Bonding bushings

Status
Not open for further replies.
J

jeff48356

Senior Member
Location
Livonia, Michigan
When are we actually required to use bonding bushings versus standard plastic ones? My understanding is whenever a metal conduit or nipple enters a meter base or service panel, and the sizes of the knockout and conduit are smaller than the largest knockout that can be removed (thus, still concentric circles left), then a bonding bushing is required. I've always used regular bushings, and have never failed an inspection on account of them. Are they required under 2008 NEC, or is this something new?
 
They are required when you encounter concentric/eccentric KO's or if you're bonding a metal raceway stubbed up into an open bottom enclosure. If the KO is punched the same size as the conduit entry then there are other methods that will work so they wouldn't be required in those installations.
 
jeff48356 said:
When are we actually required to use bonding bushings versus standard plastic ones? My understanding is whenever a metal conduit or nipple enters a meter base or service panel, and the sizes of the knockout and conduit are smaller than the largest knockout that can be removed (thus, still concentric circles left), then a bonding bushing is required. I've always used regular bushings, and have never failed an inspection on account of them. Are they required under 2008 NEC, or is this something new?
Click to expand...

The "ring knockouts remaining" issue is voltage-dependent. Both the circle ring and "crescent moon" types. 250 Volts nominal to ground is the cutoff. 250V and less, and your remaining ring KO's can be used with standard locknuts as your bonding means. Over 250V nominal to ground, and you need a bonding bushing or other method of connecting a bonding jumper, in order to not rely on the thin metal between the ring KOs. There do exist enclosures with ring KO's specifically listed for higher voltage bonding, usually a lot more robust and challenging to remove than standard KO's.

Another place you would need a bonding bushing, is a bit more obvious. Metal conduit used with nonmetal enclosures.

Service conductor raceways need a bonding bushing on at least one side, even if neither of the above rules apply. Raceways containing a GEC need it on both sides.
 
Listed Metallic Outlet Boxes covered under UL Product Category QCIT with concentric and eccentric knockouts have been investigated for grounding and bonding above and below 250 volts. The Guide Information for this Product Category says in part;

"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."

There is also a UL publication addressing this issue that can be found HERE. I am in the process of updating this Publication to the 2017 NEC.

Cabinets and cutout boxes with concentric and eccentric knockouts have not been investigated for bonding above 250 volts and would require bonding bushings to be used if all the rings are not removed.

Chris
 
Bonding bushings are never required - but are usually the easiest way to bond when more then just a standard locknut is required.
 
kwired said:
Bonding bushings are never required - but are usually the easiest way to bond when more then just a standard locknut is required.
Click to expand...

In some installation types they are the only thing that will be code compliant, not an explict requirement but a defacto one.
 
infinity said:
In some installation types they are the only thing that will be code compliant, not an explict requirement but a defacto one.
Click to expand...
You should have the option of an external bonding jumper and pipe bonding clamps nearly all the time. Might not like the looks of it but is an option.
 
Status
Not open for further replies.
Top