Consider a typical 120/240V single-phase service with a separate cast meter socket and a rigid steel conduit nipple connecting the meter socket to the service equipment enclosure. The meter socket, the nipple and the service-equipment enclosure are required by 250.92(A)(1) and (2) to be grounded through the methods in 250.92(B), including bonding to the grounded conductor of the service. The cast meter socket has a built-in grounding means connecting the neutral conductor to the socket base. The main bonding jumper in the service equipment bonds the grounded (neutral) conductor to the enclosure and to the equipment-grounding means.
The conduit nipple and the neutral conductor are in parallel for the (usually) short distance between the meter socket and the service enclosure. This is an undesirable condition, allowing normal neutral current to travel on the conduit nipple. The argument against allowing neutral current to travel on the conduit system is twofold: First, the impedance of the neutral conductor will be increased, and second, the conduit may be in contact with building steel, other piping, etc., creating other alternate paths. Loose or corroded couplings, loose locknuts or casual contacts somewhere in the path of the conduit may allow sparking and arcing that would be a fire hazard. This is also the reason given against installing a main bonding jumper in a panelboard within the same building but remote from the service location.
In spite of this well-understood Code violation, the installation described is very common. It is probably safe because the short nipple is not subject to the problems that could occur in a long run of conduit throughout a premise.
NEC 250.6(B) sets out possible corrections for objectionable currents. One of these would be to replace the rigid steel conduit nipple with rigid nonmetallic conduit (PVC), thus eliminating the parallel path for the neutral current.
For separately derived systems this problem is specifically addressed in 250.20(B) and 250.30(A), where a transformer meeting the definition of a separately derived system in Article 100 (having no direct electrical connection between the primary and secondary circuits) is permitted to have the secondary neutral conductor grounded and bonded to the grounding-electrode conductor and the equipment-grounding conductors either at the source (the transformer secondary terminals) or at the first disconnect or overcurrent protection. Whichever location is chosen must be the same one as the connection of the grounding-electrode conductor.
Exception No.1 to 250.30(A)(1) says a bonding jumper can be installed at both the source (the transformer terminals) and the first disconnect provided a parallel path for the grounded conductor is not established. It is strange that this same warning does not appear in the Code for a utility-supplied service.
The correction, again, is to replace the steel conduit run between the transformer and the first disconnect (usually the main in a panelboard) with a rigid nonmetallic raceway (PVC). If the bonding jumper is installed at the transformer secondary terminals, the neutral bar in the enclosure for the first disconnect should be insulated. Otherwise, even with a PVC raceway connection there could be a parallel path for the neutral conductor through building steel to which both the transformer enclosure and the enclosure for the first disconnect are fastened.
On a different subject, 250.64(B) does not permit flexible metal conduit as the enclosure for grounding electrode conductors AWG 6 and smaller. The reason for this is the conductor and enclosing raceway are in parallel—the raceway must be connected at both ends as required by 250.64(E). Metal flex has a much higher resistance than the other raceways permitted in 250.64(B) because the oil used in the manufacture of the flex makes in effect the resistance based on the cross section of the steel pulled out to its entire length, approximately three times the length of the run. Due to the skin effect, especially in the larger wire sizes, only 3 percent of the current flows in the grounding electrode conductor, while 97 percent flows in the enclosing metal raceway. The relatively high resistance of the flex makes it a poor choice to enclose the grounding-electrode conductor. The question then arises, why does 250.64(B) not specify which raceways shall be used for AWG 4 and larger conductors? It is more important that flex not be used for the larger conductors, but no such requirement is mentioned in 250.64(B). EC
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at email@example.com.