brother
Senior Member
I always thought that ground bushings were required on concentric knockouts even for voltages under 250 for conduits over 3/4 inch, But no where in the code can i find it. Any help would be appreciated thanks.
Ground bushings Required
- Thread starter brother
- Start date
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frizbeedog
Senior Member
- Location
- Oregon
250.92 (B) last paragraph
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
The rule is for over 250 V to ground only. The concentrics can't withstand the heat generated by a 277V ground fault, at 1 ohm 277V generates 75,000 watts of heat, at 120V its about 15,000 watts
- Location
- Massachusetts
tom baker said:
I agree, but concentric KOs have pretty much disappeared.
Now we deal with eccentric KOs which are generally rated for grounding above 250 without the need for a bonding bushing.
CHWflorida
Member
Does the rule not always apply to bonding at services?
elohr46
Senior Member
- Location
- square one
CHWflorida said:
read 250.92(B)
izak
Senior Member
- Location
- Springfield, MO
If I am not mistaken, it also applies to reducing washers in all locations at greater than 250volts to ground
izak
Senior Member
- Location
- Springfield, MO
which brings me to a question.... lets say i am coming out of a 4 square lighting box with an MC or FMC whip to a light, and I use reducing washers to put connector in a 3/4 hole. if the voltage is 277, do i need a bond bushing on the connector? and will it really do ANY good with MC or FMC(flex) ?
just curious as to everyones opinion
just curious as to everyones opinion
- Location
- Massachusetts
izak said:
No, oddly enough UL considers reducing washers as 'suitable for grounding'
And that is not really an opinion as the information can be found in the UL white book.
paul
Senior Member
- Location
- Snohomish, WA
How is the other end of your raceway terminated and is this raceway in lieu of your grounding conductor?
- Location
- Massachusetts
POWER_PIG said:
You got it.
The concentric..eccentric...'stuff' is great. However...it isn't common knowledge that most eccentric KO's are rated for over 250 volts..so..
...just have proof of what others are tossing around before an interaction with an inspector who questions the lack of bonding bushings.
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Why isn't this main stream knowledge yet?
...just have proof of what others are tossing around before an interaction with an inspector who questions the lack of bonding bushings.
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Why isn't this main stream knowledge yet?
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
UL White Book
METALLIC OUTLET BOXES (QCIT)
CONCENTRIC OR ECCENTRIC KNOCKOUTS
All boxes with concentric or eccentric knockouts have been investigated for bonding and are suitable for bonding without any additional bonding means around concentric (or eccentric) knockouts where used in circuits above or below 250 V, and may be marked as such.
For circuits other than services, the reference in the NEC is 250.97, especially read the second portion of the first sentence of the Exception.
Also remember this is for fairly new boxes, as the "older" boxes would not fall within these parameters and most likely will require some kind of bonding around the eccentric/concentric KOs, for over 250 V circuits.
Reducing Washers, fall under
OUTLET BUSHINGS AND FITTINGS (QCRV)
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with the ANSI/NFPA 70, "National Electrical Code". ...for non-service conductors only. Reducing washers may be installed in enclosures provide 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 bonding purposes may be used with reducing washers without all knockouts being removed.
So, if you are using reducing washers (doughnuts as they are called here in lower NY), be cautious when installing them in existing "older" boxes, as they most likely will require bonding around them.
METALLIC OUTLET BOXES (QCIT)
CONCENTRIC OR ECCENTRIC KNOCKOUTS
All boxes with concentric or eccentric knockouts have been investigated for bonding and are suitable for bonding without any additional bonding means around concentric (or eccentric) knockouts where used in circuits above or below 250 V, and may be marked as such.
For circuits other than services, the reference in the NEC is 250.97, especially read the second portion of the first sentence of the Exception.
Also remember this is for fairly new boxes, as the "older" boxes would not fall within these parameters and most likely will require some kind of bonding around the eccentric/concentric KOs, for over 250 V circuits.
Reducing Washers, fall under
OUTLET BUSHINGS AND FITTINGS (QCRV)
Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with the ANSI/NFPA 70, "National Electrical Code". ...for non-service conductors only. Reducing washers may be installed in enclosures provide 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 bonding purposes may be used with reducing washers without all knockouts being removed.
So, if you are using reducing washers (doughnuts as they are called here in lower NY), be cautious when installing them in existing "older" boxes, as they most likely will require bonding around them.
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