Hi all, I have a question for you about when and why a person would use a ground bushing...it has never really been explained to me.
One guy I worked with wanted one at either end of a run of Teck cable, but he couldn't explain the reasoning for this. The other day I was talking with another co-worker about this and he says it isn't necessary.
Back when I was doing residential work it was necessary to use a ground bushing on the connector inside the panel to properly bond the service entrance conduit. I guess the connector doesn't provide adequate bonding in this situation?
The code wording doesn't make sense to me so I figured you might be able to explain it a lil' better.
Bonding bushing should be used anytime a metal conduit enters any type of enclosure when the conductivity can not be ensured across where the conduit and enclosure meet. One example is a cocked conduit or a conduit is not square to the enclosure. this will prevent the locknut(s) from mating squarely to the box. Another example is the concentric / eccentric knockouts. Although they are usually rated for carry fault current, some times they limited on the rating or the concentric / eccentric sometimes get banged around during installation. When in doubt, throw on a bonding bushing.
Service conduits require bond bushings because you simply can't count on the lock nuts to handle fault current of service conductors. There was a picture floating around IAEI a while ago of a 4" conduit that had burned out the whole area of the enclosure where the lock nuts were.
Sparkyinak and gfretwell nailed it on the head. I'll elaborate a bit further: The reasoning for requiring a bonding bushing on a service entrance is that locknuts need to bite through paint or rust in order to contact bare metal and give good continuity to ground. Since we're talking about service entrances there's a high fault current available. Should a conductor short against the connector or conduit you want this to have good ground continuity.
Another example of requiring a bonding bushing or ground wedge would be when using PVC boxes and having teck cable or metal conduit run between them. The conduit or the armour of the cable would not be grounded by just using locknuts against a PVC box.
"Will it be cheaper if I drill the holes for you?"