt Enterprises, Inc.
Incorrect Motor Temperature Ratings?
I enjoyed the September 2000 Motor Facts article, The Hot Issue of Motor Temperature Ratings. However, I believe the insulation class temperature ratings in the chart are incorrect (by a factor of 10).
Senior Electrical Engineer,
Ultramar Diamond Shamrock
You're absolutely right! Thanks for pointing that out. Now I have no place making fun of those NASA engineers who crashed the little spaceship into Mars in 1999.
Technical Editor, EC&M
Improperly Bonded Conduits
I have been mildly disappointed that someone has not written about the Code violations associated with the photo of the September 2000 issue. The grounding conductor running through each of the rigid galvanized conduits is required to be bonded to the conduit on each end.
ETA Engineering and Consultants
The two conduits shown coming DOWN the steel column will be effectively bonded via the bolted clips that support them. This will happen when the grounding electrode conductor (GEC) is attached to the column via the column-grounding clamp (i.e., the copper wedge with two studs). The conduit coming UP from the slab is another matter. Here, I think you're correct. The conduit needs a grounding bushing so that it's effectively bonded per Sec. 250-92 (a)(3) of the NEC. Sometimes, these mistakes happen when manufacturers "stage" construction photos. Nevertheless, we should have caught this.
Editorial Director, EC&M