This article was posted 11/11/2005 and is most likely outdated.

Grounding vs Bonding Online Training
 

 
Grounding vs Bonding: 250.32 Buildings or Structures Supplied by a Feeder or Branch Circuit Continued

November 11, 2005 

  View Comments | Add Comments  

Many of you have completed our Online Continuing Education Courses which include video instruction and know this is a very fast way to obtain your CEU credits from the comfort of the chair you're sitting in. For the rest of you we are sending a weekly series of newsletters featuring an instructional page complete with videos.

To view the videos you will need to download the free Real Player Download Real Player plugin for your browser - if you don't already have it. After installing Real Player, you can view the videos by clicking on one of these icons:

  • Click on this icon Hi Res - Cable/DSL to view the video

Below is the full course page which includes these videos:

Grounding versus Bonding
PART II.  Hi Res - Cable/DSL [4392Kb]       Page 11 of 13
Sections

250.32 Buildings or Structures Supplied by a Feeder or Branch Circuit Continued

The equipment grounding (bonding) conductor, if of the wire type, must be sized in accordance with 250.122, based on the rating of the feeder protection device.

CAUTION: To prevent dangerous objectionable current from flowing onto metal parts of the electrical installation, as well as metal piping and structural steel [250.6(A)], a building or structure disconnecting means supplied by a feeder must not have the grounded neutral conductor bonded to the building or structure disconnecting means. Figures 250–85 and 250–86
(2) Grounded Neutral Conductor. When an equipment grounding (bonding) conductor is not run to the building or structure disconnecting means, the building or structure disconnecting means can be bonded to a grounded neutral conductor installed with the feeder conductors. This is only permitted where there’s no continuous metallic path between buildings and structures, and ground-fault protection of equipment isn’t installed on the supply side of the feeder.

Where the grounded neutral feeder conductor serves as the effective ground-fault current path, it must be sized no smaller than the larger of:

(1) The maximum unbalanced neutral load in accordance with 220.61.

(2) The available fault current in
accordance with 250.122.

CAUTION: Using the grounded neutral conductor as the effective ground-fault current path poses potentially dangerous consequences and should only be done after careful consideration. Even if the initial installation doesn’t result in dangerous objectionable current on metal parts, there remains the possibility that a future installation of metal piping or cables between the buildings or structures could create unwanted parallel neutral current paths.

Author’s Comment: The preferred practice (or at least my preferred practice) is to not use the grounded neutral conductor as the effective ground-fault current path, but to install an equipment grounding (bonding) conductor with the feeder conductors to the building or structure in accordance with 250.32(B)(1).
(E) Grounding Electrode Conductor. The grounding electrode conductor for a separate building or structure disconnecting means must terminate to the grounding terminal of the disconnecting means and it must be sized in accordance with 250.66, based on the largest ungrounded feeder conductor.

Question: What size grounding electrode conductor is required for a building disconnect that is supplied with 3/0 AWG? Figure 250–87

(a) 4 AWG    (b) 3 AWG    (c) 2 AWG    (d) 1 AWG

Answer: (a) 4 AWG, Table 250.66

Author’s Comment: Where the grounding electrode conductor is connected to a ground rod, that portion of the conductor that is the sole connection to the ground rod isn’t required to be larger than 6 AWG copper [250.66(A)]. Where the grounding electrode conductor is connected to a concrete-encased electrode, that portion of the conductor that is the sole connection to the concrete-encased electrode isn’t required to be larger than 4 AWG copper [250.66(B)].



Figure 250–85
(Click on image to enlarge)


Figure 250–86
(Click on image to enlarge)


Figure 250–87
(Click on image to enlarge)

   

Grounding versus Bonding Library - DVDs
Grounding and bonding problems are at epidemic levels. Surveys repeatedly show a high percentage of power quality problems are due to poor grounding and bonding. Electrical theory has been applied to this difficult to understand Article, making it easier for students to grasp the concepts of grounding and bonding. Additionally, Mike has color coded the graphics so you can easily differentiate between grounding and bonding. For a limited time, you can order the Entire Grounding versus Bonding Library including the textbook, 2 videos, MP3 Audio CD and the Online Program for this great price. You save over $200!

Product Code: 05GBDVD
ISBN: N/A
MultiMedia: Tape & CDROM & DVD

DVD/CD Return Policy: Unopened sealed DVD's or CD's may be returned within 10 days and we will credit your credit card or issue a refund for the price of the item(s). Opened items may not be returned unless they arrive defective.
Table of Contents
Sample Pages
Sample Graphic

Copyright © 2005 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)

 

Comments

Add Your Comments to this Newsletter
* Your Name:
   Your name will appear under your comments.

* Your Email:
   Your email address is not displayed.
* Comments:

This newsletter is closed to new comments.

Email Notification Options:
Notify me when a reply is posted to this comment
Notify me whenever a comment is posted to this newsletter