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#93362 - 05/19/05 02:41 PM 250-97
dannynova Offline
Member
Registered: 12/22/03
Posts: 18
Loc: new york
is it required to bond around concentric ko's over 250 volts to ground if a egc is pulled.
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#93363 - 05/19/05 03:37 PM Re: 250-97
resqcapt19 Offline
Member
Registered: 11/10/00
Posts: 2148
Loc: IL
yes
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Don(resqcapt19)
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#93364 - 05/19/05 05:37 PM Re: 250-97
George Little Offline
Member
Registered: 01/18/04
Posts: 1488
Loc: Michigan USA
I agree with Don. As for a small explanation: If you are pulling in an equipment grounding conductor, I suspect you are connecting it to the enclosure on both end of the conduit run, which is nice but if ther is a ground fault inside the conduit there is no effective path accross the concentric knock-out(s) and the metallic raceway will not properly conduct fault current.
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George Little
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#93365 - 05/20/05 05:12 PM Re: 250-97
dannynova Offline
Member
Registered: 12/22/03
Posts: 18
Loc: new york
after reading the code again I agree and i also agree with george's reason but what about the exception to 250-97 where a box with concentric ko's is listed for the purpose of grounding. another words there are boxes or enclosures with concentric ko's that are tested and listed for equipment grounding without a bonding jumper? Is it correct that bonding around reducing washers (donuts) is always required b/c they are not tested or listed to conduct fault current at any voltage?
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#93366 - 05/20/05 06:04 PM Re: 250-97
Joe Tedesco Offline
Member
Registered: 10/07/00
Posts: 2749
Loc: Boston, Massachusetts USA
Read this information for the rest of the story from UL:
http://www.ul.com/regulators/knockouts.cfm

Are concentric or eccentric knockouts evaluated for bonding?
By Chuck Goetz
Section 250.97 of the National Electrical Code® (NEC®) requires a bonding-type fitting with a bonding jumper to be used when conduits are installed within a concentric or eccentric knockout for non-service conductors in systems rated over 250V. The exception to Section 250.97 allows products that have been Listed for the purpose to have conduits connected to a concentric or eccentric knockout without having to provide additional means to bond the conduit.

In the past, only metallic outlet boxes were Listed by UL for use in these applications. However, UL has now investigated concentric and eccentric knockouts in equipment enclosures covered under other product categories for bonding conduits for non-service conductors without additional means.

The following table summarizes the box and enclosure categories where such an evaluation is available and how to identify compliant products that can be installed according to the exception to Section 250.97.

Category Code
Product Category
Concentric and eccentric knockouts evaluated for bonding?

BGUZ
Boxes, Junction and Pull
Only when product is marked*

CYIV
Cabinets and Cutout Boxes
Only when product is marked*

QCIT
Metallic Outlet Boxes
All products comply


* Products Listed for these applications are provided with a marking, such as "Concentric (or eccentric) knockouts suitable for bonding conduits for non-service conductors."

Cabinets and cutout boxes are used in many end-product applications, including "Enclosed Switches (WIAX)," "Panelboards (QEUY)," and "Industrial Control Equipment (NIMX)," and may additionally be evaluated for bonding through concentric or eccentric knockouts. Installers and regulatory authorities should therefore look for the above referenced marking to determine if the concentric or eccentric knockouts have been investigated for bonding in lieu of requiring bonding jumpers.

For more information on bonding applications involving concentric or eccentric knockouts in cabinets, cutout boxes, junction and pull boxes, contact Chuck Goetz in Northbrook, Ill., by phone at +1-847-664-3163; or by e-mail at Charles.A.Goetz@us.ul.com.

For more information on bonding applications involving concentric or eccentric knockouts in metallic outlet boxes, contact George Walbrecht in Northbrook, Ill., by phone at +1-847-664-3126; or by e-mail at George.F.Walbrecht@us.ul.com.

Contact Us
If you have an immediate question or need assistance, please contact Regulatory Services.
_________________________
Joe Tedesco, NEC Consultant
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#93367 - 05/21/05 04:44 AM Re: 250-97
dannynova Offline
Member
Registered: 12/22/03
Posts: 18
Loc: new york
thats for that informational link, my second question was is it required to bond around donuts or reducing washers at any voltage b/c they are not tested or listed as maintaining continuity of the egc as per 250-96(a)
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#93368 - 05/21/05 04:55 AM Re: 250-97
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
Surprisingly reducing washers are suitable for grounding.

From the 2004 UL General Directory

OUTLET BUSHINGS AND FITTINGS (QCRV)

Quote:
GROUNDING
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with ANSI/NFPA 70, ‘‘National Electrical Code.’’


We also have to think about 250.96

Quote:
250.96 Bonding Other Enclosures.
(A) General. Metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings, and other metal non–current-carrying parts that are to serve as grounding conductors, with or without the use of supplementary equipment grounding conductors, shall be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them. Any nonconductive paint, enamel, or similar coating shall be removed at threads, contact points, and contact surfaces or be connected by means of fittings designed so as to make such removal unnecessary.


I keep a cheep wire wheel in my cordless case for removing paint when needed.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#93369 - 05/28/05 12:57 PM Re: 250-97
dana1028 Offline
Member
Registered: 01/09/02
Posts: 163
Loc: San Carlos, CA
We also have to think about 250.96

quote:250.96 Bonding Other Enclosures.
(A) General. Metal raceways, cable trays, cable armor, cable sheath, enclosures, frames, fittings, and other metal non–current-carrying parts that are to serve as grounding conductors, with or without the use of supplementary equipment grounding conductors, shall be effectively bonded where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them. Any nonconductive paint, enamel, or similar coating shall be removed at threads, contact points, and contact surfaces or be connected by means of fittings designed so as to make such removal unnecessary.

I keep a cheep wire wheel in my cordless case for removing paint when needed.


In Soares - Book on Grounding - Chap. 8 (8th ed.) there is reference to a UL report showing virtually all conduit fittings perform satisfactorily under fault conditions on painted surfaces. There is an exception: die cast fittings using locknuts having flat or ribbed surfaces did not perform as well. Quote (p. 130) "It appeared as though locknuts with serrations consistently penetrated through the enclosure paint and provided better electrical contact between the fitting and the metal of the enclosure than did the locknuts with flat or ribbed surfaces." ...and "All of the conduit fittings that were constructed of steel bodies and steel locknuts completed the test with acceptable results."

This UL test was done in 1992, I don't know for sure, but I haven't seen the flat or ribbed type locknuts in quite a while.
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