Grounding vs Bonding Online Training

This article was posted 02/10/2006 and is most likely outdated.

Grounding vs Bonding: 250.66 Grounding Electrode Conductor—Size

February 10, 2006

View Comments | Add Comments

PART III. GROUNDING ELECTRODE SYSTEM AND GROUNDING ELECTRODE CONDUCTOR

VIEWING Video FILE requires Real Media Player [ FREE ]

Page 12 of 14

Sections

250.66 Grounding Electrode Conductor—Size.

Except for a ground rod electrode [250.66(A)], a concrete-encased electrode [250.66(B)], or a ground ring electrode [250.66(C)], the grounding electrode conductor must be sized based on the largest service-entrance conductor or equivalent area for parallel conductors in accordance with Table 250.66

Question: What size grounding electrode conductor is required for a 1,200A service that is supplied with three parallel sets of 600 kcmil conductors per phase? Figure 250–116

(a) 1 AWG (b) 1/0 AWG (c) 2/0 AWG (d) 3/0 AWG

Answer: (d) 3/0 AWG

The equivalent area of three parallel 600 kcmil conductors is 1,800 kcmil per phase [Table 250.66].

FPN: Because the grounded neutral service conductor is required to serve as the low-impedance ground-fault current path back to the source, it must be sized no smaller than that shown in Table 250.66 [250.24(C)(1)]. Of course, it must be sized to carry the maximum unbalanced load as calculated by 220.61.

(A) Ground Rod. Where the grounding electrode conductor is connected to a ground rod, that portion of the grounding electrode conductor that is the sole connection to the ground rod isn’t required to be larger than 6 AWG copper. Figure 250–117

Author’s Comment: See 250.52(A)(5) for the installation requirements of a ground rod electrode.

(B) Concrete-Encased Grounding Electrode (Ufer Ground). Where the grounding electrode conductor is connected to a concrete-encased electrode, that portion of the grounding electrode conductor that is the sole connection to the concrete-encased electrode isn’t required to be larger than 4 AWG copper. Figure 250–118

Author’s Comment: See 250.52(A)(3) for the installation requirements of a concrete-encased electrode.

(C) Ground Ring. Where the grounding electrode conductor is connected to a ground ring, that portion of the conductor that is the sole connection to the ground ring isn’t required to be larger than the conductor used for the ground ring.

Author’s Comment: A ground ring encircling the building or structure in direct contact with earth must consist of not less than 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4)].

Figure 250–116
(Click on image to enlarge)

Figure 250–117
(Click on image to enlarge)

Figure 250–118
(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

Comments

I support the idea that a larger service size will have a lower supply impedance, therefore a higher available ground fault, however I doubt if there's anything official to back up increasing the GEC beyond 4AWG. The reasoning behind the 4AWG size is that the impedance of the UFER ground alone is high enough that the division of ground fault current between the UFER path and the neutral path is such that 4AWG is adequate. If the electrode length and GEC size are increased, thereby lowering the path's impedance, more of the fault will follow the UFER path. I wouldn't want that. I do see an advantage of more UFER capacity in the event that the neutral path is interrupted; then there'd be safety problem. To guard against that the size would probably need to be much greater.

james wilson

Reply from: Mike Holt
I've done the math, the impedance of the Ufer ground does not have any impact on the fault current to the source and it has no effective impact if the service neutral is open. So to close this topic, the purpose of the Grounding Electrode System is not to assist in clearing a ground, it's purpose is to ground metal parts to the earth to reduce transient overvoltage... but this is another topic.

Reply from: James Wilson
The UFER ground is in parallel with the grounded conductor back to the source. In case of a ground fault a portion of that fault, although a small amount, will flow thru the UFER electrode back to the grounded source. If the UFER ground is increased in length, say from 20ft to 40ft, the resistance of the earth connection is reduced by 26%. That's using a formula from IEEE Std 142 "The Green Book" on grounding. This reduction would result in a slight increase in the return current thru the UFER. As I stated I see no advantage in increasing the capacity of the UFER path. There is nothing in the previous comments that implied that the grounding electrode, be it a UFER or any other approved type, is used to assist in clearing a ground fault. The NEC doesn't require that it be sized to guard against a neutral being open, which, if that occurred would be influenced by a lower UFER resistance.

Reply to this comment


* 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