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More Than One Neutral Conductors on a Terminal - Update

Re: http://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/More-Than-One-Neutral-Under-a-Terminal~20040730.php

Comment No. 1 Is it ok to place a neutral and ground wire under the same lug in a panel or sub-panel?

Mike's Answer: No, see 408.20.

Comment No. 2 I don't understand the reasoning for not allowing multiple neutral conductors on the same lug in a panel. The neutrals are all linked to one bar so what difference does it make. Please enlighten me about this.

Mike's Answer: The following is extracted from my textbook, Understanding the National Electrical Code.
408.41 Grounded (Neutral) Conductor Terminations
Each grounded (neutral) conductor within the panelboard must terminate in an individual terminal. Figure 408-15

Author's Comment: If two grounded (neutral) conductors were connected to the same terminal, and someone removed one of them, the other might unintentionally be removed as well. If that happens to the grounded (neutral) conductor of a multiwire circuit, it could result in excessive line-to-neutral voltage for one of the circuits. See 300.13(B) for details. Figure 408-16

300.13 Splices and Pigtails
(B) Pigtail Neutrals. In multiwire branch circuits, the removal of a wiring device, such as a receptacle, must not interrupt the continuity for the grounded (neutral) conductor. Therefore, the grounded (neutral) conductors must be spliced together, and a pigtail must be provided for device terminations. Figure 300-31

Author's Comment: The opening of the ungrounded (hot) conductors, or the grounded (neutral) conductor of a 2-wire circuit, during the replacement of a device doesn't cause a safety hazard, so pigtailing of these conductors isn't required [110.14(B)].

CAUTION: If the continuity of the grounded (neutral) conductor of a multiwire circuit is interrupted (open), the resultant over- or undervoltage could cause a fire and/or destruction to electrical equipment.

Example: A 3-wire, 120/240V circuit supplies a 1,200W, 120V hair dryer and a 600W, 120V television. If the grounded (neutral) conductor of the multiwire circuit is interrupted, it will cause the 120V television to operate at 160V and consume 1,067W of power (instead of 600W) for only a few seconds before it burns up. Figure 300-32

Step 1. Determine the resistance of each appliance, R = E2/P.
R of the Hair Dryer = 120V2/1,200W
R of the Hair Dryer = 12 ohms

R of the Television = 120V2/600W
R of the Television = 24 ohms

Step 2. Determine the current of the circuit, I = E/R.
I = 240V/36 ohms (12 ohms + 24 ohms)
I = 6.7A

Step 3. Determine the operating voltage for each appliance, E = I x R.
Voltage of Hair Dryer = 6.7A x 12 ohms
Voltage of Hair Dryer = 80V

Voltage of Television = 6.7A x 24 ohms
Voltage of Television = 160V

WARNING: Failure to terminate the ungrounded (hot) conductors to separate phases could cause the grounded (neutral) conductor to become overloaded, and the insulation could be damaged or destroyed by excessive heat. Conductor overheating is known to decrease insulating material service life, which creates the potential for arcing faults in hidden locations and could ultimately lead to fires. It isn't known just how long conductor insulation will last, but heat does decrease its life span. Figure 300-33

Understanding the NEC, Volume 1 Book — 2002

Understanding the NEC Volume 1 was written to provide insight into, and an understanding of, many of the technical rules of the NEC. This book, printed in four color, contains clear graphics, examples, and 1200+ summary questions. Based on the 2002 NEC.

Product Code: 02UND1
ISBN: 0-9710307-1-5
Pages: 430
Illustrations: 830

Table of Contents
Sample Pages
Sample Graphic

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