Concentric and Eccentric

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joeyww12000

joeyww12000

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Chatsworth GA
My question of the day is, What is the skinny on concentric and eccentric knockouts. I havent been on a job yet where foreman has called for bonding bushings to be used where these KO's have been encountered. This goes for raceways with service conductors and raceways with conductors over 250v to ground. I just havent seen it. Can someone fill me in when proper bonding of raceways do to concentric and eccentric KO's is required.
 
Take a look at 250.97 and 250.92(B)(4). For what it's worth many boxes now are listed as suitable for bonding for over 250 volts with concentric or eccentric KO's.
 
There are a number of considerations.250.97, pay attention to 250.97(3).

But more so, you need to see (QCRV) of the UL WhiteBook, page 259 in the 2008 version.

For those who do not have the book:

"Grounding
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250V and where installed in accordance with ANSI/NFPA 70, "National Electrical Code." Reducing washer are intended for use with metal enclosures having a minimum thickness of 0.053 in. for non-service conductors only. Reducing washers may be installed in enclosures provided with concentric or eccentric knockouts, only after all of the concentric and eccentric rings have been removed. However, those enclosures containing concentric and eccentric knockouts that have been Listed for grounding purposes may be used with reducing washers without all knockouts being removed."


With this all being said, there are more restrictions to the installation use of reducing washers ("doughnuts" as some call them) then most people realize.


1. not on the line side of service disconnecting means


2. In older installations/enclosures, most likely you will need to remove all of the rings, as those boxes/concentrics are not listed for the purpose. call the manufacturer if you really need to know


3. Most newer enclosures have listed eccentric knockouts for grounding, again check with the manufacturer. (somewhere in the neighborhood of the last 7 years or so)
 
nakulak said:
correct me if I'm wrong, but I believe that tangential would fall under "eccentric" as far as NEC
Click to expand...


I don't believe that they're the same. If so why would we need another name for an eccentric KO? If you compare the two they look similar but ultimately not the same.
 
Pierre C Belarge said:
I agree. There are 2 types of KOs, Concentric or Eccentric. Tangential are a form of the Eccentric type. JMHO
Click to expand...


I would disagree simply because they're not mentioned as being the same in the NEC. Even if you examine them closely they resemble a combination of both concentric and eccentric KO's. The smallest KO does not break out and leave the KO edge on the metal box like an eccentric. The first KO is like a concentric since it is dependent on the second KO. Just my 2 cents.
 
The purpose of the bonding bushing is to make sure the metal raceway going between 2 enclosures stays grounded if it relies on a KO that can become a bigger KO thus losing it's ground if not bonded.


Only my opinion based on exp.
 
I have a question on application, can you use a grounding LOCKNUT on reducing washers per UL, panel manufacturer and NEC and maintain the effective ground fault protection path.
Pierre C Belarge said:
There are a number of considerations.250.97, pay attention to 250.97(3).

But more so, you need to see (QCRV) of the UL WhiteBook, page 259 in the 2008 version.

For those who do not have the book:

"Grounding
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250V and where installed in accordance with ANSI/NFPA 70, "National Electrical Code." Reducing washer are intended for use with metal enclosures having a minimum thickness of 0.053 in. for non-service conductors only. Reducing washers may be installed in enclosures provided with concentric or eccentric knockouts, only after all of the concentric and eccentric rings have been removed. However, those enclosures containing concentric and eccentric knockouts that have been Listed for grounding purposes may be used with reducing washers without all knockouts being removed."


With this all being said, there are more restrictions to the installation use of reducing washers ("doughnuts" as some call them) then most people realize.


1. not on the line side of service disconnecting means


2. In older installations/enclosures, most likely you will need to remove all of the rings, as those boxes/concentrics are not listed for the purpose. call the manufacturer if you really need to know


3. Most newer enclosures have listed eccentric knockouts for grounding, again check with the manufacturer. (somewhere in the neighborhood of the last 7 years or so)
Click to expand...
 
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