CONCENTRIC KNOCK OUTS
- Thread starter chief4219
- Start date
- Status
- Location
- Bremerton, Washington
Re: CONCENTRIC KNOCK OUTS
For services see 250.92
For circuits over 250 Volts to ground see 250.97
For hazardous locations see Article 500 and 250.92(B)
Otherwise bonding around eccentric and concentric knock outs is not required, but can be done.
Also bonding is required around reducing washers if the metal raceway is used as an equipment grounding conductor per 250.118
Also always used assured bonding for circuits over 250 V to ground, even with clean KO's.
For services see 250.92
For circuits over 250 Volts to ground see 250.97
For hazardous locations see Article 500 and 250.92(B)
Otherwise bonding around eccentric and concentric knock outs is not required, but can be done.
Also bonding is required around reducing washers if the metal raceway is used as an equipment grounding conductor per 250.118
Also always used assured bonding for circuits over 250 V to ground, even with clean KO's.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: CONCENTRIC KNOCK OUTS
Scott
I have seen first hand as to why this is required we had a 3/4" conduit comming from a range receptical that had shorted, and the conduit was through 3/4" X 1 1/2" reducer washers. the emt connector on the end just arced to the point it caught the stud next to the panel on fire. when we saw it the fire department had shut the meter off and opened the wall by the panel the two reducer washers were melted to nothing.
All it takes is a little crosion to have this to happen.
Scott
I have seen first hand as to why this is required we had a 3/4" conduit comming from a range receptical that had shorted, and the conduit was through 3/4" X 1 1/2" reducer washers. the emt connector on the end just arced to the point it caught the stud next to the panel on fire. when we saw it the fire department had shut the meter off and opened the wall by the panel the two reducer washers were melted to nothing.
All it takes is a little crosion to have this to happen.
- Location
- Bremerton, Washington
Re: CONCENTRIC KNOCK OUTS
Scott, reducing washers do not allow the low impedance path as required in 250.4(A)(3). They have to be bonded around.
Scott, reducing washers do not allow the low impedance path as required in 250.4(A)(3). They have to be bonded around.
dana1028
Senior Member
- Location
- San Francisco Bay area
Re: CONCENTRIC KNOCK OUTS
Tom - could you provide additional information about reducing washers not being an adequate low impedance path?
Your comment caused me to go 'alooking'.
In Soares 'Book on Grounding' 6th, 7th and 8th editions they give an emphatic "reducing washers are not suitable for bonding and should always be bonded around to provide an adequate fault-return path."
However they give no reference to tests, footnotes or studies, etc. Soares is just chocked full of tests, standards and documentation about what does and does not work [grounding] but give nothing for reducers.
I confess I don't have a clue where to look in any of the UL books (white - green) for information.
thanx for any additional information you can provide.
Tom - could you provide additional information about reducing washers not being an adequate low impedance path?
Your comment caused me to go 'alooking'.
In Soares 'Book on Grounding' 6th, 7th and 8th editions they give an emphatic "reducing washers are not suitable for bonding and should always be bonded around to provide an adequate fault-return path."
However they give no reference to tests, footnotes or studies, etc. Soares is just chocked full of tests, standards and documentation about what does and does not work [grounding] but give nothing for reducers.
I confess I don't have a clue where to look in any of the UL books (white - green) for information.
thanx for any additional information you can provide.
- Location
- Bremerton, Washington
Re: CONCENTRIC KNOCK OUTS
Its in Mike Holts Grounding and Bonding text, page 53. He states Reducing washers can not be used to maintain the effective ground ffault current path (as required in 250.96(A).
I highly recommend his grounding and bonding text. Mike is very knowledgeable on the topic.
Its in Mike Holts Grounding and Bonding text, page 53. He states Reducing washers can not be used to maintain the effective ground ffault current path (as required in 250.96(A).
I highly recommend his grounding and bonding text. Mike is very knowledgeable on the topic.
dana1028
Senior Member
- Location
- San Francisco Bay area
Re: CONCENTRIC KNOCK OUTS
Thanx Tom, I have Mike's book 'Grounding & Bonding', I'd forgotten that information.
However - this isn't what I'm getting at.
With the inspector/electrician scenario:
With bonding around concentric KOs - it's pretty simple - there is a particular code section that spells out this requirement.
With a single ground rod failing the 25 ohm test there is a particular code section and an ohm value given as criteria - pass or fail.
With NM cable - 'fastened in place within 12" of each box' - there is a specific measurement.
However, with reducing washers and the arrow pointing to 250.4(A)(3) "effective ground-fault path" - you've got to admit this is a pretty vague [i.e. not black & white] indictment against reducing washers.
Electrician: "Who says reducing washings don't establish an effive ground-fault path"...
Inspector: "Mike Holt on pg. 53 of his 'Grounding and Bonding' book." !!
Tom - I want to make it clear I do not dispute the accuracy of your statement: reducing washers do not establish an effective ground-fault path.
It's just that there is usually some Ansi, UL, etc. standard that is applied to test and prove this position. I haven't seen any such standard being mentioned in Soares or Holt.
[ May 05, 2003, 05:14 PM: Message edited by: dana1028 ]
Thanx Tom, I have Mike's book 'Grounding & Bonding', I'd forgotten that information.
However - this isn't what I'm getting at.
With the inspector/electrician scenario:
With bonding around concentric KOs - it's pretty simple - there is a particular code section that spells out this requirement.
With a single ground rod failing the 25 ohm test there is a particular code section and an ohm value given as criteria - pass or fail.
With NM cable - 'fastened in place within 12" of each box' - there is a specific measurement.
However, with reducing washers and the arrow pointing to 250.4(A)(3) "effective ground-fault path" - you've got to admit this is a pretty vague [i.e. not black & white] indictment against reducing washers.
Electrician: "Who says reducing washings don't establish an effive ground-fault path"...
Inspector: "Mike Holt on pg. 53 of his 'Grounding and Bonding' book." !!
Tom - I want to make it clear I do not dispute the accuracy of your statement: reducing washers do not establish an effective ground-fault path.
It's just that there is usually some Ansi, UL, etc. standard that is applied to test and prove this position. I haven't seen any such standard being mentioned in Soares or Holt.
[ May 05, 2003, 05:14 PM: Message edited by: dana1028 ]
dana1028
Senior Member
- Location
- San Francisco Bay area
Re: CONCENTRIC KNOCK OUTS
An additional point - I see in 250.97 'Bonding for Over 250 Volts' - Exception - they add the word ' oversized' ...."where oversized, concentric, eccentric KOs are encountered..."
It would seem for bonding over 250 volts the NEC is addressing the reducing washers issue, however they don't use the term 'oversized' elsewhere [that I have found].
An additional point - I see in 250.97 'Bonding for Over 250 Volts' - Exception - they add the word ' oversized' ...."where oversized, concentric, eccentric KOs are encountered..."
It would seem for bonding over 250 volts the NEC is addressing the reducing washers issue, however they don't use the term 'oversized' elsewhere [that I have found].
Re: CONCENTRIC KNOCK OUTS
I was told by a project manager for the Corp of Engineers that the information on reducing washers is found in the building materials list. The code only says that equipment will be installed in compliance with it's listing and labeling. Notice that the NEC specifically allows listed raceways as EGCs but for hubs and washers you have to look at the listing.
110.3 Examination, Identification, Installation, and Use of Equipment.
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
--
Tom
I was told by a project manager for the Corp of Engineers that the information on reducing washers is found in the building materials list. The code only says that equipment will be installed in compliance with it's listing and labeling. Notice that the NEC specifically allows listed raceways as EGCs but for hubs and washers you have to look at the listing.
110.3 Examination, Identification, Installation, and Use of Equipment.
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
--
Tom
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: CONCENTRIC KNOCK OUTS
Bennie:
T&B Reducing Washers: UL File No. E-13938, CSA File No. 2884
Bennie:
T&B Reducing Washers: UL File No. E-13938, CSA File No. 2884
- Location
- Illinois
- Occupation
- retired electrician
Re: CONCENTRIC KNOCK OUTS
Jim,
Does the listing say that reducing washers are suitable for grounding?
Don
Jim,
Does the listing say that reducing washers are suitable for grounding?
Don
- Location
- Bremerton, Washington
Re: CONCENTRIC KNOCK OUTS
I suspect the answer is the reducing washers are not listed as a ground fitting, but are listed as an accessory such as a condcuit strap. But I'll find out. Good questions all.
I suspect the answer is the reducing washers are not listed as a ground fitting, but are listed as an accessory such as a condcuit strap. But I'll find out. Good questions all.
- Location
- Bremerton, Washington
Re: CONCENTRIC KNOCK OUTS
OK, here is what Mike Holt replied about using donuts and bonding around them:
Great Question! The best I can offer is:
250.96 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.
Raceways, connectors and couplings are listed to have the capacity to conduct safely fault current. Reducing bushing are not listed, therefore they are not identified as having the capacity to conduct safely fault current.
God Bless, Mike Holt
OK, here is what Mike Holt replied about using donuts and bonding around them:
Great Question! The best I can offer is:
250.96 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.
Raceways, connectors and couplings are listed to have the capacity to conduct safely fault current. Reducing bushing are not listed, therefore they are not identified as having the capacity to conduct safely fault current.
God Bless, Mike Holt
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