Q: Does the grounding-electrode conductor to driven ground rods have to be unbroken to both rods or may a splice be made at the first rod, then continue to the second ground rod by installing two ground clamps at the first ground rod, then continuing to the second rod with a 6 AWG copper grounding-electrode conductor?
A:Where required to install two ground rods because a single rod has a resistance to earth of more than 25 ohms, and there are no other grounding electrodes at the site, the grounding-electrode conductor must be in an unbroken length from the service to the two ground rods. If spliced, an irreversible compression-type connector listed as grounding and bonding equipment or exothermic welding must be used to make the grounding-electrode conductor continuous.
Where it takes two ground rods to make a single grounding electrode because the resistance-to-earth of a single rod is more than 25 ohms, two driven rods are required to make one Code-recognized grounding electrode.
Where a single driven ground rod has a resistance-to-earth of 25 ohms or less, the grounding-electrode conductor is permitted to be run to any convenient electrode that is a part of the grounding-electrode system. This also applies to two or more ground rods. References for these requirements are in 250.50, 250.64(C) and 250.64(F).
Q: Is it permissible to connect the grounding-electrode conductor at the service to a lug on the bottom and outside of the service-disconnect enclosure?
A: Not if the service is supplied from a grounded AC system. A grounded service conductor of an AC system must be connected to the grounding-electrode conductor as outlined in 250.24(A)(1) through (A)(5).
Here is what part (A) of 250.24 requires: “Grounding Service-Supplied Alternating-Current Systems: A premises wiring system supplied by a grounded AC service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through 250.24(A)(5).
“Part (A) allows connection of the grounding electrode conductor to the grounded circuit conductor at the load end of the service drop conductors or service lateral including the terminal bus to which the grounded circuit conductor is connected in the service disconnecting means. Part (A)(4) allows the grounding electrode conductor to be connected to the equipment-grounding bus where the main bonding jumper is connected from the grounded conductor bus to the equipment-grounding conductor terminal bar in the service equipment enclosure.”
Where the AC system is ungrounded, the grounding-electrode conductor is permitted to terminate on the outside of the service-disconnecting means enclosure. This permitted construction is mentioned in 250.24(E).
Q: What wiring methods are permitted in a large church that can accommodate about 300 people?
A: This answer is based on the type of construction used to build the church. Generally, Article 518-Assembly Occupancies applies. If the structure is not required by the local building code to be fire-rated, any wiring methods in Chapter 3 that can be installed to comply with the appropriate article for the wiring method are acceptable.
This permission appears in 518.4(B). Where the local building code requires that the church building be of fire-rated construction, the wiring methods are limited to metal raceways; nonmetallic raceways encased in a minimum of 2 inches of concrete; or Types MI, MC or AC cable containing an insulated equipment-grounding conductor.
Where the interior walls, floors and ceiling provide a thermal barrier of material that provides a 15-minute finish rating, electrical nonmetallic tubing or rigid nonmetallic conduit may be installed where protected by the finish-rating.
Tap conductors length
Q: I have a job to reconnect the wiring for three new air-conditioning units that will replace existing units that were installed about 20 years ago. There is an existing 200A feeder that has three taps (one for each air conditioner) of 6 AWG Type THWN conductors. These taps vary in length from 32 to 45 feet. The electrical inspector is requiring overcurrent protection so that the length of each tap does not exceed 25 feet. Is this necessary since the feeder and taps are existing?
A: I looked at the tap rules in the 1984 edition of the National Electrical Code, and the requirements for the 25-foot tap rule are similar to those appearing in the 2005 edition. The electrical inspector is asking for correction of a Code violation that was created by the original installer of the wiring for the air conditioners.
In the 1984 edition, taps not over 25 feet long are mentioned in Exception No. 3 to 240-21.
Here is a summary: Tap conductors cannot be longer than 25 feet; tap conductors must have an ampacity that is at least one-third the ampacity of the feeder conductors or overcurrent protection, the tap conductors terminate in a single set of fuses or a circuit breaker that will limit the current to the ampacity of the tap conductor, and tap conductors are protected from physical damage and are installed in a raceway.
The tap conductor ampacities are not quite equal to one-third of 200A. The ampacity of 6 AWG Type THWN copper is 65A. One-third of 200 is 66.66A. Also a change in 240.21(B) in the 2005 edition of the NEC does not allow the overcurrent protection for the tap conductors to be increased to the next larger size where the ampacity of the tap conductors does not match a standard-size fuse or circuit breaker. Finally, there are locations where tap conductors are permitted to be longer than 25 feet-one is a high-bay manufacturing building and the other where taps are located outside. [see 240.21(B)(4) and (5).]
Q: Where there is no buried metal water pipe, and the one-family dwelling unit is wood-frame construction, may a concrete-encased electrode as defined in 250.52(A)(3) be used as the sole grounding electrode to ground the neutral at the service?
A: At least 20 feet of half-inch or larger steel reinforcing bars-located in the concrete foundation that is in contact with the earth-is a recognized grounding electrode that is connected to the grounded circuit conductor with a 4 AWG copper grounding-electrode conductor. This grounding electrode is the only one required where no other grounding electrodes are present.
Pool grounding and bonding
Q: What grounding and bonding requirements apply to an air-conditioning remote-condensing unit located about four feet from the inside wall of an in-the-ground swimming pool? There is no other space in the backyard of this single-family dwelling to locate this air-conditioning unit.
A:All metal parts of equipment and metal-wiring methods that are within five feet horizontally or the inside walls of the pool must be bonded. Equipotential bonding must be provided by connection of the metal enclosure of the air conditioner to the swimming pool reinforcing steel with a 8 AWG solid copper bonding conductor.
Metal raceways or metal-sheathed cables that contain the branch-circuit conductors must provide an insulated equipment-grounding conductor for grounding the AC unit.
Metal-wiring methods must also be bonded to the steel reinforcing bars if not properly bonded to the air-conditioner enclosure. References for these requirements are in 680.21(A) 680.25 and 680.26.
Single neutral bus terminals
Q: While making inspections, I have noticed a few installations where two branch circuit neutral conductors are inserted in the same neutral bus terminal. There is no marking in the panelboard to indicate this is permitted or not permitted. I have criticized this arrangement, but I am finding out that this is common practice in this area. What is your advice? Should I stop writing this violation notice?
A: No, you should not. Grounded branch-circuit conductors (neutrals) are required to be terminated individually in a terminal on the neutral bus. Terminating two or more neutral conductors in a single hole on the neutral bus in a panelboard was a problem that initiated a proposal, which was accepted for the 2002 edition of the NEC.
The new material in 2002 edition appears in 408.21 and reads like this: “Grounded Conductor Terminations: Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.” For the 2005 edition, the same words appear in 408.41.
There is no marking in the panelboard because UL Standards do not require any markings where the wire terminals are only acceptable for a single conductor. This sentence is in the 2004 edition of the “General Information for Electrical Equipment Directory” published by Underwriters Laboratories Inc. under Guide (AALZ): “Terminals-Product terminals, including wire connectors and terminal screws are acceptable for connection of only one conductor, unless there is marking or a wiring diagram indicating the number of conductors which may be connected.” EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.