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Topic:   Double taps at grounding bars - 1428 visits (1 today, 2 this week)

Gary Boyet
Member


Posts: 20
From:St. Louis, Missouri
Registered: Feb 2003

posted December 17, 2005 05:40 AM            

I know we've discussed this several tiimes but I cannot locate. Can someone point me in the direction of why double taps at grounds and neutrals are wrong. Having a problem with the electrical contractor on a 1 year inspection of new home. Thank you in advance

Scott Patterson
Member



Posts: 2959
From:Ridgeland (Jackson), MS
Registered: Mar 2001

posted December 17, 2005 06:18 AM            

One reason is that unless the panel manufacture says it is OK, it is not. I bet if you look at the panel schematics it will tell you that each line should be under its own lug. Keep in mind that current can travel through the lines so if multiple lines are attached to a single terminal or lug, you could have arcing.

Is it likely? No it is not. Is it possible? Yes it is.

John Arnold
Member


Posts: 605
From:Philadelphia PA
Registered: Dec 2001

posted December 17, 2005 07:43 AM            

Most panels, if you read the labels carefully, indicate that only one "neutral" can be under a terminal screw, and that certain combinations of grounding wires are ok under the same screw (but not with the neutral), depending on size.
Also, try searching in "electrical systems" with the words "neutral double tapped".

[This message has been edited by John Arnold (edited December 17, 2005).]

John Arnold
Member


Posts: 605
From:Philadelphia PA
Registered: Dec 2001

posted December 17, 2005 07:55 AM            

NEC 2002: 408.21 "Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor."

John Arnold
Member


Posts: 605
From:Philadelphia PA
Registered: Dec 2001

posted December 17, 2005 07:58 AM            

More! From Mike Holt -
408.21 Grounded Conductor Terminations (Open Neutral)
Intent: This new section should ensure that grounded (neutral) conductors terminate within the panelboard to an individual terminal. This has been a UL requirement (UL Std. 67 – Panelboard Standard) for some time, and the addition to the NEC is intended to bring this information to the installers. Technically, this is covered by 110.3(B), which requires all equipment to be installed in accordance with the manufacturer's installation instructions and markings, but nobody knew it existed. UL Std. 67 (Panelboard Standard) permits up to three 10 AWG equipment grounding conductors to terminate on a single terminal, if the terminal is marked for this purpose.

The intent of this requirement is to ensure that the grounded (neutral) conductor of a multiwire branch circuit is not momentarily disconnected, which could result in the destruction of electrical equipment and fires from overvoltage.

John Steinke
Member


Posts: 122
From:Reno, Nv.
Registered: Jul 2003

posted December 17, 2005 09:18 AM            

As a general rule, there is no problem with as many as three ground wires under the same screw. This information is spelled out in the UL "White book," and is probably buried in the fine print of the panel maker's literature.

Since the terminals are suitable for more than one ground wire, why are they not also suitable for more than one neutral ("groundED conductor")? Because, as the Mike Holt cite explains, an open (or loose) neutral that is shared by more than one circuit ("multiwire circuit") will result in those circuits adding their voltages- with the result of 220 volts being applied to 110 volt appliances.

This was a matter clarified, I believe, in the '96 NEC, which was the first to specify that neutrals be landed separately.

John Arnold
Member


Posts: 605
From:Philadelphia PA
Registered: Dec 2001

posted December 17, 2005 09:26 AM            

quote:

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As a general rule, there is no problem with as many as three ground wires under the same screw.

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[This message has been edited by John Arnold (edited December 17, 2005).]

Richard Shaffer
New Member


Posts: 3
From:Marblehead, Ma.
Registered: Nov 2005

posted December 19, 2005 11:38 AM            

It is never a good idea to double tap.However, Double D breakers are set up to receive two wires.