bonding bushings

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M

mortimer

third party inspector
Location
New England
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retired
Are bonding bushings required from the main switch gear to a MDC 3' away? .................................................
 
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Jerramundi said:
Is it really only for circuits over 250V? I was always told it was for all concentric knockouts, which made sense to me to ensure electrical continuity in case the pipe broke one of the remaining concentric rings of the KO and thus bonding continuity.
Click to expand...

As more time marches on, you will be surprised at how much of the stuff you were told was BS.
 
Jerramundi said:
Is it really only for circuits over 250V? I was always told it was for all concentric knockouts, which made sense to me to ensure electrical continuity in case the pipe broke one of the remaining concentric rings of the KO and thus bonding continuity.
Click to expand...
Concentric KO's are approved for bonding under 250 volts to ground. Some are even approved for over 250 volts to ground.
 
electrofelon said:
As more time marches on, you will be surprised at how much of the stuff you were told was BS.
Click to expand...
Oh, I'm aware. This isn't the first time something I learned from another electrician turned out to be BS, lol.
That's part of why I decided to enroll in night classes and purchase a copy of the NEC for myself.

It just made sense to me so I never questioned this particular one because a bonding bushing with a jumper would ensure electrical continuity if the remaining concentric ever broke, for whatever reason.

Perhaps not NEC required and going a little "above and beyond," but the logic still seems sound.
 
Jerramundi said:
Oh, I'm aware. This isn't the first time something I learned from another electrician turned out to be BS, lol.
That's part of why I decided to enroll in night classes and purchase a copy of the NEC for myself.

It just made sense to me so I never questioned this particular one because a bonding bushing with a jumper would ensure electrical continuity if the remaining concentric ever broke, for whatever reason.

Perhaps not NEC required and going a little "above and beyond," but the logic still seems sound.
Click to expand...
Code aside, I do hate concentric/eccentric KO's and almost always get rid of them all and use reducing washers and use a BB if its a metal raceway. Those heavier duty KO's you typically find on meter sockets are ok and Ill typicall skip the RW, but since that is usually a service, you will need a BB anyway.
 
For raceways in general, bonding bushings are only required if you have missing or impaired continuity at the conduit termination. An example of missing continuity would be terminating on a plastic enclosure. An example of impaired continuity, is ring knockouts remaining, when over 250V to ground. Unless listed otherwise (as occurs on 4S Boxes), the sections of metal between ring knockouts are only "strong enough" for carrying the fault current, when driven by 250V to ground or less. So for 277/480V circuits, you'd need a bonding bushing and bonding jumper, to produce an alternative fault current path around the impaired continuity of the chokepoint.

The special exceptions are raceways with service conductors and GEC's. Service raceways require a bonding bushing on at least one side. Conduits containing a GEC, require it on both sides.
 
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