RMC conduit going into box w/concentric KO's

[zeb7k]

I'm using rigid steel conduit with steel boxes where I have elected to use the conduit as the grounding conductor. I have a 1/2" conduit going into a masonry box with concentric 1/2 & 3/4 knockouts. I'm not crazy about concentric KO's so I thought I'd knock out to 3/4 and use a 3/4 to 1/2 reducing bushing where the conduit enters the box. (In this case it would actually be an "expanding" bushing.) ... and a locking nut on both sides of the connection.

Isn't what I propose better than using the 1/2 knockout then praying that the 3/4 knockout doesn't come loose? -- better for insuring a good ground? The code does not say much about reducing bushings. I really rather not run a separate grounding conductor.

--zeb

[RBM]

Sure it is, and just spin on a 1/2" threaded grounding bushing and you'll be well grounded

[John Grabowski]

I agree. The reducing bushing will look kind of hokey. Put a 1/2" bonding bushing on after the locknut and attach a grounding pigtail to it. Once the box and conduit are fastened securely the concentric KO should not work itself loose.

I think running a separate grounding conductor is the best way to go.

[Rick-Meister]

[John Grabowski]

*Check article 344.60

[terry]

Yes that also would be my understanding for any 'new' work; to current codes. Existing conduit previously used as grounding 'might' be grandfathered although some jurisdictions inspections may reject it if other work is being done in same area.

[John Grabowski]

*Check article 344.60 And also 250.118

[RBM]

Yes that also would be my understanding for any 'new' work; to current codes. Existing conduit previously used as grounding 'might' be grandfathered although some jurisdictions inspections may reject it if other work is being done in same area.

It's perfectly acceptable by current NEC

[The Daring Dufas]

Here's a little tip that me and my brother use when dealing with loose concentric KO rings. We take a conduit punch and turn it backwards so the flat parts are on either side of the ring. When the screw is tightened, the ring is mashed back into place and is tighter than it was to start with.

TDD

[zeb7k]

Thanks all for the replies.

BTW I got my permit in 2006 at which time my county was using NEC

2002.

I will definitely be using the grounding bushing to insure box-to- conduit ground. John mentioned to use the grounding bushing after the locknut. Couldn't the grounding bushing be used by itself? The ones I've seen are pretty beefy compared to the locknuts. Is it code or personal preference that you use both?

--zeb

[John Grabowski]

I've always used the locknut then a bushing. I guess you would have to find out from the manufacturer if their bushing was approved to function as a locknut would. You may find that the bushing will not screw all the way down enough to lock the pipe in place.