Bushing or no bushing? Voltage is 480

ctaylo360860

Senior Member
Location
colorado
Occupation
ME
Inspector says I can’t use exception 3 or 4 in 250.97 because I’m using liquid tight flexible metal conduit.
Race way has wire type egc and ul listed fitting with a shoulder is seated to the box. It’s going into a concentric hole.
Even then the bushing would only bond the raceway if the connector is connected with the lock but and the fitting that secures the raceway to the connector.
Bonding bushing only hits the threads on the connector if lock button isn’t tight it’s not making a good connection. What you guys say?!
 

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Carultch

Senior Member
Location
Massachusetts
The issue is that the narrow connections between the ring knockouts are a choke-point in the grounding path that is "not strong enough" for a fault driven by over 250V to ground, unless the box is specifically listed for bonding at a higher voltage (e.g. 4S boxes that have heavy duty knockouts).

If you use up all of the ring knockouts at a particular conduit entry or enter at your own punched hole in the box, you get to treat it as if the ring knockouts were never there in the first place. It is when the knockout rings remain at the entry in question, that this rule applies. I cannot see from the photo whether the conduit entry in question has used up all the knockout rings or not.

The reason we attach a bonding bushing is to enable the connection of a bonding jumper between its lug and the grounding terminal of the equipment. Just installing a bonding bushing isn't enough.
 

ctaylo360860

Senior Member
Location
colorado
Occupation
ME
I understand a 4 square box wouldn’t require bonding bushings. But it seems silly to me to have to use one in a concentric hole…. But by golly if it’s code I guess I’ll have to find some more 1/2 bonding bushings to be compliant. ….
 

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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I don't believe that any cabinets or enclosures other than specific Article 314 boxes meet the suitable for bonding over 250 volts requirement. Disconnect switches do not. Based on the photo in post #5 I agree with the inspector.
 

Carultch

Senior Member
Location
Massachusetts
Sometimes I see grounding bushing and bonding bushing used interchangeably. I assume it's the same thing?
I use them interchangeably, and I'm not aware of any manufacturer who has any difference between the two terms, if both are used. I'm accustomed to calling it a bonding bushing, based on its purpose.
 

paulengr

Senior Member
I’ve seen the damage from these things. It can melt the whole side of the box out. In mining NEC is optional so it’s a little more “loose” with some rules. I have no idea why NEC used a voltage limit but those little tabs simply can’t handle above a certain amount of fault current. The rule we always went by is drill your own hole or remove all rings. Handy boxes (4x4) are the worst because it’s all rings.

Here is what UL says. Looks like if short circuit current is under 1530 A you should be OK. That’s a pretty low fault current limit considering Code minimum for devices is 5,000 A. So with a 3% impedance that’s 1530 x 0.03 = 46 A x 240 x 1.732 (I’m being very generous here) I get 19 kVA. Doesn’t seem like knockouts are a good idea even in residential service. Seems like they’re assuming fault current is very limited somehow. Everything I do is industrial so that’s a bad assumption.

 
Last edited:

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I’ve seen the damage from these things. It can melt the whole side of the box out. In mining NEC is optional so it’s a little more “loose” with some rules. I have no idea why NEC used a voltage limit but those little tabs simply can’t handle above a certain amount of fault current. The rule we always went by is drill your own hole or remove all rings. Handy boxes (4x4) are the worst because it’s all rings.

Here is what UL says. Looks like if short circuit current is under 1530 A you should be OK. That’s a pretty low fault current limit considering Code minimum for devices is 5,000 A. So with a 3% impedance that’s 1530 x 0.03 = 46 A x 240 x 1.732 (I’m being very generous here) I get 19 kVA. Doesn’t seem like knockouts are a good idea even in residential service. Seems like they’re assuming fault current is very limited somehow. Everything I do is industrial so that’s a bad assumption.

The real key in the article is that only metallic outlet boxes are evaluated for carrying fault current and the the cabinets used for panelboard enclosures are not.
 
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