AskCodeMan

Codeman:

I understand that the NEC allows only one neutral conductor or two grounding condutors under each lug in the panelboard bus bar. Is there a prohibition concerning the practice of inserting one common and one groudning conductor under the same lug?

Thanks,

Aaron

Hi Aaron,

Actually the National Electrical Code (NEC) addresses the number of conductors in a single terminal in two basic ways, as shown in the following NEC code references.

From the 2008 NEC. (underlining and bold are mine)
- ARTICLE 110 Requirements for Electrical Installations
- - 110.14 Electrical Connections.
- - - (A) Terminals. Connection of conductors to terminal parts shall ensure a thoroughly good connection without damaging the conductors and shall be made by means of pressure connectors (including set-screw type), solder lugs, or splices to flexible leads. Connection by means of wire-binding screws or studs and nuts that have upturned lugs or the equivalent shall be permitted for 10 AWG or smaller conductors.
- - - - Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.

The above section specifies that each terminal may only have one conductor terminate in it, with the exception that if the terminal is to be used for more than one conductor, the terminal must be identified for more than one conductor (i.e., the terminal has been tested, listed, and labeled, for use with more than one conductor). Normal standard terminals are seldom, if ever, identified for more than one conductor - except for when used for equipment grounding conductors. There are a few designs of terminals which allow for more than one conductor, and those are typically obvious visually as well as being identified for more than one conductor, and those terminal will state the number of conductors and the sizes allowed in that terminal.

The other part of your question is specifically regarding neutral conductors and whether one neutral conductor can have any additional conductors in the same terminal with it.

That is addressed in the following code section. (underlining and bold are mine)
- ARTICLE 408 Switchboards and Panelboards
- - III. Panelboards
- - - 408.41 Grounded Conductor Terminations.
- - - - Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.
- - - - - 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.

The above specifically states, with only one exception, that one grounded conductor (i.e., a neutral conductor), and only one grounded conductor, is allowed in one terminal, regardless of whether another conductor is a grounded conductor, an equipment grounding conductor, or otherwise.

The one exception is for parallel conductors when installed in a terminal designed, tested, listed, and labeled, for the installation of more than one conductor, but only under the conditions which allow for parallel conductors.

The short answer for this question: "Is there a prohibition concerning the practice of inserting one common and one grounding conductor under the same lug?" is:
- Yes, NEC 408.41 prohibits any other conductor to be in the same terminal with a grounded (neutral) conductor (with that one very limiting exception for parallel conductors).

Codeman