Originally Posted By: jtedesco
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UL-67 was developed to cover panelboards and I have just been advised (I am at a meeting) by a very qualified NEMA representative that the rule now in 408.21 was in that UL Standard from the beginning. You can go to
www.ul.com and search for a representative and ask an engineer.
Here is the new rule and commentary:
408.21 Grounded Conductor Terminations.
Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.
In accordance with their listing and the requirement of 110.14(A), conductor terminations are suitable for a single conductor unless the terminal is marked or otherwise identified as suitable for more than one conductor. This requirement applies only to the termination of grounded conductors in panelboards. The use of a single termination point within a panelboard to connect more than one grounded conductor or to connect a grounded conductor and an equipment grounding conductor can be problematic when it is necessary to isolate a particular grounded conductor for testing purposes. For example, if the grounded conductors of two branch circuits were terminated at a single connection point and it was necessary to isolate one branch circuit for the purposes of troubleshooting, the fact that the circuit not being tested remains energized can create an unsafe working condition for service personnel disconnecting the grounded conductor of the circuit that is being tested. There are panelboard instructions that permit the use of a single conductor termination for more than one equipment grounding conductor. See 408.20 for the requirements on panelboard terminations for grounded and equipment grounding conductors.
Exception: Grounded conductors of circuits with parallel conductors shall be permitted to terminate in a single terminal if the terminal is identified for connection of more than one conductor.
408.20 Grounding of Panelboards.
Panelboard cabinets and panelboard frames, if of metal, shall be in physical contact with each other and shall be grounded. Where the panelboard is used with nonmetallic raceway or cable or where separate grounding conductors are provided, a terminal bar for the grounding conductors shall be secured inside the cabinet. The terminal bar shall be bonded to the cabinet and panelboard frame, if of metal; otherwise it shall be connected to the grounding conductor that is run with the conductors feeding the panelboard.
A separate equipment grounding conductor terminal bar must be installed and bonded to the panelboard for the termination of feeder and branch-circuit equipment grounding conductors. Where installed within service equipment, this terminal is bonded to the neutral terminal bar. Any other connection between the equipment grounding terminal bar and the neutral bar, other than allowed in 250.32, is not permitted. If this downstream connection occurs, current flow in the neutral or grounded conductor would take parallel paths through the equipment grounding conductors (the raceway, the building structure, or earth, for example) back to the service equipment. Normal load currents flowing on the equipment grounding conductors could create a shock hazard. Exposed metal parts of equipment could have a potential difference of several volts created by the load current on the grounding conductors. Another safety hazard created by this effect, where subpanels are used, is arcing or loose connections at connectors and raceway fittings, for example, creating a potential fire hazard. Exhibit 408.4 illustrates the connection of the grounded conductor (neutral bar) to the metallic service equipment enclosure via the main bonding jumper.
Exhibit 408.4 A split-bus lighting and appliance branch-circuit panelboard supplying an individual residential occupancy.
Exception: Where an isolated equipment grounding conductor is provided as permitted by 250.146(D), the insulated equipment grounding conductor that is run with the circuit conductors shall be permitted to pass through the panelboard without being connected to the panelboard?s equipment grounding terminal bar.
Grounding conductors shall not be connected to a terminal bar provided for grounded conductors (may be a neutral) unless the bar is identified for the purpose and is located where interconnection between equipment grounding conductors and grounded circuit conductors is permitted or required by Article 250.
The grounding of electronic equipment as well as overall power quality is of concern to the electrical industry. Sensitive electronic equipment used in industrial and commercial power systems may fail to perform properly if electrical noise is present in the equipment grounding conductor.
An isolated equipment grounding terminal is permitted if required for the reduction of electrical noise on the grounding circuit. This equipment grounding terminal must be grounded by an insulated equipment grounding conductor that is run with the circuit conductors. The isolated equipment grounding conductor is also permitted to pass through one or more panelboards (without connection to the panelboard grounding terminal), but it is very important that the equipment grounding conductor terminate directly at the applicable separately derived system or service grounding terminal. If the isolated equipment grounding conductor is run in a separate building, however, 250.146(D) requires the isolated equipment grounding conductor to terminate at a panelboard within the same building.
A connection to only a separate grounding electrode that places the earth in the fault return path may prevent sufficient current for opening overcurrent protection when a ground fault occurs. See the commentary following 250.146(D) FPN and 250.54.
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Joe Tedesco, NEC Consultant
www.nachi.org/tedescobook.htm
